BIOMECHANICS OF WORK

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– 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: 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 Text1. Convert mass to Force

20kg*9.8 m/s2 = 196 N

2. Divide by # of hands.

196N/2 hands = 98N/hand

98 N

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]

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

• 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 (single 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

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

• Biomechanical criteria

• Epidemiological criteria

• Physiological criteria

Lifting Equation

• RWL=LCxHMxVMxDMxAMxFMxCM

• LC: load constant, maximum recommended weight

• 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

Lifting Equation

• Multipliers can all be obtained from tables (11.1, 10.2, 10.3, 11.2, 11.3)

• Multipliers are unitless

• Multipliers are always less than or equal to 1 (they reduce the maximum load or load constant)

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.

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.

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