BIOMECHANICS OF WORK

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 bodys 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 (åF0, å Moments0), 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 20kg9.8 m/s2 196 N 2.
Divide by of hands. 196N/2 hands 98N/hand
98 N
13
Problem in Text
1. Convert mass to Force 20kg9.8 m/s2 196 N 2.
Divide by of hands. 196N/2 hands 98N/hand 3.
Calculate F elbow. åF0 Felbow 16N 98N
0 Felbow 114N up
Felbow
16 N
98 N
14
Problem in Text
1. Convert mass to Force 20kg9.8 m/s2 196 N 2.
Divide by of hands. 196N/2 hands 98N/hand 3.
Calculate F elbow. åF0 Felbow 16N 98N
0 Felbow 114N up 4. Calculate M
elbow. åM0 Melbow-16N.18m (-98N).36m0 Melbow
38.16Nm
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 workers 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 inevitable
Injuries rare
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
  2.2-4.7kcal.min

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Lifting Equation

• RWLLCHMVMDMAMFMCM
• General form
• RWL max possible load modifiers
• Modifiers reduce the RWL so that
• RWLltLC
• (all modifiers lt1)

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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)
VM
HM
Originating height
DM
AM
CM
FM
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

• h16
• v44
• d18
• A80degrees
• F3 lifts/minute
• Cgood
• job duration 8 hours/day
• weight 15lbs

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Multipliers

• HM (T11.1) 10/h10/16.625
• VM (T11.1)(1-.0075v-30).895
• DM (T11.1) (0.821.8/d)0.821/8/18.92
• AM (T11.1) 1-.00032a1-.00032x80.744
• FM(T11.2) 0.55 (vlt75, work 8hrs, 3lifts)
• CM (T11.3) 1 (good, vlt75cm)

28
Calculation of RWL

• RWLLCxHMxVMxDMxAMxFMxCM
• RWL51lbx.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 gt1 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