biomechanics of work

BIOMECHANICS OF WORK

Chapter 11 in your text

The Musculoskeletal System

Bones, muscle and connective tissue supports and protects body parts maintains posture allows movement generates heat and maintains body

temperature

Bones

206 bones Body “framework” Protective: rib cage and skull Provide for action: arms, legs linked at joints by tendons and ligaments Tendons: connect bone to muscle Ligaments: connect bone to bone

Joints

Connection of two or more bones Movement

no mobility joints (e.g. in skull) hinge joints (elbow) pivot joints (wrist) ball and socket joints (hip and shoulder) 3DOF

Muscles

400 muscles 40-50% of your body weight half of your body’s energy needs

Muscles

Muscle Composition

bundles of muscle fibres, connective tissue and nerves

fibres are made of long cylindrical cells cells contain contractile elements (myofibrils) both sensory and motor nerves motor nerves control contractions of groups of

fibres (motor unit)

Muscle Contraction

Concentric: (also called isotonic) muscle contracts and shortens

Eccentric: muscle contracts and lengthens (overload)

Isometric: muscle contracts and stays the same length

Muscle Strength

proportional to muscle cross-section usually measured as torque

force applied against a moment arm (bone) to an axis of rotation (joint)

Static strength: measured during isometric contraction

Dynamic strength: measured during movement

Basic Biomechanics

Statics model (F=0, Moments=0), isometric contraction

Force at the point of application of the load Weight of the limb is also a force at the

center of gravity of the limb F can be calculated

Problem in Text

20kg

Person holding a 20kg weight in both hands. What are the force and moment at the elbow?

Given:Mass =20kg

Force of segment = 16N

Length of segment = .36m

Assume:

COG of segment is at the midpoint!

Problem in Text

1. Convert mass to Force

20kg*9.8 m/s2 = 196 N

2. Divide by # of hands.

196N/2 hands = 98N/hand98 N

Problem in Text

1. Convert mass to Force

20kg*9.8 m/s2 = 196 N

2. Divide by # of hands.

196N/2 hands = 98N/hand

3. Calculate F elbow.

F=0

Felbow – 16N – 98N = 0

Felbow= 114N [up]

98 N

16 N

Felbow

Problem in Text1. Convert mass to Force

20kg*9.8 m/s2 = 196 N

2. Divide by # of hands.

196N/2 hands = 98N/hand

3. Calculate F elbow.

F=0

Felbow – 16N – 98N = 0

Felbow= 114N [up]

4. Calculate M elbow.

elbowm +(-98N)*.36m=0

elbow=38.16N*m

98 N

16 N

Felbow

.18m.36m

Multi-segment models

Repeat for each segment, working the forces and moments back

How would you work out the Force and Moment in the shoulder?

What information would you need?

Lower Back Pain

estimated at 1/3 of worker’s compensation payments

may affect 50-70% of the population in general

Both in high lifting jobs and jobs with prolonged sitting

Biomechanics of Lower Back Pain

Calculation in text 300N load to 5458N back compressive force

Back must support many times the lifted load, largely due to the moment arms involved

Calculation of compressive forces vs. muscle strength can identify problems

NIOSH Lifting Guide

Sets numbers that are associated with risk of back injury

Two limits (for simple lifts) Action limit (AL): small proportion of the

population may experience increased risk of injury

Maximum permissible limit (MPL): Most people would experience a high risk of injury. 3xAL

ALInjuries rare Injuries inevitable

MPL

Weight

NIOSH Lifting Guide

Recommended Weight Limit (RWL): a load value that most healthy people could lift for a substantial period of time without an increased risk of low back pain

Covers more complex lifts Biomechanical criteria 3.4kN at L5/S1 Epidemiological criteria show damage at 4.4kN Physiological criteria to set repetition rate at 2.2-

4.7kcal.min

Lifting Equation

RWL=LC*HM*VM*DM*AM*FM*CM

General form RWL = max possible load * modifiers

Modifiers reduce the RWL so that RWL<=LC (all modifiers <1)

The Modifiers

LC: load constant, maximum recommended weight for a simple lift HM: horizontal multiplier, decreases weight with distance from spine VM: vertical multiplier, lifting from near floor harder DM: distance multiplier, accommodates for vertical distance that must

be lifted AM: assymetric multiplier, reductions for torso twisting CM: coupling modifier, depends on whether loads have handles for

lifting FM: frequency modifier, how frequently is the load lifted

Modifiers (diagrammatically)

HM VMDMOriginating height

AM CMFM

Lifting Equation

Multipliers can all be obtained from tables (11.1, 11.2, 11.3)

Multipliers are unitless Multipliers are always less than or equal to 1

…why?

Example in the Text

A worker must move boxes from 1 conveyor to another at a rate of 3 boxes/minute. Each box weighs 15lbs and the worker works for 8 hours a day. The box can be grasped quite comfortably. The horizontal distance is 16 inches, the vertical is 44 inches to start and 62 inches to finish. The worker must twist at the torso 80 degrees.

Example in the Text

A worker must move boxes from 1 conveyor to another at a rate of 3 boxes/minute. Each box weighs 15lbs and the worker works for 8 hours a day. The box can be grasped quite comfortably. The horizontal distance is 16 inches, the vertical is 44 inches to start and 62 inches to finish. The worker must twist at the torso 80 degrees.

FMWeight

CM

duration

HMVM

DM

AM

Information

h=16” v=44” d=18” A=80degrees F=3 lifts/minute C=good job duration = 8 hours/day weight = 15lbs

Multipliers

HM (T11.1): 10/h=10/16=.625 VM (T11.1):(1-.0075|v-30|)=.895 DM (T11.1): (0.82+1.8/d)=0.82+1/8/18=.92 AM (T11.1): 1-.00032a=1-.00032x80=.744 FM(T11.2): 0.55 (v<75, work 8hrs, 3lifts) CM (T11.3): 1 (good, v<75cm)

Calculation of RWL

RWL=LCxHMxVMxDMxAMxFMxCM RWL=51lbx.625x.895x.92x.744x.55x1 RWL= 10.74lbs The load is greater than the RWL so there is a risk

of back injury Lifting Index = RWL/Load IF >1 then the load is too high LI= 10.74/15 = 1.4

Designing to avoid back pain

More importantly, NIOSH equation gives ways to reduce injury reduce horizontal distance keep load at waist height reduce distance to be travelled reduce twisting add handles reduce frequency of lifts