Development of a Biomechanical Model and Validation of Assessment Tools for Personal Load Carriage Systems

BY

Wm. Alan H. Rigby

A thesis submitted to the School of Physicai and Health Education in confonnity with the requirernents

for the degree of Master of Science

Queen's University at Kingston Kingston, Ontario, Canada

September, 1999

copyright 8 Wm. Alan H. Rigby, 1999

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Abstract

This study was part of a larger military project to improve personal load carriage

systems for soldiers. The goal of this study was to develop and validate a personal ioad

carriage system biomechanical model. The mode1 would serve as the basis for a personal

load camage system design tool, which would provide a better understanding the pack-

person interface and in tum help development of new systems for soldiers. A sub-

problem of this study was to develop and validate an improved pack testing system for

evaluation of the biomechanical modei and for fbture scientific and field studies.

A load distribution mannequin and force platform were part of the comprehensive

testing system designed to provide the necessary measurements to validate past and

current personal load carriage system biomechanical models. In addition, hkro new

devices, a strap tension probe and an instmmented test pack, capable of measuring pack

strap tensions and lumbar contact forces respectively, were created. These new

measurement tools were validated within a 5% average error and suggestions were made

for funher irnprovements.

The current biomechanical model was the third iteration in a series of persona1

load carriage system models. The current model employed two different techniques to

predict pack-person interface variables. The first technique used the principles of static

equilibrium of the pack-person interface to determine unknown variables and preûict

contact forces between the pack and the human form. Tension in the upper shoulder,

lower shoulder, and load lifter straps supported much of the pack mass. Through fiction

and anatomical geometry the waist belt and lumbar region provided vertical lie. Al1

unknown elements of the equilibrium needed to be solved. The second technique used

logical relationships between different elements of the pack-person geometry, interna1

forces and moments, and contact forces to predict unknown variables and pack-person

contact forces. Again, tension in the upper shoulder, lower shoulder, and load lifter

straps supponed much of the pack mass. The relationship between the shouider straps

was modeled using a modified pulley equation. The waist belt and lumbar pad lie forces

were predicted bas4 on fiictional contributions and vertical components of reaction

forces.

The current model could not be used as a robust xientific tool. Equilibrium

predictions of pack-person interfiice variables were quite poor compared to measured

values (average p-values less than 1 . 0 * 1 ~ ~ ) . The large coefficients of fiction at the

shoulder, lumbar region, and waist caused the predicted ranges of the regional models to

be so large that their ability to contain the associated validation measurements were

suspect, despite the predicted ranges encompassing an average of 7 1.3% of the rneasured

values. On the other hand, the geometric components of the model were valid as the

model predictions and measured values were statistically correiated (average p-values of

0.99). The sensitivity analysis proved that the equilibrium expression that predicted the

outputs were highly sensitive to input variables, implicating the load lifter strap model as

a potential cause of the "ill-conditioned" system. Regional model and geometric outputs

were less sensitive to changes in input variables.

In general the model, modeling process, and sensitivity analysis provided insight

into and qualitative information about the pack-person interface. In addition, two new

measurement tools were validated and can be added to the personal load carriage system

battery of test. Future directions outlined in the thesis showed how the current rnodel and

validation techniques could be used for the next iteration of the model, other scientific

studies, and field tests.

Acknowledgements

I would like to acknowledge the Defense and Civil Institute of Environmental

Medicine for their financial and materials contributions to this thesis. 1 found

this work very stimulating and enjoyable, and 1 appreciate DCiEM's and Major Linda's

Bossi's help in making this research possible.

1 would also like to take this opportunity to thank a number of individuals,

without whom this thesis would no€ have been possible: Dr. Joan Stevenson, for ail your

help, guidance, and encouragement over the past 4 years; Dr. Ron Pelot and Dr. Tim

Bryant for your invaluable contributions and advice; Gerry Saunders of the Clinical

Mechanics Group for your precision and tireless efforts in construction of the strap

tension probe; The gang in the lab: Jon, Wayne, Pat, Derek, and Sue, for making

yourselves so available for al1 the day to day help.

1 would also like to recognize my many friends and family, who's support has

rneant so much. A special thank you to my mother, Sandy Rigby, for everything you

have done, your credits are too long to list.

Finally, 1 would like to acknowledge Jenn Ellis. Your motivation, support, love,

and understanding throughout this thesis and in life continue to be my inspiration. Thank

you, Jenn. 1 love you.

................................................................................................................. Abstract

................................................................................................. Acknowledgments

....................................................................................................... List of Figures

......................................................................................................... List of Tables

......................................................................................... Chapter 1 : Introduction

General Project Focus ................................................................................

Review of Literature ..................................................................................

..................................................................... Biomechanical Model

................................ Objective Evaluation of a Pack's Effectiveness

General Review of Pack-Person Interface Literature .......................

Chapter 2: Development and Validation of a Biomechanical Assessrnent Tool ........

Introduction ...............................................................................................

Test Mannequin .........................................................................................

Force Platforrn ...........................................................................................

Test Pack ...................................................................................................

................................................................................... Strap Tension Probe

.................................................................. Strap Tension Probe Validation

Accu racy ........................................................................................

Reliability ....................................................................................... . . Precision .........................................................................................

.............................................................................. In Yivo Analysis

...................................................... Strap Tension Probe Validation Resuhs

Calibration ...................................................................................... ................................................................. Accuracy and Reliability

In Vivo Analysis ..............................................................................

................................................ Strap Tension Probe Validation Discussion

................................................ Strap Tension Probe Validation Conclusion

1

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Chapter 3 : Biomechanid mode1 of a Personal Load Caniage System ....................

Introduction ...............................................................................................

The Mode1 .................................................................................................

....................................................................... General Pack Model

......................................................................................... Notation

The Shoulder and Related Pack Elements ...................................................

.................................................................................. The Shoulder

................................................................... Geometric Calculatiorts

....................................................................... The Shoulder Model

The Waist and Related Pack Elements ........................................................

The Waist .......................................................................................

................................................................................ The Waist Belt

The Lumbar Region and Related Pack Elements .........................................

The Lumbar Region ........................................................................

The Lumbar Pad .............................................................................

The Rote of Friction ...................................................................................

Chapter 4: Biomecha~cal Mode1 Validation Methods ............................................

............................................................................................... Introduction

.................................................................................. Validation Procedure

The Model ......................................................................................

............................................................... The Measurement System . . .

Sensitivity Analysis .....................................................................................

..................................................................................... Statisticd Analysis

................................................................................................. Chapter 5: Results

............................................................................... Coefficients of Friction

...................................................................................... Model Predictions

....................................... ......................... Geometric Predictions .... Shoulder Mode1 ..............................................................................

Lumbar Pad Mode1 .........................................................................

Waist Belt Model ............................................................................

List of Figures

Figure 1-1 : Phase 1 shoulder based pack-person interface mode1 ............................ ..................................................................... Figure 1-2: Phase II waist belt model

................................................................... Figure 1-3 : Phase II lumbar pad modd

....................................... Figure 1-4: Phase II personal load carriage system model

.......................................................... Figure 2- 1 : The load distribution mannequin

............................................................................... Figure 2-2: Modified test pack

........................................................................ Figure 2-3: Modified surgical pliers

Figure 2-4: Modified pliers open and closed Mews .................................................

......................................................... Figure 2-5: Schematic open and closed views

............................. Figure 2-6: Geometncally simplified strap tension probe system

Figure 2-7: Change in fùnctional strap length .........................................................

Figure 2-8: Stifiess of pack system .......................................................................

Figure 2-9: Strap tension probe validation setup .....................................................

Figure 3-1 : Elements of a standard commercial pack and the biomechanical mode1 .

...................... Figure 3-2: Biomechanical model of a personal load carnage system

Figure 3-3: Determination of strap and wrap angles ...............................................

................................................................... Figure 3-4: Net shoulder contact force

......................... Figure 3-5 : Anatomical simplification of a transverse waist section

................................ Figure 3-6: Wedge shape of hips in Frontal and sagittal planes

............................ Figure 3-7: Complete anatomical simplification of the hip region

Figure 3-8: Liff capability of the waist belt .............................................................

... Figure 3-9: Transverse view of a typical quarter section of a waist belt in tension

.............. Figure 3 - 1 0: Anatomical simplification of the lumbar region, sagittal Mew

.................................................... Figure 3- 1 1 : Lumbar region - lumbar pad model . . ............................................................................. Figure 3 - 1 2: The role of fhction

vii

List of Tables

th Table 2- 1 : 50 percentile male human fonn mannequin .......................................... 18

Table 2-2: Calibration coefficients and error measures ............................................ 31

Table 2-3: Accuracy and reliability results with respect to tension .......................... 32

Table 2-4: Accuracy and reliability results with respect to stifiess ......................... 32

Table 2-5: Precision analysis results ....................................................................... 33

Table 2-6: In vivo analysis results ........................................................................... 33

Table 4- 1 : Contribution of fnetion over the shoulder, waist belt, and lumbar pad .... 61

Table 4-2: Test setup configurations ...................................................................... 63

..................................................... Table 5- 1 : Coefficient of Ection determination 66

Table 5-2: Shoulder geometry model predictions compared to actual measured values 67

Table 5-3: Upper shoulder strap tension (Tt) mode1 validation results .................... 69

Table 5-4: Shoulder contact force ( ~ 3 mode1 validation results ............................. 70

Table 5-5: Shoulder contact force (sNX) mode1 validation results ............................ 71

Table 5-6: Shoulder contact force (!Snz) mode1 validation results ............................ 72

Table 5-7: Shoulder fnction force (FR) mode1 validation results ............................ 73

Table 5-8: Lumbar contact force (Fx) mode1 validation results ............................... 75

Table 5-9: Lumbar pad lifi force ( F ~ ~ ) model validation results .............................. 76

Table 5-10: Waist belt lift force (FZ) mode1 validation results ............................... 78

Table 5- 1 1 : Waist belt - lumbar pad complex lifi force (Fz) model validation results 79

Table 5- 12: Summaiy of sensitivity analysis for geometric outputs ......................... 80

Table 5- 13 : Summary of sensitivity anaiysis for regional mode1 outputs .................. 81

Table 5- 14: Summary of sensitivity analysis for equilibrium expression outputs ...... 82

Mii

Chapter 1

Introduction

General Project Focus

Many different devices exist to improve the load carriage capability of humans.

History has shown the backpack to be the most comrnon choice for both civilians and

military personnel. Throughout its military use, the pack's basic design undenvent very

little change. However, the ment recreational boom has led to many vked designs that

have magnifieci the diflerence between state-o'the-art civilian packs and current military

systems. Only by understanding the effect of these changes can design advances be made.

Furthermore, understanding what factors make pack designs effective pnor to

construction of prototype designs would be even more valuable for cornfort and fit of

systems. Until now advances in pack design have corne from costly, time-consuming, and

oAen subjective evaluations of prototype systems. A more effective method of evaluating

current and future pack designs is necessary.

The impetus for this project came as part of a larger Canadian Forces endeavor to

better outfit rnilitary personnel under the auspices of Defense and Civil Institute of

Environmental Medicine. Queen's Ergonomics Research Group has contributed to this

endeavor over the last five years. Their research has encompassed biomechanical,

physiological, and subjective analyses of persona1 load carriage systems (as well as load

carriage webbing and vests) and the pack-person interface. The main thrusts of the

Queen's Ergonomics Research Group contracts were to create a comprehensive battery of

personal load carriage system evaiuations. Part of that work included a personal load

canlage system biomechanical model. This model was to be the basis of a design tool for

evaluating and improving cunent and proposed pack designs.

If an objective method of evaluating pack designs dunng the initial stages of the

design process could be developed, then poor designs could be discarded, retaining

potentially effective ones, thus saving time and money. Not only could packs be evaluated

eariy in the design process, but dso insight could be gained into how design variations

affect a pack's usefùlness. To develop such a method, three essential elements must be

known. One, the variables of a pack-person interface that determine a pack's effediveness

must be understood. Two, objective methods of measuring the elements deemed

important must be created. And three, a method of predicting the objective measures,

based solely on pack design critetia and user-controlled inputs must be generated.

Work done by Stevenson et al. (1995, 1996, 1997, and 1998) succeeded in

completing the first two requirements of such a personal load carriage system evaluation

tool. Through subjective assessment of packs, experienced user interviews, user focus

groups, and in-field measurements, Stevenson et al. (1995, 1996, and 1997) developed a

comprehensive list of variables that predicted pack performance. Furthemore, Stevenson

et al. (!995, 1996, 1997, and 1998) created and validated a battery of objective measures

of pack-person interface variables. Finally, Bryant et al. (1997) was able to relate these

objective measures to the subjective evaluations thereby providing a method for

evaluating pack designs. If, therefore, a method of predicting the objective outcomes,

based solely on design criteria and hypothetical pack components could be achieved,

personal load cmiage systems could be evaluated objectively prior to the construction of

a single prototype.

The general purpose of this work was to develop and validate such a predictive

tool. Specifically, it was the author's intent to develop and validate a biomechanical

mode1 of the pack-person interface. Such a model would theoretically predict the pack-

person interface variables, such as forces and moments, that previous researchers, found

to be important objective measures (Stevenson et al., 1997).

Essentially, biomechanical models are variable relations and equilibrium

expressions that are used to represent a physical system. They can be used to predict the

values of variables that can not otherwise be measured thus providing insight into the

system by iliustrating the relationships between pack elements or predicting the values of

variables. While it is this latter hnction of the model, or the former two goals, dl

objectives are of interest to the author. Being able to describe variables that could not

otherwise be measured provides remchers with the ability to better evaluate current

designs and improve the ability to collect data in al1 situations. Insight gained fiom a

biomechanical model can lead to a better general understanding of the pack-person

system. The relationship between elements of the pack-person system, how specific

elements determine pack eflectiveness, the sensitivity of the system to input variables, and

the major processes that drive the pack-person system can al1 be better understood.

Finally, the modeling process itseif can generate unique perspectives and novel approaches

to new and improved personal load carriage systems.

Review of Literature

Biomechanical Modtl

This work represents a more comprehensive model than the two previous

generations of the pack-person interface biomechanical model. Stevenson et al. (1995)

initially developed a shoulder-based model for suspension of a simple pack, which was

designated Phase 1. MacNeil(1996) funhered this work by validating t his shoulder-based

model. A Phase II biomechanical mode1 was developed by Rigby (1997) as a continuation

of this work.

The Phase 1 model was based on a simple bag-and-straps representation of a pack.

Figure 1 - 1 illustrates the shoulder-based model and Equations 1 - 1 through 1 -3 detail the

equilibrium expressions. The upper and lower shoulder straps hold the load carriage

system vertically and against the body while their couples apply force to the shoulder. In

fact, MacNeil(1996) was able to show that the upper shoulder strap exerted more force

than the lower, and the difference between these two strap segments, which are connected

over the surface of the shoulder, was equal to the force of fiction on the surface of the

shoulder. As a result he was able to relate the tension in the upper and lower shoulder

strap via the pulley qwtion (Equation 14). MacNeü also showed that a third force

existeci at the lower back-lumbar pad contact point. This force wuntered the net

horizontal force created by the shoulder straps, thus satisfjing the moment equilibrium

about the centre of the pack.

Figure 1 - 1 : S houlder based pack-person intertace model, adapted fiom MacNeil(1997)

and Stevenson et al. (1995).

The following equations were taken Grom MacNeil's (1996) work and, for

continuity were adapted to use notation found in the current model. Please refer to the

notation descriptions given in Appendix 1.

Equation 1 - 1 : -Tl*(cosei) - T2-cos(û2) + Fx - W*sin(P) = O

Equation 1-2: Tl*(sin&) + T2*sin(e2) - Wgcos(fl) = O

Equation 1-3 : TI-(cos~I)*(v~ - d3+ddi) - Ti+inei)-(vX) + Fx*(vrd3) -

TI-cos(~~)-(vz-~,-~~) - T2-sin(8+(vs) = O

Equation 1-4: Ti = ~ 2 . p

During Phase II of the model, Rigby (1997) used MacNeil's (1996) shoulder-based

model and added a hip Mt-lumbar pad complex model to create a more comprehensive

version. Rigby (1997) modeled the waist as a partial half cone where the siope of the cone

represented the anatomical dope of the hips. The hoop stress equation was used to

determine the net compressive force generated by the tension in the waist belt. The

vertical reaction force component of the net compressive force generated vertical lift and

fiction was proposed to resist the tendency of the pack to slide down the hips. Similady,

the lumbar region was modeled as a flat faced surface with a sagittal plane angle

equivalent to the angle created in the lower back by natural lordosis. The horizontal force

at the lumbar pad was proposed to be converted to vertical lie by the geometry of the

lower back and friction resisted the tendency of the pack to slide dom the back (Rigby,

1 997).

Figures 1-2 and 1-3 illustrate the waist belt and lumbar pad models respectively

and Equations 1-5 through 1- 10 detail the mathematical relationships. The waist belt-

lumbar pad complex was added to MacNeil's (1996) equilibrium expressions. The

resulting Phase II pack mode1 is displayed in Figure 1-4 and the equilibrium equations are

detailed in Equations 1 - 1 1 though 1 - 13.

Low Back

Abdomen

Waist Belt

Hip

Figure 1-2: Phase II waist belt model, adapted âom Rigby (1997).

6

Figure 1-3: Phase ïI lumbar pad mode1 adapted from Rigby (1997).

Figure 1-4: Phase II personal load carriage system, adapted fiom Rigby (1997).

The following equations were taken fiom Rigby's (1997) work and, for continuity

were adapted to use notation found in the current model. Please refer to the notation

descriptions given in Appendix 1.

Equation 1-5: Fz = FBz + F~~

Equation 1-7:

Equation 1-8:

Equation 1-9:

Equation 1 - 10:

Equation 1 - 1 1 :

Equation 1 - 1 2:

Equation 1 - 1 3: O = Fx*(vz-d,) - T2-(cos8&(vz-drd3) - Fz(vx) - T2*(sinûz)*(vx) - Tl-(sine ~)-(vx) + Tl.(cosû &(d l-d3+vz)

Rigby (1 997) predicted and measured the forces and moments associated with the

pack-person interface of six civilian-style packs. Ali measurements were within a self-

imposed 10% error limit with two exceptions. The lumbar contact force error was 28%

and one waist belt - lumbar pad Lüt force error was 433.7%. The lumbar contact force

error was, however, within the accutacy range of the measurement device used (TekscanTM

pressure sensing system) and was considered acceptable and the high error in the lift force

was considered an anomaly. The entire Phase II model validation, however, was limited

by the measurement system and the relatively low statistical power resulting fiom a small

number of packs tested. Specifically, no tools existed to accurately measure the tension in

the upper shoulder strap or determine the vertical lia contribution generated by the waist

belt independent of the lumbar pad and vice-versa.

While the work done by Rigby (1997) provided unique insight into the waist belt-

lurnbar pad complex, the validation technique lefi the Phase II model with certain

limitations. MacNeilts (1996) validation of the shoulder strap tensions was accepted, the

vertical lia of the waist belt-lumbar pad complex was validated as a unit, and the

understanding of some of the intemal mechanisms of the system was lefi unclear.

Objective Evaluation of' a Pack's Effcetiveness

As part of the larger military load carriage project, Bryant et al. (1997) developed

an evaluation method for pack performance. Basically, soldien experienced in using

persond load camage systems subjectively evaluated a series of packs and objective

measures of those same packs were made. A Pearson correlation matrix was created

using the two data sets and the objective measures that accurately predicted the user's

evaluations of the packs were determined (Bryant et al., 1997). The subjective and

objective tests will be described in more detail below.

Subjective (human factors) measures were gathered as part of the larger DCIEM

project (Stevenson et al., 1995, 1996, 1997, and 1998). Experienced pack users from the

Canadian Amed Forces conducted numerous physicai tasks while wearing a series of

packs. Activities included: long distances marching, obstacle course running, donning the

pack, doffing the pack, agility circuits, mobility exercises, and lethality exercises (Doan,

1998). Upon completion of the exercises, each user completed comprehensive

questionnaires, rankings, and ratings regarding the pack and its effect on these activities.

Stevenson et al. (1995, 1996, 1997, and 1998) developed a number of objective

measurement tools for packs and the pack-person interface. The Load Carriage Simulator

rnakes dynamic measures of forces and moments at the L3 spinal level, contact pressures,

and relative motions of the pack during simulateci human motions. The Cornpliance Tester

measures the relative stEness of a pack system and the Load Distribution Mannequin

measures the relative load transmitted fiom the pack to the person over different locations

on the body (Stevenson et al., 1996, 1997, and 1998).

At an alpha level of 0.05, r > 0.67 represented signifiant correlations when the

subjective and objective measures were compared using a Pearson correlation matrix of 76

variables (Bryant et al., 1997). Significant correlations were observed between specific

human factors measures and Il displacementlforce load carriage simulator measures and

10 pressure/sti&ess variables. Bryant et al. (1997) noted that: pack displacement was

strongly correlated with posteiior hip discomfon; force and moment averages and

amplitudes were correlated with mobility and cornfort; and, a high correlation existed

between vertical force amplitude and overall pack ratings in the human trials. At lower

correlation levels, Bryant et al. (1997) reported that pressure measures were correlated

with discomfort scores and pack stifiess measures were correlated with mobility and

agility scores.

Objective measures that were shown to correlate significantly with human factors

were placed in a benchmark pool and the IO' and 90' deciles were determined using

means, standard deviations, and the t-distribution statistic (Bryant et al., 1997). In this

way, the objective measures of future packs could be evaluated against this benchmark

pool. In other words, below the 1 0 ~ decile a pack would be considered poor, above the

90' it would be considered excepiional, and scores between were deemed average (Bryant

et al., 1997).

Bryant et al. (1997) also developed a first generation threshold limit value

detemination for objective measures of pack-person interface pressures. A linear

regression (8 = 0.31, a = 0.05) of pressure measures correlated highly with subjective

measures of pressure discomfort and the confidence interval was determined for pressure

scores at which 9% of users reported discomfort. Thetefore, 9W of users perceived

discomfort at 20 kPa and it was suggested that average skin contact pressures should not

rise above this threshold lirnit value (Bryant et al., 1997). These threshold lirnit vdues

were also in the same range as those reported for blood occlusion and bedsores (Holloway

a al., 1976), other factors which are associated with tolerance to skin contact pressure.

Since objective measures of pack-person interface variables could now be used to

evaluate pack designs, the role of the biomechanical model becomes more important. If a

model existed that could predict these objective measures, packs could be evaluated based

only on inputs to the model. Prototype designs could be evaluated without the cost and

time of constmcting an actual prototype. This would certainly be of significant advantage

to pack designers and evaluators.

Gtneral Rtview of Pack-Person Interface Literature

The majority of previous literature falls into one of three categories; physiological

studies, subjective appraisal studies, and biomechanical studies (Pelot et al., 1995). The

following bief discussion has been classified according to these genres.

Numerous physiological studies have been conducted to determine the maximum

load that can be camed by individuals. Sagiv et al. (1994) had 26 male subjects walk for 4

hours at 4.5 km/hr. Based on heart rate scores, blood pressure, and perceived exertion,

Sagiv et al. (1994) determined that individuals were capable of carrying as much as 66%

of their body weight. Similarly, Holewijn (1989) indicated that as much as 37 kg for

males and 22 kg for females could be carrieci with limited impact on metabolic systems.

Epstein et al. (1988) supported Holewijn (1989) and Sagiv et al. (1994) by concluding,

based on V02 maximal tests (the maximum amount of oxygen that is consumed in

milliliters per minute per kilogram of body weight), that 40 kg would ultimately lead to

fatigue. Conversely, based on heari rate, temperature, VOI maximum and perceived

exertion, Shoenfeld et al. (1977) suggested that 25kg was the maximum sustainable load

for a 5 to 6 kmhr march. And Yu and Lu (1990) concluded that, for Chinese soldiers, an

even lower maximum value of 20 kg was indicated by energy requirement and heart rate.

These extremes seem to be best represented by Goslin and Rorke (1986) and Patton et al.

(1991) who showed that exertion increased linearly with load and speed of marching.

Maximal load carriage has also ken evaluated by subjective analysis. During a

retrospective study of self-reported perception of combat loads, Hunter and Turl (1964)

suggested 18 kg was a maximum. The Canadian Department of National Defense (1982)

determined, fiom a self-reported fatigue study, that soldiers wukl carry up to one third of

their body weight. In terms of performance ratings on an obstacle course, Nelson and

Martin (1982) indicated that performance related inversely to load rnass. Despite these

low payload values (compared to physiological limitations), specific trades in the Canadian

Armed Forces denote required loads of 66 to 70 kg. Similady, the Amencan Anned

Forces required soldiers to cany 54 to 66 kg (Iverson 1987).

In tenns of load placement, varying opinions exist in packing kit into a load

carriage system. During a biomechanical study Martin and Nelson (1982) measured

postural sway on a force plate and determined that loads placed ia the middle of the back

were easier to balance than high or low loads. Hinrichs et al. (1992) conducted a

mechanical study of pack inertia and found that loads closer to the back were more

cornfortable for the user.

Physioiogical analysis of load placement revealed slightly different outcomes than

the biomechanical studies. By measuring balance and performance on an obstacle course.

Holewijn and Lotens (1992) found that an evenly distributed load produced better results

than a concentrated load of the sarne mass. Legg et al. (1992) collected heart rate. and

oxygen uptake data and determined that carrying load in a pack was superior to

supporting load directly on the shoulders, though biomechanical analysis contradicted

these results. Bobet and Nonnan (1984) discovered that heart rate was not dependent on

a load placement for extended marches. However, Neumann and Cook (1984) found that

high load placements increased the EMG level of the gluteus medius muscle in order to

counteract the higher adduction force during gait. Yet, in a more comprehensive EMG

study, Bobet and Nonnan (1992) showed that motor patterns varied between subject

making intra-subject anal ysis difficult .

Subjective perception of load location suggests that loads placed high in the pack

are more advantageous for long straight hikes, and loads placed low in the pack are best

for shorter hikes that repuire more maneuverability (Jenkins, 1992). Conciusions made by

Jenkins (1992) pointed out that the high loads reduced the amount of famuard trunk lean

ta get the centre of mass over the base of support, reducing energy costs for long

marches. However, the lower centre of gravity aided in maneuverability by decreasing the

moment about the long and transverse axes of the body.

Numerous studies were also dedicated to rnethods of load carriage; shoulders,

waist, head, hands, etc. In an early physiological study, Bedale (1924) found that load

c d e d in a pack was significantly less energetically costly than the same load d e d by

the hands. Datta and Ramanathan (1970) further suggested that equally balanced loads on

the front and back of the torso were superior in tenns of oxygen consumption. However,

these front-back carriers were detrimental to cote body temperature. The majority of

literature on this topic was fiom a biomechanical perspective.

Martin and Nelson (1982) used force plate measures to conciude that intemal

fhne packs allow for greater postural stability relative to extemal frames. Somewhat

surprisingly, Kinoshita and Bates (1983) found no significant difference in force plate

measures between single and double carrier style packs. Similady, Martin et al. (1982)

reported no difference in gait patterns between vanous types of load caniage systems.

HoleWijn ((1990) showed that while physiological measures produced no significant

difference between shoulder-based systems and packs with a waist belt, the waist belt style

packs demonstrated significantly lower shoulder pressures. Presumably the waist belt

supported some of the load, thus reducing the net shoulder force. Kram (1991) reviewed

a unique load carriage system. Cornpliant "spnngy" poles that traversed the shoulder

supporting the loads at the ends of poles produced smaller peak shoulder forces and

ground reaction forces than standard packs. The author suggested that the poles created a

resonance situation in which the load was moving in unison with the user thereby

minirnizing inertial forces during vertical directional changes (Kram, 199 1).

The majority of subjective analyses of pack type are centered on variables

secondary to the pack-person interface such as style of pocket closures, colour, and

volume. However, Martin et al. (1982) found that users prefer a long pack frame relative

to a short fiame, despite equivdent centre of gravity positions. Yet, Kirk and Schneider

(1982) found that self repoited perceiveci exertion scores showed no significant difference

between different pack styles. Many studies did identiS, pack preferences based on

subjective measures even if no obvious objective reason existed for these preferences and

conclusions suggest soldiers can identify preferences based on codon.

The goal of this study was to develop and validate a personal load carriage system

biomechanical model. The model would serve as the basis for a personal load carriage

system design tool, which would provide a better understanding the pack-person interface

and in turn help development of new systems for soldiers. A sub-problem of this study

was to develop and vaiidate an improved pack testing system for evaluation of the

biomechanical model and for fiiture scientific and field studies.

Chapter 2

Development and Validation of a Biomechanical Model

Assessrnent Tool

Introduction The difficulty in validating a biomechanical model lies in making accurate

measures of the predicted values. As was discussed in Chapter 1, Rigby (1997) was not

able to h l l y validate his modei due to the inability to measure the necessary pack-person

interface variables. By definition a biomechanical mode1 is designed to predict values

that are otherwise not practically measurable. h, therefore, requires the generation of a

unique testing setup that will provide the opportunity to measure these forces. Under

these specific conditions, accurate measures can be made and used to validate the

biomechanical model. Then the model can be used in general, where these measures can

not be made.

The unique testing jig developed for the purpose of vaiidating a personal load

camage system biomechanical model combines four measurement tools. The first, a 50'

percentile male human form mannequin instrumented to measure the body reaction

forces. The second, a force platform to measure ground reaction force. The third, a strap

tension probe that was used to measure the tension in the four straps of the pack system.

The foutth, a modified personal load carriage system. The pack was designed to both

rneasure the lumbar pad contact forces and be adjustable so that the geometry of the pack

wuld be changed to create unique test setups. Each of these measurement devices are

outlined in detail below. The techniques used to measure the described forces are

explained and the rationale for determining indirectly measured forces are given.

Test Mannequin The mannequin was anthropometrically representative of a 5oth percentile male

based on height, and circumference measures (Table 2- 1). The head, arms and legs were

removed to make it easy to don and doff packs. The mannequin was also covered with 5

mm thick Bocklite, a substance used in prosthetics, to represent human skin (MacNeilBt

Rigby, 19%). Fixed to the base of the mannequin was a rotating vice that could be

adjusted to provide the forward lean demonstrated by experienced pack users under

heavy loads (Stevenson et al., 1995).

To measure the body reaction forces, the mannequin was cut transversely at the

T 1 0 spinal level and a six-degree-of-freedom load ceIl was inserted. The axis of the load

ce11 was onented at the antenor-posterior and medial-lateral mid-line of the mannequin

and at the level of the split. In this arrangement, the load ce11 measured the net body

reaction force acting on the torso above the leve! of the load cell. The mannequin was

not representative by weight due to the hollowing of a 15cm diameter by 20cm high

cylinder to house the load cell.

The load ceIl was an AMTIm (Boston, Mass.) MCSTM series multi-cornponent

transducer capable of measuring six channels: Fx, Fv, Fz, MI, MY, and Mz. The device

was outfitted with four-arm bridges to minimize thermal effects and the cell was designed

to minimize cross talk between the channels.

The test mannequin, known as the Load Distribution Mannequin and shown in

Figure 2- 1, was evaluated and validated by Stevenson et al. (1995). It has a capacity far

beyond the required levels for load carriage evaluations. The non-linearity and hysteresis

of the ce11 are 0.2% full-scale output (AMTI, 1991). The sensitivity is 0.13 mV/(V*lb*tt)

FG 0.50mV/(V*lbtft) Fx and Fy, and 0.20 mV/(V*in*lbtft) Mx, My, and Mz (AMTI,

1991). The ViewdacTM data acquisition system was used to apply the voltage to the load

cell, and measure the output voltage, apply the calibration equation, and display the

outputs. The data were read fiom the screen and recorded by hand.

Table 2-1 : 5 0 ~ Percentile male human fonn mannequin.

Anthropometric Measurements 50"' Percent ile Male Human-Fonn

~ o d e l ' Mannequin

Neck circumference (cm) 40.8 39.5

Acrornial height, sitting (cm) 59.8 61.5

Chest circumference, maximum (cm) 99.1 101.6

Chest circumference, axi llary (cm) 102.3 101.2

Waist circumference, omphalion (cm) 86.2 84.5

Biacromial breadth (cm) 39.7 38

Back length2, C7 to L4L5 (cm) 50.6 45.3

Hip circumference (cm) 89.5

Buttock circumference (cm) 98.4 95.4

1 - Mode1 data were generated by the SafeworkTM Program

2- Back length = (sitting height) - (menton to top of head) - [(waist height) - (buttock

height)]

Figure 2-1 : The Load Distribution Mannequin.

Force Platform The mannequin described above was placed on the force platform, which was

embedded in the floor of the testing facility. By measuring the ground reaction force of

the entire test mannequin, body contact forces below the load ce11 were detennined by

simple subtraction. Subtracting the anterior-posterior force of the upper body from the

net anterior-posterior force reveals the anterior-posterior force acting on the lower body.

Similarly, subtracting the longitudinal force of the upper body fiom the net longitudinal

force reveals the longitudinal force on the lower body. Therefore, by simple subtraction

horizontal and longitudinal body reaction forces could be measured directly and

compared to the model ' s predicted values.

The force platform, AMTITM model LG6-4-lm, consisted of four measurement

gauges at each corner of a 610 mm x 1220 mm metal plate, on which the mannequin was

mounted. The platform simultaneously measured three force components and three

moment cornponents related to the three geometric axes, X, Y, and 2. The measurements

were made by a series of foi1 strain gauges attached to proprietary load cells at the four

corners of the platfonn. The gauges were based on Wheatstone bridges with output

voltages proportional to the force applied to the system. Note: the tnie axis of the

platform is actually located 5.5 cm below the top face (AMTITM, 1989). The LabviewTM

data acquisition system was used to measure the output voltages, apply the calibration

equations and display the force and moment outputs.

In an unpublished report, Potter (1998) was able to show that the force plate has

an average percent emr in vertical force measurements of 0.078% and a standard

deviation of 0.34%. Potter was also able to show a resolution of 0.002 kg, and a linearity

of 0.999 R~. Over a multi-day analysis there was no signifiant difference between data

under the same conditions. Finally, Potter reported a precision of 0.374 mV standard

deviat ions for the vertical force measure.

Test Pack

The test pack consisted of a modified DACMEnl pack board (a 510 mm x 345

mm x 9 mm polyunthane shed), standard 2.54 cm nylon webbing straps, a DACMEN

waist belt, a six-degreeof-feedom load cell, and a variable payload. A 153 mm x 1 53

20

mm cut was made in the bottom-centre of the board and a six-degree-of-freedom load ce11

was mounted in this opening with an alurninum frame. The load cell was onented such

that the Z-axis was perpendicular to and the X-axis was parallel to the long axis of the

pack tiame. Then a 145 mm x 145 mm lumbar pad was fixed to the top of the load cell.

Figure 2-2 shows the modified test pack. The top of the load ceIl protmded 21 mm fiom

the antenor surface of the pack.

Figure 2-2: Modified test pack.

Essentially the lumbar pad was isolated fiom the rest of the pack by the six-

degree-of-fieedom load cell. As a result, the forces transmitted though the lumbar pad

could be directly measured. ~ h e Xsutput of the load cell was a direct measure of the lifi

the lumbar pad transferred to the pack and 2-output of the load cell was a direct measure

of the horizontal Iumbar contact force. This load ceIl was also used to determine the

vertical lift component provided by the waist belt alone. The vertical component of the

lumbar pad was measured directly and the net vertical lift of the pack by the waist belt

and lumbar pad cornplex wss measured by the force plate. Therefore, simple subtraction

revealed the vertical lift provided by the waist belt alone.

Standard 2.54 cm nylon webbing straps were attached to the pack board ta

shoulda straps and load lifter straps. As well, a DACMEm waist belt was fixed

21

mate

to the

user face of the board. A payload of 28.8 kg or 23.8 kg were attached to the back face of

the board, creating total loads 35.0 and 30.0 kg for the test configurations. The centre of

gravity of the pack was determined by reaction board method. The nature of the

D A C W pack board allowed al1 of these elements of the pack (straps, waist belt, and

payload) to be moved, creating unique pack geometry for the validation tests.

The load ceil was the same model as the one embedded in the test mannequin and

the ViewdacTM data acquisition system was used to measure and record the output

voltage, apply the calibration equation, and display the force output of the gauge.

Stevenson et al. (1995) evaluated and validated this system and determined equivalent

linearity, hysteresis, sensitivity, and accuracy.

Strap Tension Probe To measure the tension of the shoulder straps, load lifter straps, and waist belt, a

unique measurement tool was designed. The in-line transducer used by Stevenson et al.

(1995, 1996, 1997, and 1998) was too large for this application. A device that could

measure tension in pack strapping as short as 40 mm was needed.

The strap tension probe that was developed and constmcted with the assistance of

the Clinical Mechanics Group (Kingston, Ontario, Canada) consisted of a pair of surgical

pliers that were modified and instmmented with a foi1 strain gauge. Fixed in CO-linear

alignment to one face of the pliers' head were two parallel stainless steel cylindrical pins

positioned 26 mm apart. Fixed to the second face was an identical single pin, positioned

between and parallel to the other two pins. The single pin was also attached to the pliers

head by rneans of a fiee rotating pin joint so that, throughout the pliers range, the three

pins remained parallel. The modified pliers head can be seen in Figure 2-3 and Figure 2-

4. Fixeâ to one handle of the pliers, 110 mm from the pivot point, was an adjustable

stop-rod to regulate the closed position of the plien. Fixed to the other handle was the

foi1 strain gauge between the pivot and the stop-rod.

Figure 2-3 : Modified surgical pliers.

Figure 2-4: Modified pliers head, open (left view) and closed view (right view).

The foi1 gauge consisted of a half Wheatstone bridge circuit. An excitation

voltage of 6 volts was applied to the bridge and the gauge output voltage was amplified

by an OmegaTM Model 465-115 Bridge sensor. The amplified output voltage was

displayed by a Goldstar OS-9029ATM 20 MHz analog oscilloscope with a significant digit

capability of 10 mV. Al1 output voltages were recorded and entered into a spreadsheet

for analysis.

In the open position, a strap under tension remained in straight alignment between

the two fixed ends, Figure 2-SA. In the closed position, the geometry of the strap

changed as illustrated in Figure 2-58. Using this static equilibrium position and some

geometric simplifications, the change in voltage output of the strain gauge was used to

determine the tension in the suap.

Figure 2-5: Pliers head in open position (A) and closed position (B).

The three geometric simplifications were: 1) The wrap angle of the strap around

the pins was assumed to be negligible becausc of the relatively small diameter of the pins.

This allowed for the three locations where the strap contacted the pins to be considered

point contacts. 2) The thickness of the strap was also considered negligible because it

was also relatively small. This allowed the strap to be treated as a line in space. 3) The

small coefficient of fiction of the stainless steel pins provided a surface around which the

friction could be considered zero. These assumptions produced the system illustrated in

Figure 2 6 and defined by Equations 2-1 through 2-23. Ci and C2 are constants of

proportionality and V is the voltage output of the strain gauge.

Figure 24-A shows that tension in the strap applies a force upward on the centre

pin and a force downward on the two outer pins. The magnitudes of these forces are a

product of the angle q created by geometry and tension in the strap (Equations 2-1 to 2-

3). To maintain this closed position a force equal in magnitude and opposite in direction

(Fi, 4, & F3) must be applied to the three pins. These forces are also defined by the

illustration in Figure 2-6-8. These forces are transmitted to the handles of the pliers

through the pivot.

Figure 2-6-A: Geometrically simplified system.

Figure 26-8: Geometricall y simplified system.

Equation 2- 1 : FI = Tsinq

Equation 2-2: FI = Tsinq

Equation 2-3 : F3 = 2Tsinq

Fr defines the bending moment of the upper handle, which resists the force acting

on the single pin (Fa). F4 defines the bending moment of the lower handle, which resists

the forces acting on the two outer pins (FI and F2). Fs and F, represent the added force

of the hands and the stop rod. Clearly, the user cannot compress the handies with the

exact closure force; these two forces represent the additional force that is involved. Both

forces cancel and are irrelevant to Further calculations. Equations 2-1 to 2-19 completely

define the equilibrium illustrated in Figure 2 6 . The change in voltage in the foi1 gauge

fixed to the upper handle of the pliers i s proportional to the bending moment created by

F6, which in tum is proportional to the tension in the strap.

Equation 2-4: CF = O = FI + FI - F3 + F4 - Fs - Fc + F7

Equation 2-5 :

Equation 2-6: F1+ F2 = F3

Equation 2-7: CF = O = F.4 - Fs - F6 + F7

Equation 2-8: XM = O = F4d2 - Fsd2 + Rd2 - F6d2

Equation 2-9: Fs = F7

Equation 2- 10: CF=O=F4-F6

Equation 2- 1 1 : ZM = O = (FJ - Fr)d2

Equation 2- 12: Fsd2 = Fsdi

Equation 2- 13 : F6 a F3

Equation 2- 1 5:

Equation 2- 16:

Equation 2- 1 7: CiV = 2Tsinq

Equation 2- 18: T=- Lt v 2 sin q

Equation 2- 19: T = C2V

An important result of weaving the strap through the strap tension probe is the

change in îùnctional length of the strap. In the open position shown in Figure 2-7 the

strap, which follows a straight line of length 2d1, is required to span the distance between

the two outer pins. In the closed position, the weave of the strap through the pins

necessitates a strap of length 2h, where h is the hypotenuse of di and dz, to span the two

outer pins. The extra strap needed between the two outer pins is taken from the

remaining swap that runs between the two outer pins and the two fixed ends of the strap.

Figure 2-7: Length change of test strap, open (left view) and closed (right view).

When both ends of the strap are fixed, shortenhg the fiinctional length of the strap

increases tension in the strap. In fact,

The stifhess of such a system can be

kI and k2 at each end, as illustrated

the increase in tension is defined by strap stifiess.

modeled as a strap with tension springs of stiffness

by Figure 2-8A The stiffness ki and kz can be

combined to form constant k at one end of the strap, Figure 2-8B. As a result of

increased tension in the strap due to the geornetry of the closed probe, the tension

detennined by Equation 2-19 (T) is larger than the actual tension of the strap (Ta). Using

simple spring theory Equation 2-22 defines this increase in tension.

Figure 2-8: Stiffness of pack system. A: variable stiffness at both ends of strap under

tension. B: consolidated stiffness of a strap under tension.

The change in length of the spring (Ax) is defined by the geometry (Equation 2-

20) and since di is fixed, it is determined by the amount of closure in the pliers (d2).

Thus, strap tension and stiffness are uniquely defined by Equation 2-23.

An equation in two unknown parameters, Ta and k, is produced. To determine the

two unknown parameters, a unique equation with the same variables was created.

Whenever there is an adjustment to the amount of closure of the pliers, the stoprod, or

changes to the geometry of the system; a unique d2 is created. This unique d2 detemines

the variable Ax, which determines the coefficient of the spring term, k. As well, the

unique dz produces a unique q, which in tum changes the coefficient C2. This produces a

unique equation with which to solve TO and k. In fact, a third closure level was also used

so that a least squares analysis could be used to produce a more accurate method with

which to determine the strap tension.

Equation 2-20: Ax = 2h-21

Equation 2-2 1 :

Equation 2-22:

Equation 2-23 :

Stnp Tension Probe Validation To calibrate the strap tension probe, the constant of proportionality had to be

detemineci for each of the three closure positions, C2,, C22, and CÎ3 respectively. This

ailowed calculation of Axi, Ax2 and Ax3 from the geornetry of the system.

With one end of a strap f i e , as with a weight hung from a fixed end, the spring

constant becomes O and Equation 2-23 is reduced to Ta = T = C2#. To determine C2),

10 known weights were hung three times each in random order and the output voltage of

the tension probe was recorded. Linear regression analyses of the strap tension and the

voltage output were performed. This procedure was repeated to determine Cl2 and C23.

To determine the validity of the strap tension probe, data were collected, recorded, and

anal yzed for resolut ion, accuracy, sensitivity, preci sion, and reliabilit y.

Accu racy

The test setup consisted of a srnall boom crane, an in-line tension strain gauge, an

in-line tension spring, a length of standard 2.54 cm webbing strap, and the strap tension

probe. The dope of the output from an [nstronTM force-displacement measurement unit

was used to detennine the stiffness of the strap-spnng system; the boom crane was

considered to be rigid. One end of the strap was fixed to the lever end of the crane, the

other end was fixed to the in-line strain gauge (Figure 2-9). The strain gauge was fixed to

one end of the spring, which was fixed to the stationary portion of the crane. The crane

was raised to produce tension in the strap (Figure 2-9). The in-line strain gauge used to

rneasure strap tension had an accuracy of S2 N. The tensions selected for analysis were

in the range of tension values reported by Stevenson et al. (1996).

To determine measurement system accuracy, a strap with known tension and

stifhess was measured with the strap tension probed three times, one for each closure

setting, and the output voltages were recorded. The three outputs were used in the system

of equations and a least squares analysis was performed to detemine swap tension and

stifiess. The predicted tension and stifhess were compared to the actual values for

accuracy. The protocol was repeated for various tensions and stifhess and the predicted

values were compared to the actual values using a t-test for the tension and stiffness.

Figure 2-9: Strap tension probe validation setup.

Relia biüty

To determine the reliability of the strap tension probe, the accuracy data were

collected over three different days between which the test set up was disassembled and

reassembled.

Precision

To determine the precision of the strap tension probe, 10 repetitions of the

accuracy data were collected for a low tension, medium tension, and high tension. The

spread of the data points was analyzed for standard deviation and percent error.

In Vivo Analysis

To determine if the strap tension probe was able to masure strap tensions in

pack-like situations and determine if the strap is modeled effectively by a spring-like

system, a series of in viw measures were d e . A standard pack, in which an in-line

strain gauge fit, was set up on the test mannequin. The shouklet strap was set to ten

different tensions and the tension probe was usod to detemine the tensions. The

predicted tensions from the strap tension probe were compared to actual tensions

measured by the in-line gauge using a paired t-test.

Strap Tension Probe Validation Resulk

Calibration

Table 2-2 illustrates the results of the linear regressions of the output voltage and

the strap tensions for each coefficient. Table 2-2 also shows the value determined for

each coefficient and the error associated with these deteminations. The associated Ax

value is also shown.

Table 2-2: Calibration coefftcients and error measures.

Coefficient C2# Value Associated Ax# R-squared Standard Error P-Value c2 I 0.258 2.3 mm 0.99 1.8 4 . 6 ~ 1 ~ " c22 0.360 1.2 mm 0.98 3.1 1.3~10" c23 0.628 0.5 mm 0.96 4.0 1 . 6 ~ 1v8

Accuracy and Reliability

Table 2-3 and 2-4 illustrate the accuracy and reliability of tension and stiffness

measurements respectively. Each table shows the prediction made by the strap tension

probe and the actual value. The average error was 5.4% for tension predictions and

587.7% for stiffness predictions. For tension and stiffness predictions respectively the

average day 1 mors were 5.03% and 689.4%, the average day 2 errors were 5.1% and

473.3%, and the average day 3 errors were 6.1% and 566.5%. A t-test comparing the

predicted tensions with the actual tensions revealed a p value of 0.38. A t-test comparing

the predicted and actual stiffness values revealed a p value of 1.03 * IO*'.

Table 2-3: Accuracy and reliability of the strap tension probe with respect to tension

measures.

Trial # Predicted Tension (N) Actual Tension (N) Percent Enor 1 37 33 12.1 2 56 60 6.7 3 46 47 2.1 4 33 33 O 5 53 60 11.7 6 46 47 2.1 7 40 42 4.8 8 30 3 1 3.2 9 23 21 9.5

Table 2-4: Accuracy and reliability of the strap tension probe with respect to stiffness

rneasures.

Trial Predicted Stiffness ( N h ) Actual Stiffness (Nlrn) Percent Error 1 5246.2 774.5 577.3

Precision

Table 2-5 illustrates the results of the precision analysis. The average prediction

of the 10 samples is shown as well as the standard deviation and percent enor of the

repetit ions.

Table 2-5 : Summary of precision anal ysis results.

Actual Tension (N) Average Predicted Tension (N) Standard Deviation Percent Emt 33 32.8 2.35 0.6 1

In Vivo Analysis

Table 2-6 illustrates the results of the in vivo analysis. The ten measurements

made by the strap tension probe, the actual tensions, and the associated error are shown.

The average error was 5.9%. A paired t-test cornparing the predicted tension to the actual

tension in the strap revealed a p-value of 0.60.

Tabie 2-6: Surnrnary of in vivo analysis results.

Preâicted Tension (N) Actual Tension (N) Percent Error 66 63 4.3 50 50 O 35 38 7.9 23 21 8.0 30 27 11.1 12 15 20.0 65 65 1.5 50 47 5.7 40 40 O 19 19 O

Stra p Tension Probe Validation Discussion

It was possible to measure the tension of a strap using the strap tension probe to

within approximately 6% of tnie tension values. Furthemore, the cornparison between

predicted strap tension and actual strap tension (p = 0.38) revealed that the two sets of

data were not significantly different.

Small standard deviations of a sample of 10 data points, as is shown in Table 2-5,

illustrate the precision of the strap tension probe. Repeated measures of the same strap

setup produced relatively similar values over a number of measures. This indicated thaî

the probe will produce equivalent results over repeated tests.

Similarly, the reliability of the system (strap tension probe and calibration

protocol) was considered acceptable. Not only was the system able to provide precise

readings of a test setup, it was also sufficiently robust to provide equivalent readings

between test setups. Three unique test setups revealed average errors of approximately 5

%, 5 %, and 6 %? for each day, illustrating that the system's accuracy was not dependent

on the test setup being used.

The probe was developed to reach hard-to-masure sites on the pack straps.

Therefore, one of the rnost important tests of the probe was its ability to provide accurate

information in vivo. Average error of approximately 6 % and non-significant differences

between predicted and actual tension reveals that the strap tension probe system was an

accurate representation of the pack system. The approximate error did not exceed the

enor experienced under controlled conditions and the two sets of data (actual and

predicted) were not significantly different.

Since the current model did not attempt to predict system stiffness the ability of

the strap tension probe to measure it is of litt!e immediate importance. However, the next

phase of the model may encompass dynamic conditions in which system stiffness will

become much more relevant. As such, prospective interest in the ability of the strap

tension probe to measure system stiffness exists. While the strap tension probe and

associated system of equations produced accurate estimates of tension, the predictions of

the stiffness were poor. Tables 2-3 and 2-4 illustrate that small changes in the strap

tension were accompanied by relatively large changes in the stiffness, k. This high

sensitivity of k may have been responsible for the inaccurate predictions of stiffness.

Therefore, a small error in tension was accompanied by a large error in k, explaining the

inaccurate predictions of stiffness associated with relatively accurate predictions of

tension.

Strap Tension Probe Validation Conclusions The strap tension probe had clear advantages: it was possible to accurately predict

the strap tension of standard 254 mm straps within the tight confines of pack design.

Results revealed an accuracy of S 5 % dunng calibration and an in vivo accuracy of

M.OO/o. The ability of the strap tension probe to measun strap tensions was considered

acceptable for this study within the range of strap tensions used during actual load

cadage conditions. Although the systmi was "ill-conditioned" for measurement of strap

stifhess future work may be able to exploit the strap tension probe to provide insight

into the stiffness of the pack-person intedace in order to improve Our understanding of

pack design.

Chapter 3

Biomechanical Model of a Personal Load Carnage System

Introduction In its simplest form, a biomechanical mode1 is a set of mathematical expressions

that define the physical relationship of forces and moments in a biological andor

mechanical system. In this case, the system is the pack-person interface. The goal of a

model is to predict values of the system that could not otherwise be measured and, in

general, allow experimentation in a controlled environment.

In a design setting, a biomechanical model can be used as an effective tool. The

model can provide insight into the relationships between variables and it can also be used

as an objective method of evaluating potential designs. As was outlined in Chapter 1,

Stevenson et al. (1996) identified a number of pack-person interface variables that have a

significant impact on the user's perception of a pack. If a biomechanical model could

predict these variables based on the physical characteristics of a design, much time and

money could be saved. Furthermore, work done by Pelot et al., (1998) used a previous

version of this biomechanical mode1 in an optimization routine. The routine was

designed to optimize pack geometry and placement of kit in that pack. The usefùlness of

their optimization routine is dependent on the effectiveness of the biomechanical model

on which it is based. lmprovement of the personal load caniage system biomechanical

model, therefore, enhances potential design tools and strengthens the basis for future

optimizat ion routines.

A model's effectiveness is bounded by the researcher's understanding of the

system the model represents. In this case, that understanding cornes from the two

previously validated models, experimental data from the Load Carriage Simulator,

experimental data fiom the Load Distribution Mannequin, and subjective input from

experienced pack users. Furthermore, a number of assumptions and simplifications are

made during the modeling process. While these assumptions and simplifications are

necessary to develop a determinate model, they lead to limitations in the model itself.

Not only must these limitations be understood but they must also be accepted. The model

description, information used to develop the model, assumptions and simplifications are

detailed in this chapter.

First, a general outline of the pack-person interface is described including al1

relevant notation. A free body diagram is presented detailing the static equilibrium of the

pack. Then, each subsection of the pack is broken down into specific subcomponents of

the model: shoulders, waist, and lumbar regions. Finally, the system of equations

generated is solved to provide a statically deteminate solution.

The Model

General Pack Model

A photograph of a typical commercial pack is shown in Figure 3-1. Each of the

three shoulder strap segments combine to create a suspension system for the pack over

the shoulder of the user. These straps have been designated, fiom bottom to top, the

lower shoulder strap, upper shoulder strap, and load lifter strap. It is the tension in these

straps around the shoulder that supports a portion of the pack's weight. The effect of

these tensions in the free body diagram are twice the recorded value as the Iefi and right

hand sides have been combined due to assumed symmetry of the system.

The waist belt and the lumbar pad elements of the pack are also important as they

help to maintain the pack's equilibrium position on the user by providing a lifi force and

horizontal reaction force at the point of contact with the lower back (Stevenson, 1995,

1996, 1997, 1998). It is assumed that both the lumbar pad and the waist belt act on the

pack at the same attachment point and provide cumulative suspension to the system.

Previous studies (Stevenson et al., 1995; MacNeil, 1996) have shown that a reaction force

exists perpendicular to the pack at the lumbar region. This force counteracts the moment

generated about the centre of gravity of the pack by the shoulder straps and the vertical

lie component of the waist belt and lumbar pad.

Figure 3- 1 : Elements of a standard commercial pack and the biomechanical model.

A fkee body diagram of the backpack is shown in Figure 3-2 and the notations

used are defined within this chapter. The suspension system elements have been

numbered fiom the top down in previous generations of the model. As new elements

were added they were numbered sequentially. The subscripts have been grouped

regionally for convenience. For example, the upper shoulder strap's location is noted di,

its tension Ti and its angle fiom the vector normal to the pack, 0,. The subscript 2 refers

to the lower shoulder strap, subscript 3 to the waist belt and lumbar cornplex, and

subscript 4 to the load lifter straps. Please note that the entire figure and the reference

coordinates have been angled at P degrees fiom the vertical to reflect the normal body

lean which occurs under heavy loading conditions (Stevenson et al., 1995). This leads to

a pack-based courdinate system angled at B from a global reference system.

Inputs to the model include: mass of the pack and its contents, position of the

centre of gravity, geometry of the pack, and tension in the straps that the users c m control

(the waist belt, the lower shoulder strap and the load lifter strap). The other variables that

are not controlled by either the user or the designer are leA as outcome masures of the

model. Al1 variables are identifiai as either inputs or outputs in the notation section.

Figure 3-21 Biomechanical mode1 of a personal load carriage system.

The pack static equilibrium equations for the force in the X-direction, force in the

2-directions and the moments about the centre of gravity of the pack can now be

simplifiecl accordingly. These three equations are presented in tems of multiple

unknown values. The remaining sections of this chapter are devoted to modeling the

specific regions of the suspension system thereby expressing unknown variables in tems

of known quantities.

Equation 3- 1 : Equilibriurn expression for forces in the X-direction:

Equation 3-2: Equilibrium expression for forces in the 2-direction:

Equation 3-3 : Equilibrium expression for moments about the pack's centre of gravity:

Notation

Following are detailed descriptions of al 1 the parameters il lustrated by the general

pack mode1 and associated regional models (shoulder, waist, and lumbar region). For

ease of location this notation is also repeated in Appendix 1.

Orienration:

X coordinate dong pack depth

2 coordinate dong pack height

Pack Container:

W the force of the mass of the pack (input)

vx horizontal position of t he centre of mass fiom the back of the pack (input)

vz vertical position of the centre ofmass fiom the bottom of the pack(input)

h horizontal dimensions of the pack container (input)

hz vertical dimensions of the pack container (input)

Bearer:

d3 distance from bottom of pack to lumbar pad contact centre (input)

d5 distance fkom lumbar pad contact centre to shoulder centre (input)

d6 distance from pack to centre of shoulder (input)

r average radius of shoulder (input)

r~ average radius of hips (input)

P body lean angle (input)

YL anatomical lower back angle fiom vertical (input)

YB anaiornical hip angle from vertical (input)

Shoitider sïraps:

tension in upper shoulder straps (LHS and RHS summed) (output)

tension in lower shoulder straps (LHS and RHS summed) (input)

tension in load lifter straps (LHS and RHS summed) (input)

distance from lumbar pad centre to attachment of upper shoulder strap (input)

distance from lumbar pad centre to attachment of lower shoulder strap (input)

distance fiom lumbar pad centre to attachment point of load lifter straps (input)

upper shoulder strap angle from the vector normal to the pack (output)

lower shoulder strap angle from the vector normal to the pack (output)

load lifter strap angle C.C.W. from the vector normal to the pack (output)

upper shoulder strap wrap angle around the shoulder (output)

load lifter strap wrap angle around the shoulder (output)

angle at which sN acts h m pack normal (output)

coefficient of fiction of strap on shoulder (input)

net force of shoulder straps acting though the centre of the shoulder (output)

X-component of sN (output)

2-component of sN (output)

force of fiction around the shoulder (output)

WQist M t :

Ts tension in waist belt (input)

d3 distance to lumbar pack centre fkom bottorn of pack (input)

TX compressive force that T3 applies around the hips (output)

TG normal force component of T3c (output)

Tm the force of fnction due to ~3~~ (output)

FBz lifl provided by the waist belt resting on hips (output)

coefficient of friction of waist belt on hips (input)

Lirm bar region:

Fx reaction force of lower back on pack in X-direction (output)

Fsn the component of Fx normal to the lower back (output)

Fxr the force of fiction due to Fx (output)

FLz lifi on the pack fiom friction and angle at lower back (output)

coefficient of friction of lumbar pad on lower back (input)

Fz total lift force at lumbar contact point of pack (output)

The Shoulder and Related Pack Elements

The Shoulder

In previous iterations of personal load carriage system biomechanical models, the

shoulder has been simplified to a cylindrical shape oriented such that the cross sectional

circumference was in the sagittal plane. MacNeil ( 1996) showed that this simplification

was appropriate for purposes of developing a biomechanical model. Therefore, this

generation of the model will continue to use the simple cylinder shape to represent the

shoulder.

Gcometric Calcu1ations

Tt is important to determine the cornplete geometric propenies of the pack system.

Figure 3-3 and Equations 3-41 though 3-8 outline this process. Frorn the input

dimensions of the pack and user, the angles of action of the thtee shoulder straps (€II,&,

and 8 4 ) were determineci. As well, the wrap angles of the upper shoulder straps (ai) and

the load litter straps (a) were detennined.

42

Figure 3-3: Determination of €Il ,&, 84, al, and ~ 4 .

The following series of 3-4 equations express the calculations needed for 81, the angle of

the upper shoulder strap relative to a normal vector to the pack.

r Equation 3 4 . 1 : cos(81) = - 4

(4 -d,)+e, - JR

Equation 3-4.2:

Equation 3-4.3:

The following series of 3-5 equations express the calculations needed for 82, the angle of

the lower shoulder strap relative to a normal vector to the pack.

r Equation 3 -5.1 : cos(&) = - - d6

( d , - d d - e , Jm~

Equation 3-5.2:

Equation 3 -5.3 : 02 = arctan (%)

The following series of 3-6 equations express the calculations needed for 04. the angle of

the load lifter strap relative to a normal vector to the pack.

Equation 3-6.1 :

Equation 3-6.2:

Equation 3-6.3 :

Equations 3-7 and 3-8 determine, through simple subtraction the wrap angles of the lower

shoulder strap to the upper shoulder swap and the lower shoulder strap to the load lifter

swap respect ive1 y.

Equation 3-7:

Equation 3-8:

The Shoulder Model

In the previous two versions of the model, the shoulder strap suspension of the

pack was modeled as a simply pulley with fnction. The pulleys represented the

cylindrical shaped shoulders and the straps were ropes that feed around the pulley.

MacNeil (1996) showed that the simple pulley equation (Equation 3-9.1) provided a

relationship between the lower shoulder strap tension (TI) and the upper shoulder strap

tension (Ti) such that the upper shoulder strap tension (Ti) was higher. The higher

tension in the upper shoulder strap indicated that the force of fnction around the shoulder

(FS) was directed down over the shoulder surface. However, during preliminary work on

the current model it was found that the direction of the shoulder frictional force (FR) was

actually a product of how the straps were tightened. If the lower shoulder strap was

tightened last, the force of friction (Fis) moved up over the shoulder surface producing a

relationship between the shoulder strap tensions defined by Equation 3-9.2. In other

words the method of d o ~ i n g or adjusting the pack dictates the direction of the force of

fiction.

Equation 3-9.1 :

Equation 3-9.2:

In this phase of the model, the load lifter strap was added to the shoulder complex

and its tension (T4) became another input variable because the user can control the

tension in the strap. Previous work by Stevenson et al., (1 996, 1997, and 1998) discussed

the fhnctional significance of the load lifter straps. They were able to show that the upper

and lower shoulder straps function almost solely as a suspension system for the pack

about the user's shoulder whereas the load Mer straps were considered to be geometnc

manipulators. Stevenson (1996) and her colleagues suggested that the load lifter straps

served to change the angle of pull and point of application of the upper shoulder strap

tension (Tl). Stevenson et al. (1996) aîso discovered that, for the most part, the upper

shoulder strap and the load lifter strap were not joined to form the lower shoulder strap

until both were r u ~ i n g tangent to the shoulder's surface. Therefore, the upper shoulder

strap tension (Ti) and the load lifter strap tension (TI) could be assumed to have sorne

type of additive effect on the lower shoulder strap tension (TI). In fact, if the pulley

analogy of the upper and lower shoulder straps was extended to the load lifter strap and

the pulley was considered fnctionless, the sum of the tensions of the load lifter strap (T4)

and the upper shoulder strap (Tl) would be equal to the lower shoulder strap (T2)

(Equation 3-10). Assuming that the load lifter strap had a similar relationship to the

lower shoulder strap as the upper shoulder strap, a unique relationship between the three

tensions was determined.

Suppose two separate pulleys existed: one for the upper and lower shoulder strap

and one for the load lifter strap and the lower shoulder strap. When donning a pack, the

lower shoulder strap is tightened and then the load lifter strap is tightened (Stevenson et

al., 1995). Since the lower shoulder strap is tightened afler the upper shoulder strap,

Equation 3-1 1.1 would define their relationship. Similarly, the load lifter strap is

tightened aAer the lower shoulder strap and Equation 3-11.2 would define their

relationshi p. Superposit ioning these equations (3- 1 1.1 and 3- 1 1.2) Equation 3- 1 1.3 was

created, producing the relationship between the three strap tensions.

Equation 3- 10: T2=T1 +T4

Equation 3 - 1 1.1 : T*' = T~. e ~ 4

Equation 3- 1 1.2: T ~ * ~ = T*/e"l

Equation 3 - 1 1.3 : Tz = TI' + T2"

Equation 3- 1 1.4: T4 T2 = Ti .ehal +- e Pta4

Since the three shoulder strap tensions are able to be expressed in terms of each

other and the straps are at known orientations to the shoulder, a net shoulder strap effect

(sN) on the shoulder was deduced and is illustrated in Figure 3-4 (Equations 3-12 through

3-15). In fact, because the shoulder is modeled as a pulley, the net shoulder strap

reaction force must run though the centre of the shoulder at a specific angle (4).

Figure 34 : Net shoulder contact force, s'.

Equation 3-12:

Equation 3-13:

Equation 3-14:

Equation 3- 1 5 :

The Waist and Related Pack Elements

The Waist

To mode1 a waist belt, an understanding and simplification of the anatomical

waist had to be achieved. Figure 3-5 shows a transverse section of the simplified waist

used by Rigby (1997) in Phase II. The major assurnptions were that the lower back and

abdominal areas were flat-faced sections in the transverse plane and the hips were two

semicircles. Although these assumptions seem to be extreme when cornpared to a typical

transverse human section, similarities can easily be drawn and such simplifications were

necessary for future calculations.

In the frontal and sagittal planes, human hips lie at a slight angle fiom vertical and

were modeled using the wedge shape illustrated in Figure 3-6. In three dimensions, this

wedge shape takes on the geometry of the midsection of a half cone.

Lumbar Region

Abdomen

Figure 3-5: Anatomical simplification of a transverse waist section (Rigby, 1997).

4%

Figure 3-6: Wedge shape of hips in frontal and sagittal planes (Rigby, 1997).

The transverse simplification of the waist (including the abdomen and lurnbar

region) was then combined with the half cone shaped hip section. The result was a cube

shaped midsection representing the abdomen and lumbar region flanked on either end by

semicircular half cones. The final model is illustrated in Figure 3-7. The exact size and

ultimate shape of the geometric simplifications are based on a 50' percentile male human

form, which compared reasonably to the SafeworkTM software 50' percentile model

outlined in Chapter 2.

.(,....O...@@ m..... m.*.. b.

Figure 3-7: Cornplete anatomical simplification of the hip region (Rigby, 1997).

Tbe Waist Btlt

The waist belt accomplishes the task of supporting the pack mass through

compression of the waist. Over the hip area the waist belt applies a compressive force

(Tx) that transmits lia to the pack through friction and body reaction force at the angle

described by the hips. The friction resists the pack fiom sliding down the body

(Equations 3-16 and 3-17) and the angle of inclination of the hips allows for a portion of

the body reaction force to transmit directly upward (Equations 3- 18 and 3- 19).

Equation 3- 1 6: F Z ~ = T=COS(YB) + sin(^^)

Figure 3-8 describes how the current model represents the waist belt. Rigby

(1997) modeled the waist belt using hoop stress. While this was effective for the specific

condition of a continuous waist belt of constant tension around the entire circumference

of the hips that applied a uniform pressure, it failed to represent a more general situation.

The current model attempts to represent a more generalized case.

The compressive force of the waist belt (Tx) is a product of the pressure and area

between the waist belt and the waist. While it is obvious the waist belt provides lif l to the

pack (F~Z), it was not initially clear how the vector sum of the compressive forces (which

is zero for a complete circle) leads to the lia. Despite the fact that the direction of the

forces lads to a vector sum in the transverse plane of zero, the magnitude of the forces

still acts perpendicular to the hips. Hence, it is the net magnitude acting perpendicular to

the hips that generates lie. Therefore, one might cal1 this vector a pseudo-vector where it

is only important that the component of the vector normal to the hips be considered in

model calculations.

To determine the relationship between the tension in the belt (Ta) and the

magnitude of the compressive force (Tx), the belt model was split into four equal

quadrants. Each quadrant is the largest sized section that will not result in the belt's

compressive forces canceling each ot her based on their direction. Furthemore, this split

makes the intemal force of the belt (tension) an extemal force that can be related to the

compressive force. Figure 3-9 illustrates a typical quarter section and Equations 3-18

through 3-23.3 detail the equilibrium expressions. This model need only assume that the

tension in the belt (Ta) be constant throughout its length and that the four quadrants are

equivalent. The point of application of the force is over the entire surface of the hips in

the fonn of pressure. This pressure is simplified to four pseudo-vectors normal to the

hips. As noted above, the global direction of the compressive forces are not important,

only their direction relative to the hipq which are four normal forces that impact the lift

of the waist belt.

Pressure between

a

1 1 2 ~ 2

Figure 3-8: Lift capability of the waist belt.

Figure 3-9: Transverse view of a typical quarter of a waist belt in tension over the hips.

Let P = the pressure between the belt and the hips for one quarter segment

Let A = the area of contact between the belt and the hips for one quarter segment

Eguation 3- 18: Fo = P-A

Equation 3-19.2: T3 = Fox

Equation 3-20.1 : DY = 0 = T3 - FOY

Equation 3-20.2: T3 = FOY

Equation 3-2 1 :

Equation 3 -22:

Equation 3-23.1 :

Equation 3-23.2

Equation 3 -23.3 :

To model the belt over the abdomen and lower back, a further simplification was

required. The waist belt was assumed to exit the pack parallel to the lumbar surface.

Since the abdomen and the low back are assumed to be flat-faced surfaces and the waist

belt runs tangent to these surfaces, no compression of the abdomen or lumbar area exists.

Therefore, they were ignored for the waist belt model. Due to the simplifications made,

the net force applied to the belt (and therefore the pack) in the transverse plane was zero,

which was important in determination of the lumbar reaction force (Fx) variable and any

media1 lateral forces (Y-axis). Finally, the belt was assumed to connect to the pack by

means of a pin joint so that no net moment is conducted to the pack systern. This is a

reasonable assumption due to the relatively sofi nature of the waist belt connections

available in current packs. Figure 3-2 illustrates how the lia of the waist belt is

transferred to the pack by this pin joint.

The Lum bar Region and Related. Pack Elements

The Lumbar Region

The lumbar region model was taken from Rigby (1997). Similar to modeling the

waist belt, modeling the lumbar pad required understanding and simplification of the

lower back anatomy. During modeling of the waist belt the lumbar region was simplified

as a fiat-faced surface. However, like the hips, the low back actually lies at a slight angle

from vertical (Figure 3- 10) and the lumbar pad i s designed to exploit this angle. Since

the waist belt forces are tangent to this surface, the waist belt forces have no effect in this

region.

Figure 3-10: Anatomical simplification of the lurnbar region, sagittal view (Rigby, 1997).

The Lumbar Pad

Although the waist belt was assumed not to contribute horizontal force to the

pack, a force is transmitted perpendicular to the low back through the lumbar pad to

maintain pack equilibrium; denoted Fx. Like the waist belt the body reaction force to FX

applies a vertical lie at the noted angle of inclination of the back. As well, friction resists

the pack nom sliding down the surface of the lumbar region. Figure 3-1 1 outlines the

lumbar pad model. Since the waist belt and lumbar pad support the volume of the pack at

the same point, the net lift provided by the lumbar pad - waist belt complex is simply a

sum of the two independent lias. This relationship is illustrated by Equation 3-25 and is

represented by Fz on the general pack model.

Finally, the sum of the waist belt lift force @zB) and lumbar pad lie force (fiL) creates the net lifi force of the complex (Fz) (Equation 3-25).

Figure 3-1 1 : Lumbar region - lumbar pad model (Rigby, 1997).

Equation 3-24.2: Fxr = F X ~ - C ~ L

Equation 3-24.2: F~ = asin( in(^^) + Fxr cos(yL)

Equation 3-25 : Fz = F~~ + F~'

The Role of Friction

Unique solutions for the model are determined based on the idea that each

element of fnction in the model finctions at its maximum value. However, this may not

necessarily be the case.

Suppose a block of known mass sits on a surface with a coefficient of static

fiction p and a tension of force T pulls at the block (Figure 3-12). It is clear that the

maximum force of fnction is the product of the normal force and the coefticient of

fiction. It is also clear that under static equilibrium conditions, the force of fiiction is

equivalent to the tension. If the tension exceeds the product of the normal force and the

coefficient of friction, the block will move. However, it is also true that as the tension

drops below the product of the nomal force and the coefficient of fiction the block will

not move in the opposite direction. In other words, the force of friction is a fùnction

ranging fiom a minimal value of zero when the opposing force is zero to a maximal value

of the product of the normal force and the coefficient of friction when the opposing force

is high.

Fr

Figure 3-12: The role of friction.

Consider this principle in the context of the personal load carriage system

biomechanical model. The fnctional forces that serve to provide lie to the pack will only

be as large as the net force tending to move the pack (down the back). For example,

suppose the pack is in equilibrium on the user with the fnctional components acting at

their maximum values. The user then tightens the shoulder straps forcing the shoulders

to bear more of the load. The result then (to keep the pack fiom moving up the user's

back) is for the hips and lower back to becorne unloaded. This unloading is a result of

the frictional force decreasing. The specific values that counter the tendency to move the

pack down the user's back are reduced. In fact, the shoulder straps can be tightened to

the point that the fnctional forces are reduced to zero.

The fact that al1 the frictional forces operate over a range makes the system of

equilibrium equations indeterminate. However, with an understanding of how the

Grictional forces Vary within those ranges and how the three frictional forces (shoulder

swaps, lumbar pad lift and waist belt lia) react with respect to one another, the model can

be predictive of the system. The model expressions that use fiictional forces can be

rewritten by including notation that represents the range of possible values for the forces

of friction (Equations 3-26 through 3-28).

Equation 3-26, from Equation 3-23.2:

Equation 3-27, fiom Equation 3-24.2:

Equation 3-28, fiom Equation 3- 1 1 :

The system of equations that represent the static pack model can now be simplified.

Equation 3- 1 becomes Equation 3-29:

Equilibrium expression for forces in the X-direction:

Equation 3-2 becomes Equation 3-30:

Equilibrium expression for forces in the 2-direction:

Equation 3-3 becomes 3-3 1 :

Equilibrium expression for moments about the pack's centre of gravity:

Chapter 4

Biomechanical Model Validation Methods

Introduction It was necessary to validate the persona1 load carriage system biomechanical

model presented in the previous chapter. Such a model would be of little use without

substantial proof that the model does, in fact, represent the pack-person the system.

Values predicted by the model had to be compared to the actual forces. If the differences

between these values were acceptably small, the model would be considered accurate and

thus representative of the pack-person interface.

The goal of this chapter is to describe the protocol for validating the model. The

techniques used to measure each of the relevant forces are explained, methods used to

collect the data are discussed, methods used to conduct a sensitivity analysis are detailed,

and the statistical analyses necessary to evaluate the data are presented.

Validation Procedure

The Model

Model inputs, which are detailed in Appendix 1, were gathered and recorded.

Geometric inputs were measured fiom the test pack.

The position of the centre of gravity was determined by reaction board method.

The mass of the pack was determined by weight on a hanging scale.

The strap tensions that the user controls (waist belt [Ts], lower shoulder strap [T2],

and load lifter strap [T*]), as inputs to the model, were gathered in a randomized

fashion. The test sehips are detailed on the test setup configuration chart (Table 4-2).

Note that the tensions listed in the Table 4-2 for shoulder straps were the tension of a

single strap. Men entered into the model the tensions were multiplied by two to

represent the combination of the le&-hand-side and the right-hand-side.

The coefficient of friction around the shoulder (ps) was determined by the rnethod

describeci by MacNeil(1997). A shoulder strap with two ftee ends was wrapped over

the shoulder of the load distribution mannequin at a wrap angle of 180 degrees. One

fkee end of the strap was fixed to the floor and fkom the other end three different

known masses were hung. The strap tension probe was used to masure the tension

of both the fixed end and the &ee end of the strap and. by subtraction, the force of

fnction around the shoulder was determined. The force of fnction and the pulley

equation were used to solve for the coefficient of fnction around the shoulder. Three

different masses were used to estimate three values for the coefficient of fiction and

an average value was used as an input to the model.

The coefficient of fiiction for the lumbar region (pL) was determined by placing three

loads of known value on an inclined surface. The load was covered with the same

material as the lumbar pad of the test pack and the inclined surface was covered with

BockliteTM to replicate the interface between the lumbar pad and the lumbar region of

the test mannequin. The surface was gradually inclined until slippage of the load

occurred. The tangent of the angle was used io detennine the coefficient of friction.

Three loads were used to determine three coefficients and an average value was used.

A similar method to the lurnbar region was used to determine the coefficient of

friction for the waist belt (pB).

The previous chapter outlined the role of fnction and how the range of fnctional

forces could Vary, effecting model outputs. Table 4-1 outlines the various permutations

of the forces of fiiction over the shoulder (FR), waist belt (T& and lumbar pad (Fsr).

Each of these combinations was irnposed (along with al1 other inputs to the

biomechanical model) on Equations 3-29, 3-30, and 3-31. As a result, eight different

model output combinations were calculated for each test setup. The maximum and

minimum values of a11 eight of the modei output permutations were recorded as the range

of output for the regional model and saved for analyses.

Table 4-1 : Permutations of fictional force values for the three anatomical regions.

Model Output Shoulder Region Waist Belt Region Lumbar Pad Region

1 Max. Max. Max.

2 Max. Max. O

3 Max. O Max.

4 O Max. Max.

5 O O Max.

6 O Max. O

7 Max. O O

8 O O O

The Measurement System

The test mannequin was setup on the force platform such that the centre of

rotation of the vice was collinear with the axis of the force platform. The rotating vice

was set at 5 degrees of forward lean (P), which was based on human trunk postures

assessed during load carriage (Stevenson et al., 1995). The force platfonn, load cell in

the test pack, and load cell at the waist of the test mannequin were zeroed so that all

measurements made were a result of the pack-person interfice only.

The first test pack configuration was setup and the pack was mounted on the test

mannequin according to methods described by experienced users (Stevenson et al., 1995).

The strap tension probe was used to ensure that the input strap tensions were set

correctly. Data were then collected in the following order: forces and moments fiom the

force platform, forces and moments fiom the load ce11 in the mannequin, forces and

moments fkom the load ceIl in the test pack, and tensions in the straps (waist belt, lower

shoulder strap, upper shoulder strap, and load lifter strap). Shoulder wrap and strap

angles were measured and rewrded using a protractor and a flexible spline rule. The

pack was removed and the procedure was repeated bvice more for a total of three trials

per pack configuration. The variables were averaged and recordeci in spreadsheet fonn

for fiirther analyses.

Table 4-2 outlines the test pack contiigurations. Configurations consisted of al1 16

permutations of pack mass (30 kg and 35 kg) and geometry (location of shoulder strap

and load lifter strap attachment points). The pack masses were show to be typical loads

in previous research (Patton et al., 1991; Iverson, 1987) and pack geometry was

constrained by the DACMETM pack-board. Three different strap tension input

combinations were then generated at randorn for each pack configuration creating a total

of 48 test configurations. The strap tension inputs, in 10 N increments per strap, were

randomized with group averages of 60 N for the lower shoulder strap, 60 N for the load

lifter strap, and 40 N for the waist belt. Stevenson et al. (1996) showed that these

tensions were the average selections of experienced pack users.

Note: the upper shoulder strap tension was measured on both the right and leA-

hand-side. The values were recorded and then summed and recorded as the single

tension Ti. The right and Ieft-hand-sides were compared using a paired t-test of the two

sets of data. Similarly, the right and lefi-hand-side geometry was also recorded

separately. The two values for shoulder strap angles (el, Cl2, and 84) and the shoulder

wrap angles (al and a) were averaged to detennine a single value for each. The right

and left-hand-side geometric measures were also compared using a series of paired t-

tests.

Table 4-2: Test setup configurations.

Sensitivity Analysis

To detemine the effect that each input variable had on the biomechanical model

output variables a sensitivity analysis was conducted. The results indicated how the

model outputs were affected by changes in inputs thus providing insight into which input

variables were of importance and which output variables were particularly sensitive to

errors.

A typical test pack setup was selected from the validation analysis and the model

outputs were m r d e d and denoted "original". Then, as each input variable was

systematically and independently increased by IV!, the model outputs were recorded.

The absolute changes in the model outputs were calculated for each model input increase

by subtracting the new output value from the original output. These absolute increases

were tabulated. The absolute changes were then normalized to determine the absolute

fractional change in the output variables as a result of systematic independent increases in

the model inputs.

The results were tabulated and outputs that varied greater than 3% (the mean

change in output values generated by 10% increases in inputs) were highlighted. The

results were then summarized to illustrate the most sensitive elements of the pack-person

intefiace.

Sta tistical Analysis

Statistical analysis needed to validate a biomechanical model was a cornparison of

predicted values from the model and actual values measured using the validation

materials (the test mannequin, test pack, load platform, and the strap tension probe). If

there were no statistical difference between the actual values and the predicted values, the

model would have been considered valid.

The equilibrium expression predictions and the geometric predictions were

compared to the matching measured values using paired t-tests. There were seven

equilibrium expression comparisons and five geometric comparisons, one for each

variable. Under normal conditions multiple paired t-tests is inappropriate. However,

because only the ability of a single prediction to represent the related measured value was

important, and not the interaction of the variables, muhi pie paired t-tests were acceptable.

The nul1 hypothesis of this analysis was that no significant difference existed between the

sample means of the two groups (predicted and measured). By standard convention, the

analysis was performed at a significant a level of 0.05. Failure to reject the nul1

hypothesis would indicate that the predicted pack-person reaction forces were not

different from those measured by the test setup, indicating that the mode1 was effective in

predicting the pack-body contact forces.

To determine the sample size, or number of packs to be tested, a power

calculation was performed. By convention, the test statistic was calculated based on 95%

confidence level, with repetition power of 80%. The most conservative estimate was

required. The force with the largest variance estimate was used to calculate the number

of pack geometries necessary. The pilot study by Rigby (1997), while using inputs of 60

N at the shoulders and 40 N at the waist, revealed that the lumbar lift force provided the

most conservative estimate of sample size, or largest variance. The population variance

was (S~,,,,,,~ lin = 17.2), grand sample mean (pg = 1 12.6), and treatment means of ( ~ i , =

116.9, p,,, = 108.2). Calculations for the number of pack test configurations were derived

from Glanz (1997). The sample size was estimated and noncentrality parameter was

calculated. The power of this study was then detennined from standard power tables. At

a power of 80 % Vd = 15 (fiom power table), therefore 14 pack iterations were required.

Since the protocoi generated a population size (different pack geometries) of 48, a power

of greater than 80% was easil y assured.

As was mentioned above the equilibrium expression predictions of the output

values required direct comparisons. The regional models, on the other hand, were only

capable of predicting their respective output forces and moments within a range

determined in part by the force of friction in these regions. Since a paired t-test only

compares the mean of the predicted and measured pack-person variables and this statistic

is not capable of comparing the measured values to the predicted ranges, the regional

models were evaiuated by inspection. The number of measured pack-person interface

variables that feil within the regional models range predictions were counted and a

percentage of the maures bounded by the predictions were calculated.

Chapter 5:

Results

Coefficients of Friction

Table 5-1 shows the average coefficient of friction and the range of the

rneasurements for the shoulder (ps), waist (pe), and lumbar regions (pL) of the pack-

person interface.

Table 5- 1 : Coefficient of fiction determination.

Area Met hod Average C( Range of p

Shoulder Equation 2-9.2, pulley equation 0.39 0.0 1

Waist Tangent of angle of inclination that created 0.32 0.05

impending slip

Lower back Tangent of angle of inclination that created 0.35 0.06

impending slip

Mode1 Predictions

Geometric Preâictions

Table 5-2 shows the strap and wrap angles of the shoulder straps predicted by the

model, the actual measured equivdents, and the associated p-values fiom sets of paired t-

tests. For example, the model prediction of the angle of the upper shoulder strap (81) was

45.3 degrees and the measured value was 44.9 degrees. Comparison of dl 16 pairs of

upper shoulder strap angles (81) (predicted and meawred) revealed a t-statistic p-value of

0.97. Comparison of the tight and lefk-hand-side of measures revealed average p-values of

0.79. The complete data set can be seen in Appendix 2.

Table 5-2: Shoulder geometty mode1 predictions compared to actual measured values,

C O

.r( Ci

Yi % bu

1

2

3

4

5

6

7

8

9

10

I l

12

13

14

15

16

Avg.

Std. Dev.

expressed in degrees. Paired 1-test comparisons are noted.

Shouldtr Modtl

Table 5-3 shows the measured tension in the upper shoulder strap (Ti) compared

to the equilibrium expression prediction of that value. A paired t-test of the measured

data and the predicted data pmduced a p-value of 3.16* 10''~. Table 5-3 also shows the

range of strap tension (Tl) predicted by the regional mode1 (Equations 3-10 and 3-1 1).

Forty-four of the 48 measures fa11 within the predicted range. Cornparison of the nght and

lefi-hand-side measures of the upper shoulder strap tension (Ti) revealed a p-value of

0.93. The complete data set can be seen in Appendix 2.

Tables 5-4, 5-5, and 5-6 show the measured shoulder contact forces (sN, sNx, and

sNZ) and equilibrium expression prediction of those forces. Each of the three predictions

(s" sNX, and sNz) were compared to the measured value by paired t-tests resulting in p-

values of 2.16 * 1 O-", l . 9 l * 1 O*", and 2.70' 1 respectively. Tables 5-4, 5-5, and 5-6 also

show the range of these forces predicted by the regional model (Equations 3-12 through

3-15). For s', s\, and sNz, 20,3 1, and 34 of the 48 measures respectively fall within the

predicted range. The complete data set can be seen in Appendix 2.

Table 5-7 shows measured shoulder fiction force (FR) compared to the

equilibrium expression prediction of that force. A paired t-test of the predicted and

measured values produced a p-value of 7.53*10? Table 5-7 aiw shows the range

predicted by the regional model (Equation 3-9 through 3-10). Forty-four of the 48

rneasures fa11 within the predicted range. The complete data set can be seen in Appendix

2.

Table 5-3: Upper shoulder strap tension (Tl) mode1 validation results (in Newtons).

Test Condition Measlïred Equilibrium Regionai Mode1 Range Prediction Prediction Minimum Maximum

1-1 71.33 19.94 27.10 80.00

Table 5-4: Shoulder contact force (sN) mode1 validation results (in Newtons).

Test Condition Measured Equilibrium Regionai Mode1 Range Preâiction Predict ion Minimum Maximum

1-1 174.63 1 14.28 121.1 0 173.96 1-2 227.41 f 54.43 156.96 239.40 1-3 148.93 77.21 111.86 131.21 2- 1 182.71 87.89 1 14.60 150.78 2-2 108.72 64.68 89.96 95.99 2-3 214.28 98.21 159.55 206.82 3- 1 t 76.25 80.1 3 131.23 147.86 3 -2 289.22 i 92.69 206.48 31 9.97 3-3 196.39 1 03.95 136.76 1 87.62 4-1 233.36 t 74.05 180,67 279.97 4-2 159.87 86.78 112.12 148.62

Table 5-5: Shoulder contact force (sN*) model validation results (in Newtons).

Test Condition Measured Equil ibrium Regional Model Range Predicîion Prediction Minimum Maximum

1-1 1 17.67 83.78 88.82 126.03

Table 5-6: Shoulder contact force (sNz) mode1 validation results (in Newtons).

Test Condition Measuted Equilibrium Regional Mode1 Range Prediction Prediction Minimum Maximum

1-1 129.00 77.72 82.32 1 19.91

Table 5-7: Shoulder fiction force (FR) mode1 validation resuits (in Newtons).

Test Condition Measured Equilibrium Regional Mode1 Range Prediction Prediction Minimum Maximum

1-1 8.67 60.06 0.00 52.90 1-2 10.00 85.31 0.00 82.76 1-3 7.67 60.14 0.00 20.70 2- 1 7.33 04.37 0.00 36.83 2-2 8.67 39.1 3 0.00 6.97

Lumbar Pad Model

Table 5-8 shows the measured lumbar contact force (Fx) compared to the

equilibrium expression prediction of that force. A paired t-test revealed a p-value of

3.20*10". Table 5-8 also shows the range of Fx predicted by the regional model

(Equations 3-24 through 3-25). Seventeen of the 48 measures fall within the predicted

range. The complete data set can be seen in Appendix 2.

Table 5-9 shows the measured lifi force of the lumbar pad ( F Z ~ ) compared to the

range predicted by the regional model (Equation 3-24.2). Twenty-four of the 48 measures

fa11 within the predicted range. The complete data set can be seen in Appendix 2.

Remember that there is no equilibrium expression representation of the lumbar pad lie

force ( F ~ ~ ) . Hence only a range predicted by the regional model exists.

Table 5-8: Lumbar contact force (Fx) mode1 validation results (in Newtons).

Test Condition Measu red Equilibrium Regional Mode1 Range Ptediction Ptediction Minimum Maximum

1-1 122.33 1 13.71 88.82 128.03

Table 5-9: Lumbar pad lift force ( F ~ ~ ) mode1 validation results (in Newtons).

Test Condition Measured Regional Mode1 Range Prediction Minimum Ma-ximum

1-1 40.33 10.91 59.58

Waist Belt Mode1

Table 5-10 shows the measured waist belt lia force (FzB) cornparecl to the range

predicted by the regional model (Equations 3- 16 through 3-23). Forty-six of the 48

measures faIl within the predicted range. The complete data set can be seen in Appendix

2. Remember that there is no equilibrium expression representation of the waist belt lie

force (IJzB). Hence only a range predicted by the regional model exists.

Table 5-1 1 shows the measured lift force of the waist belt and lumbar complex

(Fz) compared to the equilibrium expression prediction. A paired t-test produced a p-

value of 3.99* l O-'. Table 5-1 l a l s ~ shows the range of Fz predicted by the regional model

(Equations 3-16 through 3-23). Forty-eight of the 48 measures fa11 within the predicted

range. The complete data set can be seen in Appendix 2.

Table 5- 10: Waist belt lifi force ('FZB) model validation results (in Newtons).

Test Condition Measuted Regional Mode1 Range Prediction Minimum &ximwn

1-1 163.36 39.89 200.1 5

Table 5-1 1 : Net lifi force (Fz) mode1 validation results (in Newtons).

Tesi Condition Measured Equilibrium Regional Mode1 Range Prediction Prediction Minimum Maximum

1-1 21 2.69 264.82 50.80 258.73

Sensitivity Amlysis

TaMe 5-12, Table 5-13, and Table 5-14 summanze the results of the sensitivity

analysis for the geometric outputs, regionai model outputs, and equilibrium expression

outputs respectively. The detailed sensitivity analysis results can be seen in Appendix 3.

The table shows the fraction of change in the output variables (rows) as the input variables

(columns) were systernatically and independently increased by 10%. Cells only contain

values if the change in output variables were greater than or equal io the 50' decile. The

most sensitive model output was clearly the equilibrium expression prediction of the upper

shoulder strap tension (Ti). One can see from Table 5-14 that small changes in model

inputs (lm) led to consistently large changes in the equilibrium predictions of the upper

shoulder strap tension (Ti). It might appear as though the shoulder reaction forces (!SN,

s\, !SNZ), and the force of friction around the shoulder (FR) were also highly sensitive.

However, the upper shoulder strap tension (Ti) was used to calculate these variables

directly, so sensitivity of shoulder reaction forces (sN, s\, and sNz) was a product of the

sensitivity of Ta. Conversely, the lumbar reaction force (Fx) and vertical lifi of the waist

belt - lumbar pad complex (FZ) were relatively insensitive to changes in the inputs.

Table 5- 12: Summary of sensitivity analysis for geometric outputs, expressed in percent

change.

Model

outputs

01

02

0 4

a1

a

Model inputs increased by 10 %

ds (mm) ds (mm) r (mm) dl (mm) 6 (mm) O. 12 0.08 0.05 0.20

0.07 0.04

0.32 0.09 O, 16

0.05

0.1 1 0.08

Table 5- 14: Summary of sensitivity analysis for equilibrium expression outputs, expresseci

in percent change.

Mode1 inputs increased by 10 %

Cbapter 6

Discussion

Coefficients of Friction Determination of the coefficients of friction about the thtee anatomical regions

(shoulder, waist, and lumbar region) revealed values of 0.39, 0.32, and 0.35 respectively.

Of note was the dramatic increase in these values fiom MacNeil's (19%) measurements.

MacNei1 (1996) used a rnilitary combat shirt, over Tekscanm pressure sensing devices,

which were mounted on the mannequin. The current work used only the mannequin.

These differences accounted for the variation in coefficients of fiction.

Geometric Predictions

Table 5-2 clearly illustrates the ability of the mode1 to accurately predict the test

pack's geometry. The predicted values and the measured values of strap geometry (el, Oz, 04, ai, and a) were not significantly different (average p-value = 0.94) and thus Rom

the same sample.

The ability of the model to accurately predict the test pack's gecmetry cannot

only be attributed to the model itself, but also the repeatability of the validation protocol.

The mannequin was marked and labeled so pack elements could be placed and replaced

in an accurate manner. Furthermore, input data corn the test pack and mannequin

geometry were accurately measured and remained unchanged between trials.

While predictions of al1 the geometric masures were very accurate, the validation

protocol revealed that the vertical stays supponing the attachment points for the load

lifter straps were quite compliant. When the load lifter straps were tightened the vertical

stays flexed, altering the geometnc relationship between the straps and the stays and the

stays and the pack body. However, angles of the load lifter straps (Or) were measured

relative to the main body of the pack itself and the compliance of the stays did not

significantly effect the geometric reiationship between the straps and the test pack body.

While this may have had a signiticant effect on other elements of the pack-person

interface, such as strap tension, it is of littie importance to the geometric predictions.

Since the relative angle of the strap to the pack was not significantly affected the factor

can be considered inconsequential. However, the cornpliance of the vertical stays is

addressed fbrt her below.

Comparison of the geometric measures on the right and left-hand-sides of the test

pack indicated that the right and lefl shoulder strap angles (81, &, and Br) and shoulder

wrap angles (al and a) (p = 0.67, 0.85. 0.92, 0.71, and 0.80 respectively) were not

significantly different. This indicated that the right-lefi symmetry assumption was

accurate for pack geometry.

Equilibrium Predictions

In general the equilibrium predictions of upper shoulder strap tensions (Tl),

lumbar reaction forces (Fs), waist belt - lumbar pad complex lift forces (Fz), shoulder

reaction forces (sN, s\, sNZ), and the forces of friction around the shoulder (FB) were

poor as evidenced by the lack of statistical relationship (p < 0.001) for al1 seven outcome

variables. However, because upper shoulder strap tensions Tl, which were poorly

predicted, were used to calculate the shoulder reaction forces (sN, sNx, and sNz) and the

forces of friction around the shoulder (FB) it was not surprising that the later predictions

were also poor. It is not entirely understood why the equilibrium expressions were so

poor in predicting upper shoulder strap tensions (TI), lumbar reaction forces (Fx), and

waist belt - lurnbar pad complex lift forces (Fz). The author hypothesizes that the

sensitivity of the equilibrium system and the accuracy of the equilibrium expressions

themselves may both have had a detrimental effect.

This hypothesis was examined by conducting a sensitivity analysis to determine

the effect that each input variable had on the biomechanical model output variables. It

was apparent from the sensitivity analysis that upper shoulder strap tensions (Ti) were

highly sensitive to changes in the model inputs, whereas lumbar reaction forces (Fx) and

waist belt - lumbar pad complex lie forces (Fz) were noticeably less sensitive. This

discrepancy in sensitivity created an "ill-conditioneâ" system. The equilibnum

expressions were not capable of depicting the pack-person interface. The details of this

hypothesis are outlined below during discussion of the biomechanical rnodel's sensitivity

anal ysis.

The second possible explanation for poor predictive power of interface variables

could be incorrect modeling. This would occur if a kinetic element that existed in the

physical equilibrium was not included in the rnodel or kinetic elements that were both a

part of the physical system and the model were misrepresented. Since previous, simpler

phases of the model were more accurate predictors (MacNeil, 1996; Rigby, 1997), the

author feels that elements added to this phase of the model were rnisrepresented. One

such element was the load lifter strap tension (T4). Inspection of the model output

surnmary table in Appendix 2 revealed that when load lifter strap tensions (T4) were

large, the prediction of upper shoulder strap tensions (Ti) became less accurate and often

becarne negative, a physical impossibility for tension. Furthermore, when load lifter

strap tensions (T4) were eliminated from the equilibrium expressions (a variable value of

O N) the equilibrium expression's predictions of upper shoulder strap tensions (TI) were

much more accurate. For example, the two closest predictions occurred on test setup 15-

3 and 16-2, both of which had T4 inputs of zero.

The author postulates that the vertical component of the load lifter strap was given

too much weight in the equilibrium expressions. The current model suggested that the

entire vertical component of the load lifter strap, the cosine of the strap tensions (T,),

acted on the pack-body by pulling the pack down. This condition would suggest that, as

the load lifter strap was tightened, the pack would effectively have a greater downward

force and thus lessen the vertical lift created by the increase in lower shoulder strap

tensions (TI). Practically, however, a wearer of the pack would not feel an increase in

"weight" of the pack when the load lifter straps were tightened. m i l e it was clear that

load lifter strap tensions (T4) were an important element of the load carriage system that

exerted some force on the pack and user, it was not entirely understwd how this

occurred.

Future work is needed to fùrther explore how the load lifter straps transfer forces

between the pack, trunk, and shoulders. One possibility is to assume that the load lifter

straps contributed only anterior-posterior force to the system and not downward force.

The downward forces that would be generated by the load lifter strap geometry may not

be transferred to the pack because the shoulders are unable to transfer the reaction force

to the load lifter straps as the shoulders do to the upper and lower shoulder straps. The

upper and lower shoulder straps wrap around the shoulders at approximately 180 degrees

and the reaction force of the shoulders can oppose the tension in the straps directly,

allowing the straps to act on the pack itself Whereas the load lifter straps wrap angle

was much smaller and the contact with the shoulder was almost entirely on the anterior

surface of the shoulder thus the reaction forces of the load lifter straps may not act in the

superior-inferior direction.

If a system of equations is "ill-conditioned" by an incorrectly represented

element, such as load lifter strap tension (T4), solutions for the other variables will be

incorrect also. The author feels that this is the case for equilibrium predictions of lumbar

reaction force (Fx) and waist belt - lumbar pad complex lifi force (Fz). Changing the

shoulder suspension model will have a significant effect on the remainder of the

equilibrium expressions. Once the representation of load lifter strap tension (TI) is

improved, the remainder of the equilibrium expression predictions will improve also.

Furthemore, because the shoulder reaction forces (sN, sNx, and sNz) and the force of

fiction (FR) were calculated directly fiom upper shoulder strap tension (TI),

improvements in the shoulder suspension model will improve predictions of the former

variables as well.

Comparison of the upper shoulder strap tensions on the tight and lefi-hand-sides

of the test pack indicated that the right and left upper shoulder strap tensions (Ti) were

not significantly different (p = 0.93). This indicated that the assumption of right-left

symmetry was accurate for strap tensions.

Regional Mode1 Predictions

The most notable outcome of the regional model predictions was the size of the

predictive ranges. Chapter 3 outlined the variable contribution of fiction throughout the

model and how these variations led to predictive ranges rather than predictive values.

Inspection of Tables 5-2 through 5- 1 1 illustrated that these ranges were quite large; on

average 152% of the minimum prediction and 56% of the maximum prediction.

Furthermore, large coefficients of fiction at the shoulder (b), the waist De), and lumbar

region (pL) the sudaces in question exacerbated this situation. Most imponantly, while

the regional model predictions encompassed most of the validation outputs, suggesting

the regional models were able to accuratel y predict pack-person interface variables, the

size of the ranges made this statement questionable.

A better understanding of key factors affecting the model could be gleaned by

eliminating the fictional contribution of the lumbar pad, waist belt, and shoulder. Since

the precision of the model was so pwr, it was dificult to access its accuracy. While

most predictions fell within the regional model ranges, the ranges resulting from

frictional components were too large to be accepted as an effective model. It is

recommended that future validations alter the test mannequin andlor test pack to ensure

that smaller ranges for the fictional forces are determined. One suggestion is that the

mannequin be covered with TenonTM sheeting, reducing the coefficients of friction and

thus the ranges of the frictional forces. While this does not provide much information

about the contribution of frictioii, it will help researchers better understand the pack-

person reaction forces.

Inspection of Tables 5-5 and 5-8 reveals that the regional model predictions of

lurnbar contact forces (Fs) were consistently high, while predictions of shoulder reaction

forces (sNX) were more accurate. Since lumbar reaction force (Fx) and shoulder reaction

forces (sNX) were in direct opposition and were modeled as counter forces to each other

they should be equivalent. However, consistently high lumbar reaction force (Fs) values

led the author to believe that, while it was not apparent during testing, the waist belt

contributed some anterior-posterior force to the pack. This would challenge the

assumption that the waist belt did not contribute to anterior-posterior forces.

Future work should attempt to either eli minate any anterior-posterior force

generated by the waist belt or include these forces into the regional model. The . attachment of the waist belt to the pack could be changed to one that allowed free

movement in the antenor-posterior plane. If the waist belt was comected to a rod that

was permitted to slide within a chamber attached to the pack-person and oriented in the

anterior-posterior plane, 2-axis, and Y-axis forces would still be transmitted and X-axis

forces would be eliminated. The other option would be to account for the anterior-

posterior force in the model by calculating it as twice the product of the belt tension and

the cosine of the transverse angle of the belt attachent point.

Despite the two factors noted above, the regional rnodel predictions encompassed

71.3% of the measured values. The most accurate predictions were of waist belt - lumbar pad complex lift force (Fz) whose predictions encompassed 10W of the

measured values while the lumbar reaction force (Fx) predictions were the least accurate,

only encompassing 35.4% of the measured values. The relatively low accuracy of the

latter was best explained by the fact that the waist belt generated some anterior-posterior

force. Other poor predictions included shoulder reaction forces (sN) and lumbar lift

forces ( F ~ ~ ) . Because net shoulder reaction forces (sN) were calculated fiom the root of

the sum of the squares of horizontal (sNx) and vertical (sNz) shoulder reaction forces, the

inherent error in the later two cornbineci to increase the enor in the former. Similarly,

lumbar lift forces ( F ~ ~ ) were derived from lumbar reaction force (Fx) predictions, which

as noted above, were relatively poor.

Sensi tivity Analysis

The sensitivity of upper shoulder strap tensions (Ti) suggested that the

mathematical relationships generated by the equilibrium expressions were "ill-

conditioned" toward upper shoulder strap tensions (Ti). Because the mathematical

relationship disproportionately weighted upper shoulder strap tensions (TI), the physical

system could not be well represented without an improved method of assessrnent for

upper shoulder strap tensions (Ti). Small errors in input values significantly altered the

output value of upper shoulder strap tensions (TI) thus leading to poor predictability.

Alternatively, insensitivity of the lumbar contact forces and waist belt - lumbar pad

complex lie forces (Fs and Fz) led to similar problems for opposite reasons. To register

change in lumbar reaction forces (Fs) or waist belt - lumbar pad complex lifi forces (Fz),

the input values must be changed drastically. It would seem that lumbar reaction forces

(Fx) and waist belt - lumbar pad complex lia forces (Fz) were more responsive to load

factors than pack geometry. The physicai system required that model predictions be

more accurately measured because of the high sensitivity of some model outputs and

relatively low sensitivity of others.

The sensitivity analysis provided support for logical model relationships:

Geometric outputs were sensitive to changes in geometric inputs. Upper shoulder swap

angles (el), lower shoulder strap angles (&), and load lifter strap angles (e4) were al1

sensitive to changes in: the lumbar contact point to shoulder center (d5), pack body to

shoulder center (&), shoulder radius (r), lumbar center to load lifter strap attachment

points (&), and lumbar center to upper shoulder strap attachment points (di) dimensions.

The shoulder wrap angles (al and a4) were also sensitive to changes in similar inputs.

Considering that upper shoulder strap tensions (Tl) were related to lower shoulder strap

tensions (TI) by the modified pulley equation, it was not surprising that upper shoulder

strap tensions (Ti) were sensitive to changes in lower shoulder strap tensions (4). The

same relationship dictated that upper shoulder strap tensions (Ti) and frictional forces

around the shoulder (FR) were sensitive to changes in the coefficient of friction around

the shoulder ($1. It was also logical that shoulder strap tensions (Ti, Tt, and T4) had

significant effects on the shoulder reaction forces (sN, sNX and sNZ). Because forces of

fiction around the shoulder (FR) were derived fiom the difference between the lower

shoulder strap tensions (TI) and the combination of the upper shoulder strap tensions (Tl)

and the load lifter strap tensions (TJ), it was not surprising that the forces of friction

around the shoulder (Fa) were sensitive to shoulder strap tensions. The lifl forces of the

waist belt (F~z) and lumbar pad ( F ~ z ) were sensitive to changes in anatomical angles (ye

and yL) or the hip and lumbar region. Waist belt forces, such as the compressive forces

(Tx and TX~) , were sensitive to increases in waist belt tension (T3) And finally, the lie forces due to fiction at the waist (Tm) and lumbar regions (Fsr) were sensitive to

changes in the coefficients of fiction in these regions (pe and pt).

Some input-output relationships were less obvious. Increasing the rnass of the

pack (W) while not increasing the shoulder strap tensions (Ti, T2, and T4), which

occuned in the sensitivity analysiq required that the extra mass be bom elsewhere.

Therefore, waist belt - lurnbar pad complex lie forces (Fz), which was essentially the

difference between the vertical shoulder suspension forces (sNX) and the mass of the pack N N (W), was sensitive to changes in pack mass. Shoulder reaction forces (S , S X, and sNz)

and lumbar contact forces (Fx) were highly sensitive to changes in the lumbar contact to

shoulder center (d5) and pack body to shoulder center (&) dimensions. More accurately, N N however, the shoulder reaction forces (S . S X, and sNz) were sensitive to the geometric

outputs, which were in tum sensitive to the geometric inputs. Changes in the angle of

pull of the shoulder straps (01, O*, and 04) would certainly e f f ~ t the distribution of force

about the shoulder. As well, because the lumbar contact forces (Fx) counteracted the

shoulder reaction forces (sNx) in the anterior-posterior plane, the lum bar contact forces

(Fx) were highly sensitive to shoulder reaction forces (sNX). Altering the tensions in the

lower shoulder strap (T2) also created the same chain reaction effect of sensitivity. The

lumbar contact forces (Fx and Fz) were highly sensitive to lower shoulder strap tensions

(Ta) because lower shoulder strap tension (T2) significantly effected the shoulder reaction

forces (sN, sNx, and s'z), which in tum effected the lumbar contact forces (FA and Fz).

The shoulder Contact forces (sN, sNy, and sN2) and lumbar contact forces (Fx and Fz)

were also sensitive to variations in the lumbar centre to upper shoulder straps (di) and

lumbar centre to load Mer stnps (6) dimensions as a result of a similar chain reaction.

Methods and Materials In validating the system, it is important to have an accurate benchmark pack as a

reliable measurement tooi. The test pack was a simplified extemal frame system with

most features of the pack-person interface sirnilar to a standard pack. It is clear that the

pack was representative of a stiff, extemal frame style pack. The results of the study

cannot currently be extended to sofier interna1 frame style packs. Further investigation

into how the mode1 represents these packs would be necessary. The main body of the test

pack was relatively rigid. However, as mentioned earlier, the vertical stays chat served as

the load lifter anachment points were much less rigid than the remainder of the pack.

While this did not effect the model's ability to predict geometry, it may have had some

effect on the force transmitted between the load lifter straps and the pack itself Under

normal static conditions a more compliant connection would not Iead to lower force

transmission. However, the relative sensitivity of the system to extraneous factors

suggested that alteration in any element of the pack-person interface test setup may have

had significant effects on output masures. Furthemore, a dynamic mode1 has been

suggested as a future goal of the research team thus making test pack cornpliance a much

more critical variable.

It is recommended that the vertical stays on the test pack be changed. Altering the

stays so that a uniform stifiess is achieved throughout the pack will reduce the

extraneous factors that may tend to effect measurements. Specifically, because the effect

of varied cornpliance is not well understood, future work should strive to eliminate such

concems. One recommendation is to mn aluminum stays along the entire height of the

pack so that al1 shoulder straps are attached to the pack through the same medium.

Another concem with respect to pack stiffness was the assumption that the waist

belt transmitted forces to the pack body by a pin joint. While this assumption was made

it was known that the connection was not actually a pin joint and a small error was

expected. However, since it has become apparent that the pack-person interface outputs

are quite sensitive to extraneous factors this assumption has become suspect. It is not

ent ire1 y understood what effects t his assumpt ion may have had on the pack-person

interface measures. However, it is believed that the assumption may have had more of an

effect than previously anticipated and moments at the waist k l t may have been

transfened to the pack. The actual rotational equilibrium of the pack would be affected

and thus the test pack may not be indicative of a standard pack in this respect.

Future endeavors should strive to consolidate the pin joint concem. Either by

gaining an understanding of how the stifhess of the waist belt effects the system or

changing the connection of the waist belt to the pack to an actual pin, this concern can be

eliminated. The most effective solution may be to reattach the waist belt to the pack with

a simple bal1 and socket style connection. This would ensure that a pin joint connection

exists and no assumptions need be made.

As was mentioned in Chapter 3, the order of tightening pack straps affected the

direction of the force of friction around the shoulder (FR). It was also discovered during

the initial stages of a broader military pack investigation that the method of donning and

dofing the pack had a significant effect on pack-person interface variables. For example,

doming the pack by tightening the shoulder straps and then the waist belt would result in

a different set of measured outputs than donning the pack by tightening the waist belt and

then the shoulder straps. The doming factor was accounted for during the validation

study by employing the same doming technique throughout the study (taken fiom

protocol employed by experience pack users). However, this result indicates the

sensitivity of the system to extraneous factors and suggests that Gare must be taken during

ftture validation studies or interpretation of results.

As was discussed above, the method used to don the pack significantly affected

the outcome measures. Whiie the same method could be used consistently throughout

future validation studies it may be more important to gain an understanding of how

donning methods affect outcornes. The validation protocol could be repeated using the

various donning procedures outlined in the literature. The outcome measures could then

be compared using a paired analysis and thus gain an understanding of the effects of

doming techniques. Not only can this information be used to better validate the

biomechanical model, but it rnay also provide more insight into the pack-person interface.

The assumption that contact between the pack body and the users' back only

occurs in the lumbar region rnay not be entirely accurate. During two test setups (3-2 and

7-3), the pack-body came into contact with the upper back. The author believes that the

contact was the result of extreme shoulder strap tensions. These two test condition setups

(Table 4-2) required strap tensions that were higher than the predicted equilibrium could

provide. The extra shoulder force pulled the pack against the upper back of the test

mannequin creating another counter force to the shoulder force. The sum of the lumbar

contact forces (Fx) and the new upper back contact force counteracted the excessive

anterior-posterior tensions of the shoulder straps (TI, TI, and T4), thereby permitting the

required tensions.

Previous literature reviews and investigations at Queen's University suggested

that experienced users oflen tighten the shoulder straps beyond the minimum necessary

tension (Pelot et al., 1995). This tension draws the pack into contact with not only the

lumbar region, but also the upper back as well. It is not surprising that this occurred

during two test setups. Therefore, it is suggested that future work attempts to understand

what conditions produce contact with the upper back and how this contact affects

equilibrium conditions. With intemal fkame packs, this issue would be exacerbated.

Some concem existed with the strap tension probe. The force transducer of the

strap tension probe was louited between the pivot and the handles. In this configuration,

the strap tension probe was sensitive to the location of the force applied to the handles. If

the force wes not applied directly over the stop rod, a bending moment was generated

about the stop rod thus altering the output of the force transducer. In this controlled

laboratory setting, a grip that concentrateci the force directly over the stop rod was

employed and was considered reliable and repeatable. However, for future field studies,

the probe should be modified so variable grips on the pliers do not affect the probe's

readings. It is suggested that the force transducer be move to the shaft of the pliers

between the pivot and the strap prongs and the pliers be re-calibrated and re-validated.

With the transducer located between the pivot and prongs no extemal bending moment

would be applied. This would ensure that any gripping technique employed would

provide accurate outputs.

Chapter 7

Future Directions and Conclusions

Future Directions Many concems were expressed and recommendations made about the model and

testing materials in the previous chapter. The concems specitic to this wotk should be

examined and decisions made to consolidate this model before significant model

applications or funher model advances are attempted. Specifically, the following

concerns and recommendations should ail be investigated fbrther:

The stiffness of the vertical stays for anachment of the load lifter straps to the pack

should be altered so that the connection of al1 the shwlder straps provide consistent

cornpliance;

Re-evaluate certain elements of the model, specifically the way in which the load

lifter strap tensions (T4) transfers forces between the shoulder and the pack body. In

fact, the waist-lumbar region should be evaluated in isolation fiom the shoulder

model. Once both elements are accurately represented independently, a combined

comprehensive model, such as the curent attempt can be realized.

The size of the predictive ranges of the regional models should be reduced and the

mode1 revalidated to better understand the pack-person reaction forces;

Eliminate the antenor-posterior force generated by the waist belt on the test pack or

include this force in the model;

Re-evaluate contact between the pack and the user's (or test mannequin's) upper

back;

Detennine the efTects of different donning methods for the test pack;

Improve the design of the strap tension probe so that it may be used in various

situations and be considered more reliable;

Improve the swap tension probe to be insensitive to gripping techniques.

Once this work is improved through consolidation of the model and associateci

validation, researchers would be better able to undertake the following two related

94

investigations: further development of the current optimization routine and development

of a dynamic personal load carriage system biomechanical model.

Previous research (Pelot et ai., 1998) outlined an optimization routine that used a

biomechanical model as its basis. The routine was designed to optimize the pack

geometry, pack materials, kit selection, and kit placement in a pack to minimize negative

effects in the pack-person interface. Currently, the optimization routine uses previous

phases of this model as a base. Once this phase of the model is improved and

revalidated, it can be incorporated in an attempt to funher the optimization routine. The

routine can only be advanced as the biomechanical mode1 is advanced.

The ultimate goal of any model is to provide a perfect representation of a system.

Since the personal load carriage system model represents a dynamic situation, eventually

a dynamic load carriage model needs to be developed. Furthemore, because this phase

of the model has begun to deal with dynamic elements, such as pack stiffness, it seems

that the next phase should attempt such an undertaking. The author suggests that the

biomechanical model be investigated in a cyclical motion, such as the vertical oscillations

associated with gait. Under these conditions, peak forces, moments and pressures could

be studied and the stiffness of certain pack elements evaluated.

Conclusions The main objective of this work was to develop and validate the next phase in a

series of pack-person interface biornechanical models. It was also expected that four sub-

objectives would also be met:

1. The biomechanical model would be the basis for the persona1 load camage system

design tool outlined in Chapter 1;

2. The equilibrium expressions and the regional models contained within the personal

load camage system model would provide researchers and designers alike with more

information about the pack-person interface;

3. The process of developing and validating the biomechanical model would provide

more insight into the pack-person interface;

4. And the load cadage system measumnent tools would be improved through the

addition of a strap tension probe and test pack.

The current phase of the biomechanicai model is not as representative of the

physical pack-person interface as the author had hypothesized. The equilibrium

predictions of the pack-person interface variables were quite poor. A number of

approaches should be taken to improve this phase of the model before it is considered

accurate and appropriate for xientific investigations of design work. The regional

models did a reasonable job at predicting the range in which the physical variables will

reside. However, the ranges were too large to be conclusive about iheir accuracy. The

author also believes that the current representation of the load IiAer strap is quite poor

and it had major impact on many other output variables in the model.

Despite the poor ability of the current model to predict pack-person contact

forces, the model proved to be an excellent predictor of pack geometry. The relative

angles of the three shoulder draps to the pack and the wrap angles of these straps were al1

accurately predicted. This aspect of the rnodel can be used in future investigations.

The biomechanical model could not considered a scientifically valid

representation of the physical pack-person system and should not yet be used to conduct

pack performance evaluations of current or prototype pack designs. However, despite the

fact that the model did not stand up to a rigorous scientific validation, many elements of

the model can provide qualitative information about pack-person interface variables.

Designers may be able to gain some insight into trends or descriptive relationships

between variables, helping the design process.

Development and validation of the current biomechanical model has improved the

understanding of the pack-person interface. Results of this study has illustrated the

following points:

The method of donning a pack can have a significant effect on the pack-person

interface variables;

The direction of the force of fiction is determined by the order in which the shoulder

straps are tightened;

The cornpliance of the pack may have a significant effect on pack-person contact

forces;

The sensitivity analysis revealed that the pack-person intertace might be quite

sensitive to design features;

5. The sensitivity analysis also illustrated that changes in most model outputs were the

logical result of changes to model inputs, illustrating some legitimacy within the

current biomechanical model.

6. Friction within the pack-person interface is a very important characteristic that must

be fiilly understood for future designs.

It is interesting to note that the complex pack-person interface may not be best

represented with an equally complex model, since previous versions of the model seemed

to better represent the system than the current model.

The strap tension probe and the validation test pack make excellent additions to

the Queen's Ergonomics Research Grou p' s battery of persona1 load carriage system

measurement tools. Validation of these tools illustrated that the devices were an effective

means of measuring strap tensions and numerous pack-person contact forces. The

current validation test pack could be used to conduct future pack-person interface

analyses. However, if the stiffness of the pack is required to be uniform, the vertical

stays that serve as the attachment point for the load lifter straps must be improved. Also,

if the waist belt connection is to be assumed a pin joint, the rigidity of that connection

must be reduced. The strap tension probe also requires some fiirther investigation before

it can be used in fùture analyses. Despite the fact that the probe was considered valid for

this study, it should be modified to be less sensitive to grip location and thus be more

versatile for field use.

In general the author feels that this work has contnbuted to the larger military

load carriage project. While the current model cannot be used as a robust scientific tool,

much information has be learned and added to the load carriage system knowledge base.

Furthermore, this work provides an excellent beginning for improvements to the current

model and the next iteration of a dynamic biomechanical model. Although the model

cannot be used as the basis for a valid design tool, qualitative information can be gleaned

€rom the model for design purposes. In addition, two new measurement tools have been

added to the load carriage system measurement battery and can be used for both fùture

validation studies as well as investigations in a larger scientific scope. Finally, the tiiture

directions outlined above detail the strategy by which the current model and measurement

tools can be used as a stepping stone to tiiture scientific work.

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Appendix 1

Biomechanical Model Notation

Notation . . .. ... .. .. .. ...... .... .... ..... . ............ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104

Notation

Following are detaiIed descriptions of al1 the parameters illustrated by the general

pack mode1 and associated regional models (shoulder, waist, and lumbar region).

Orientation:

X coordinate along pack depth

Z coordinate along pack height

Pack Container:

W the force of the mass of the pack (input)

vx horizontal position of the centre of mass (input)

vz vertical position of the centre of mass (input)

hx horizontal dimensions of the pack container (input)

hz vertical dimensions of the pack container (input)

Bearer:

d3 distance from bottom of pack to lumbar pad contact centre (input)

d5 distance from lumbar pad contact centre to shoulder centre (input)

d6 distance fiom pack to centre of shoulder (input)

r average radius of shoulder (input)

r~ average radius of hips (input)

P body lean angle (input)

y~ anatomical lower back angle fiom vertical (input)

YB anatomical hip angle from vertical (input)

Shmdder strqs:

TI tension in upper shoulder straps (LHS and RHS summed) (output)

T2 tension in lower shoulder straps (LHS and RHS summed) (input)

T4 tension in load lifter straps (LHS and RHS summed) (input)

di distance fiom lumbar pad centre to attachment ofupper shoulder strap (input)

d2 distance from lumbar pad centre to attachment of lower shoulder strap (input)

4 distance fiom lumbar pad centre to attachment point of load lifter straps (input)

81 upper shoulder strap angle fiom the vector normal to the pack (output)

& lower shoulder strap angle Grom the vector normal to the pack (output)

8 4 load lifiet strap angle C.C.W. fiom the vector n o d to the pack (output)

al upper shoulder strap wrap angle around the shoulder (output)

a4 load lifter strap wrap angle around the shoulder (output)

4 angle at which sN acts from pack normal (output)

PS coefficient of fiiction of strap on shoulder (input)

sN net force of shoulder straps acting though the centre of the shoulder (output)

sNx X-component of sN (output)

sNz 2-component of sN (output)

Fm force of fnction around the shoulder (output)

Waist belt:

T3 tension in waist belt (input)

d3 distance to lumbar pack centre tiom bottom of pack (input)

TX compressive force that T3 applies around the hips (output)

T X ~ normal force component of Tu: (output)

T x ~ the force of friction due to T ~ C * (output)

~~z lifl provided by the waist belt resting on hips (output)

coefficient of fiction of waist belt on hips (input)

Ltcmbar region:

Fx reaction force of lower back on pack in X-direction (output)

F X ~ the component of Fx normal to the lower back (output)

Fxr the force of friction due to Fx (output)

FLz lift on the pack fiom fiiction and angle at lower back (output)

PL coefficient of tnction of lumbar pad on lower back (input)

Fz total l i e force at lumbar contact point of pack (output)

Appendix 2

Summary Data Sets

....................................................................................... Geornetric measurements 1 07

......... Geometric mode1 predictions and validation measurements: Group averages 1 10

....................................................... Mode1 predictions by equilibrium expressions 1 11

Regional model predictions ................................................................................... 1 12 . ....................................................................................... Validation measurements 1 14

............................................................ Validation measurements: Group averages 1 17

..................................................................... Sample data set of model predictions 1 18

Minhum Mpdmum MWmun Markmim Minhum Mixlmum Minimum W m u m Minimum 27.10 37.24 1934 23.1 7

13.03

31.6s

20.14 46.50

27.51

40.60

21 .?O

23.03

4-43

21 .O4

13.05

27.62 15.29

4.43

1 7.35

22.28

49.65

32.23

2 7.29 33.50

12.74

40.06

26.m 20.86 25.S 34.87

16.07

35.66 18.1 7

31.0t

26.20 39.8s 46.06 38.06

nat 22.44 17.57

13.66

20.1 1

40.22

29.72 18.73

n.n

X s*, f, Pt Pz Modmum Mlnimum Msximum Minimum Mprknum Mnknum W m w n Minimum Msximurn

irwu of pck input, Symbd M a Unib W 33.00 kg 1 up9ll8h. Stnp q k from n m .

V W t l c r l p o J t k n o f ~ o f m r u h o i f i o r i t o l ~ o f ~ o f m r u wrtkrl dimemion of pack horimtal dl- of prck lumbr contrct to di. C m pack to rti. Center dius of rh0Uw.r fadiw of h i p body ban angb low b c k angk hip angle bwer stl. Stmp tsmkn laid lifter tendon lumbar Mer to upper sh. Stmp lumtmr Wdl to kwr sh. Strsp lumôar Ebntw to kad iiitff drap eoelflcknt of friction iround 8h. Waia belt tam&n lumbar psd mter to bottom of prck coefficht of fricth of wrht bsn coeflicknt of frktbn of lumôar pnâ

force of fricth wwnd diouidar

c o m p r ~ v e r01C-e of WaM belt normal fana componsnt of T3C force of friction due to f X

Fx 115.09 N Fz 236.63 N TI 26.11 N lift on pack from lumbar cornpiex

Statk Equilikium Output8 [~meripcim ~yrnimi ~ a t a Units

force of friction due Co fXN

-ci0 min )LS

~ ) i ô

min Ps Ps

min Cis lfux Ci6 min )rs - PB min PB

lfuxcie min W c i e min )y

m)ie

min )is

WIis min Ps

Cie min )is mmt Cis min Ci8

m)is

min Ps Ci6

min

-)is

min r s

min Mi -Cis -)ib

M C i e fnin P?l

inhl Pa

Appendix 3

Cornpiete Model Sensitivity Analysis Results

Unique variable outputs as input variables increase by 1 û% ................................... 121

N o m l i z e d increase in output variables as input variables increase by 10% .......... 123

Appendix 4

Mode1 Validation Protocol

Mode1 validation protocol ... .. . . .. .... .. . . . . ... ... . ... . .. ... . ......... ........ .. . ... . . . .. ... .... .. . . . .. .. . . . .-. 126

Mode1 Validation Protocol

refer to Table 4-2 and setup the test pack according to the designated test condition

(strap position and pack mass)

using the reaction board method, measure and record the centre of gravity of the test

pack in the current setup

position test mannequin on force platform and adjust forward lean to required body

lean angle

zero the force platform and the test mannequin load ceIl

mount the test pack on the mannequin such that the centre of the lumbar pad contacts

the mannequin at the centre of the lumbar region (marked with an asterisk)

tighten the waist belt

feed the shoulder straps over the shoulder and tighten the lower shoulder swap

ensuring that the straps contact the shoulder along the path marked with black lines

tighten the load lifter strap

using the strap tension probe and a zeroed digital volt meter, measure the change in

voltage for the three closure positions (noted in Chapter 2)

10. using a least squares method (typical spreadsheet fùnction) and the coefficients

provided in Chapter 2, determine the strap tension for the waist belt and the right and

lefi lower shoulder strap and load lifter strap

1 1. adjust the pack straps in order: waist belt, lower shoulder strap, and load lifter strap

12. repeat strap tension measurement and strap adjustment until the input strap tensions

are within 1 N of the input tensions noted in Table 4-2

13. using a protractor measure the angle between the test pack main board fiame and the

right and left lower shoulder straps, upper shoulder straps and the load lifter straps

14. feed a sheet of paper between the lower shoulder strap and the test mannequin and

mark the point of resistance of the paper (the point at which the strap contacts the

mannequin)

15. repeat between the upper shoulder swap and the mannequin and the upper shoulder

strap and the load lifter strap

16. using a flexible spline rule, measure the wrap angle of the upper shoulder straps and

the load lifter straps on both the right and left sides

17. record the force data from the force platform

18. record the force data from the test mannequin load ceIl

19. record the force data tom the test pack load ceIl

20. using the strap tension probe measure and record the strap tension of the upper

shoulder strap

21. remove the test pack and randomly select a new test condition fiom Table 4-2

22. repeat data collection