1. BIOMECHANICS OF WORK
Chapter 11 in your text

2. The Musculoskeletal System
Bones, muscle and connective tissue
supports and protects body parts
maintains posture
allows movement
generates heat and maintains body temperature

3. 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

4. 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

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

6. Muscles

7. 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)

8. Muscle Contraction
Concentric: (also called isotonic) muscle contracts and shortens
Eccentric: muscle contracts and lengthens (overload)
Isometric: muscle contracts and stays the same length

9. 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

10. Basic Biomechanics
Statics model (åF=0, å 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
åF can be calculated

11. Problem in Text
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!
20kg

12. 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
98 N

13. Problem in Text åF=0 1. Convert mass to Force Felbow
20kg*9.8 m/s2 = 196 N
2. Divide by # of hands.
196N/2 hands = 98N/hand
3. Calculate F elbow.
åF=0
Felbow – 16N – 98N = 0
Felbow= 114N [up]
Felbow
16 N
98 N

14. Problem in Text Felbow 16 N 98 N 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.
åF=0
Felbow – 16N – 98N = 0
Felbow= 114N [up]
4. Calculate M elbow.
åM=0
Melbow-16N*.18m +(-98N)*.36m=0
Melbow=38.16N*m
Felbow
.36m
.18m
16 N
98 N

15. 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?

16. 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

17. 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

18. 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
Weight
Injuries rare
Injuries inevitable AL
MPL

19. 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 kcal.min

20. 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)

21. 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

22. Modifiers (diagrammatically)HM VM
Originating height DM AM FM CM

23. 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?

24. 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.

25. Example in the Text FM
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.
Weight
duration CM VM HM DM AM

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

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

28. 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

29. 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