Download - 5 Biomechanics
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About 1/3 of U.S. workers perform tasks thatrequire high strength demands
Large variations in population capacity (strength)
Basis for understanding and preventingoverexertion injuries
simulate design alternatives
Wide application potential Orthopaedics
Rehabilitation
Sports science
Vehicles
maintain: D < C D: task Demands (forces and moments) C: human Capability (strength, tissue
tolerance)
ot va ues are g y var a e an cu t tomeasure and predict
"strength" = not one thing!; here, typicallyuse max. joint moment (and is a function ofposture, time, etc.)
The annual incidence of WMSDs in all industries is about
300-400 per 100,000 workers for all workers; substantial
differences between industries.
A recent NIOSH study estimated 22% of all VDT workers
a some ype o pro em.
Average Worker's Compensation cost per claim and total
costs per injury:
All WMSDs: $14,726/$20,000
Carpal Tunnel Syndrome: $29,000/$100,000
Enormous increase in complaints: 1983->1988, from 35%
to 67% of all occupational illnesses. Recent numbers have
stabilized around 25-30%.
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Source Total Cost ($) Notes
In-plant med ical visitsand treatment s
14, 050 ~$50 pe r v isit
Emplo yee ab sences 127 ,905 Each 1 -week absencerequired 1 r eplacement w orker
Work restriction s 16, 192 1/2 of the work restrictions
Job c hanges initiatedby employe e
13, 9 84 E ach j ob c hange r eq uir edretraining fo r 2 workers
Tota l 172 ,131
Estimated costs associated with 93 cases of shoulder disordersreported to the plant medical department of an automobileassembly plant.
From: Punnett, L. et al. (2000) Scand J Work Environ Health
Meat Packers Thigh Boning Operations
thighs suspended 44" from floor
1 thigh every second turkey
one hand grabs thigh
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knife hand 2-3 cuts to separate meat
knife hand 1 cut to separate tendon
7,560 turkeys per shift
Keyboard Entry Work keys/shift: very low force, very high reps
Assembly in electronics and light manufacturing
Sewing Operations
Packing Operations
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From: Business Week, 02/23/99
De Quervain's Disease -- Stenosingtendinitis of the flexor tendons of thethumb.
From Putz-Anderson, V. (1988). CumulativeTrauma Disorders: A Manual for MusculoskeletalDiseases of the Upper Limbs. London, UK: Taylor& Francis., Fig. 3
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Tendon injuries can produce localizedswelling in the area of the injury.
The inflammation associated with tendoninjuries can interfere with nerve function andeven permanently harm nerve tissue.
Carpal Tunnel Syndrome (CTS): Tendonitis ofthe flexor tendons of the fingers as they passthrough the carpal tunnel resulting incompression of the median nerve.
Carpal Tunnel Syndrome accounts for lessthan 10% of the total number of WMSD cases.
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Back injuries account for nearly 20% of all injuries and illnesses in
the workplace and nearly 30% of all workers compensation
payments.
The lifetime incidence of low-back pain is estimated to be 60% with
about 4% incapacitated for over 6 months. Annual and point
prevalences are 40% and 20%.
It has been estimated that 6.5 million people stayed home each day
due to low-back injuries, with 1.5 million new injuries per month.
Chronic low-back pain affected ~35 million people.
The cost to U.S. industry includes 170 to 240 million lost work days
each year and ~$5 billion in workers compensation payments.
The total direct costs of back injuries is ~ $30 billion, with indirectcosts possibly doubling that figure.
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Cervicalvertebrae
Thoracicvertebrae
Lumbarvertebrae
Sacral/Coccygealvertebrae
L5/S1
Nordic Questionnaire
Biomechanics Models
Material Handling: NIOSH Guideline forManual Lifting Task
Quick Assessment Tool
Hanya dijawab jika jawaban pada kolom 1 ya
Apakah Anda mempunyai keluhan (nyeri,ngilu, pegal) selama 12 bulan terakhir pada
anggota tubuh berikut?
Apakah dalam 12
bulan terakhir,
masalah tersebutmengakibatkan
Anda tidak dapat
bekerja secara
normal?
Apakah Anda
mempunyai
masalah yangsama dalam 7
hari terakhir?
Apakah menurut
Anda masalah
tersebutberhubungan
dengan pekerjaan
Anda di sini?
Leher tidak ya tidak ya tidak ya tidak ya
Bahu tidak ya, sebelah kanan tidak ya tidak ya tidak ya
ya, sebelah kiri
ya, keduanya
Siku tidak ya, sebelah kanan
ya, sebelah kiri
ya, keduanya
tidak ya tidak ya tidak ya
Pergelangan tangan
tidak ya, sebelah kanan
ya, sebelah kiri
ya, keduanya
tidak ya tidak ya tidak ya
Punggung Atas tidak ya tidak ya tidak ya tidak ya
Punggung Bawah tidak ya tidak ya tidak ya tidak ya
Paha tidak ya(salah satu atau keduanya)
tidak ya tidak ya tidak ya
Lutut tidak ya
(salah satu atau keduanya)
tidak ya tidak ya tidak ya
Pergelangan kaki tidak ya(salah satu atau keduanya)
tidak ya tidak ya tidak ya
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Two-DimensionalStatic
Two-DimensionalDynamic
Three-DimensionalStatic
Three-DimensionalDynamic
Translational equilibrium (a = 0):
Rotational equilibrium ( = 0):
Forces = 0
Moments = 0
Free-body diagrams are schematicrepresentations of a system, identifying allforces and all moments acting on thecomponents of the system.
Here, we will differentiate between external andinternal forces and moments (see below)
Anything can be chosen as the free body!
Examples: whole body, arm, hand,
Choose wisely, and solving biomechanics
problems becomes much easier.
Resultant or External what the world does to the body
gravity
contact loading (e.g. with ground)
Reactive or Internal w a e o y oes n response
muscle activation
ligament stretch
joint contact forces
In equilibrium: Resultant + Reactive = 0
F = 0 M = 0Reactive = - Resultant (Or, internal = -external)
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External Forces and Moments
F, M
Internal Forces and Moments
F, M
+Y
+X
Equilibrium:
F = 0 F + F = 0 F = -F (Translational Equilibrium)
M = 0 M + M = 0 M = -M (Rotational Equilibrium)
+Z
X
17.0 cm
35.0 cm
180 N
10 N
From Chaffin, DB et al (1999) OccupationalBiomechanics. Fig 6.2
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From Chaffin, DB et al (1999) OccupationalBiomechanics. Fig 6.7
10 N180 N
FB?
5 cm
HANDCOMELBOW
Unknown values: Biceps and external elbow force (FB and FE), and any joint contact
force between upper and lower arms (FJT, an internal force) External elbow moment (ME), and internal ME
Lower arm selected as free body Isolates elbow forces and moments Results in a single unknown (see below)
35.0 cm17.0 cm
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1. Establish coordinate system (signconvention)
2. Draw Free Body Diagram, including knownand unknown forces/moments
3. Solve for external moment(s) at joint
4. Determine net internal moment(s), and solvefor unknown internal force(s)
5. Solve for external force(s) at joint [can alsobe done earlier]
6. Determine net internal force(s), and solve forremaining unknown internal force(s)
FBD:
E H
WLA
=mLA
g=10N
FH=m
Hg=
180N
FB=??
FJT
=??
ME=??
+Y
+X
+Z
_ _ME = 0 = ME + ME -> ME = -ME
ME = MLA + MH = (WLA x maLA) + (FH x maH)
ME = (-10 x 0.17) + (-180 x 0.35) = -1.7 -
63
ME = -64.7 Nm (or 64.7 Nm CW)ME = -ME -> ME = 64.7
ME = (FJT x maJT) + (FB x maB) = FB x 0.05
FB = 1294 N (up)
_ _
Externalmoment is dueto externalforces
Internalmoment is due
to internalforces
_ = 0
FE = 0 = FE + FE -> FE = -FE
FE = WLA + FH = -10 + (-180)
FE = -190 N (or 190 N down)
FE = - FE -> FE = 190
_ _
_ _
_
Thus, an 18 kg mass (~40#) requires 1300N(~290#) of muscle force and causes1100N
(250#) of joint contact force.
Tips: 1 kg = 2.2 lb-mass; 4.45N = 1 lb-force
FE = FJT + FB
FJT = 190 - 1294 = -1104 N (down)
Links are rigid
Joints are frictionless
No motion
No out-of-plane forces (Flatland)
Known anthropometry (segment sizes andweights)
nown orces an rec ons
Known postures
1 muscle
Known muscle geometry
No muscle antagonism (e.g. triceps)
Others
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Biomechanical analysis yields external moments atselected joints
Compare external moments with joint strength (maximuminternal moment)
Typically use static data, since dynamic strength data arelimited
Use appropriate strengt ata (i.e. same posture)
Two Options:
Compare moments with an individuals joint strength
Compare moments with population distributions to obtainpercentiles (more common)
Multi-joint systems & multi-axis loads:
Determine the weakest link, or where the highest (relative)loads exist
Percent of population with sufficient joint
strength
Moment demands imposed by task (M)
Mean population strength (S)
Variability of population strength (V)
Assuming a normal distribution
z = (M - S) / V
z = (X - )/
from stats table z => %-ile
z-score reflects cumulative probability P(X Xi);normalized based on mean and std. dev.
From table of cumulative probabilities of the normaldistribution (z-table), the percentile corresponding to thez-score can be found
= 40 Nm; = 15 Nm (from strength table)
z = (15.4 - 40)/15 = -1.64 (std dev below the mean)
From table, the area A corresponding to z = -1.64 is 0.95
Thus, 95% of the population has strength 15.4 Nm
If ME = 15.4 Nm, what % of the population has sufficientstrength to perform the task (at least for a short time)?
Note:Both M and S vary considerably with posture and task conditions
Quick and Easy Evaluation of Lifting Tasks
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Start End
Factors affecting workload?
A load constant is the maximum recommendedweight for lifting at the standard lift location underideal conditions.
LOAD CONSTANT = 23 kg (51 lbs)
Decrease the load constant to account for theinfluence of known risk factors using 6 multipliers: vertical location (VM) vertical travel distance (DM) asymmetry (AM) frequency (FM) coupling (CM)
All Multipliers are 1
Recommended Weight Limit (RWL) =
23kg HM VM DM AM FM CM
HD
VO
VD
HO
HM = (25/H)
H = horizontaldistance (in cm)of the handsfrom themidpoint
HD
ankles.
Measure at theorigin anddestination oflift.
extreme cases HM accounts for
Low-Back Loads
HO
Example?
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0.6
0.8
1
1.2
Multiplier
20 30 40 50 60 70 800
0.2
.
Horizontal Distance (cm)
If H 25, HM = 1
Relatively big, non-linear effect
25
VM = (1-(0.003|V-75|))
V = vertical distance (incm) of the hands from thefloor. Measure at theorigin and destination of
.
VM accounts foracceptability and capacity(strength) as a function ofposture
VO
VD
Example?
0.8
1
1.2
Multiplier
torso flexion overhead reach
0 20 40 60 80 100 120 140 160 1800
0.2
0.4
.
Vertical Distance (cm)
Moderate, non-linear effect
DM = (0.82 +(4.5/D))
D = vertical travel distance(in cm) between the originand destination of the lift.
D = |VD-VO|
accoun s or me a o cdemand, task dynamics, butnot lift vs. lower
D
Example?
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0.6
0.8
1
1.2
Multiplier
0 20 40 60 80 100 120 140 160 1800
0.2
.
Distance Moved (cm)
Relatively small, non-linear effect
AM = (1-(0.0032|A|))
A = angle (deg) ofasymmetry angulardisplacement of theload from the
A
sagittal plane.Measure at theorigin anddestination of lift.
AM accounts fordecreased strengthand increased spineloads in asymmetricpostures.
sagittalplane
Example?
0.6
0.8
1
1.2
Multiplier
0 20 40 60 80 100 120 140 160 1800
0.2
0.4
Asymmetry Angle (deg)
Moderate, linear effect
V
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V 3 likely have increased risk
Some believe that if workers are properly screened(based on the task requirements) and trained, thatthey can safely work at lift indexes greater than 1but less than 3.
What are ideal lifting conditions?? Maximize RWL (keep load close to the body, )
Manual work activities other than lifting are assumed to beminimal
The equation does not account for unpredictable situationssuch as shifting loads
A favorable ambient environment is assumed (19- 26 C or66 - 79 F)
Risk of slips not accounted for (good floor surface assumed)
Lifting and lowering tasks are assumed to pose the same riskof injury
Tasks involving one-handed lifts, lifting while seated orkneeling, or lifting in a constrained work area are not
appropriate for this model Does not account for individual anthropometric differences
Start End
H = 13.0 cm
V = 13.5 cmA = 0 deg
H = 41.5 cm
V = 89.0 cmA = 0 deg
D = 75.5 cm; F = 1/min; Couplings = Fair
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HMStart = (25/13) = 1HMEnd = (25/41.5) = 0.60
VMS = (1-(0.003|13.5-75|) = 0.82VME = (1-(0.003|89-75|) = 0.96
DM = (0.82+(4.5/75.5)) = 0.88
AMS = AME = (1-(0.0032)(0)) = 1
CMS = [Fair, V