Occupational

1
Chapter 6

• Occupational
• Biomechanics
• Models

2
Why Model?

• Models
• simple representation of complex systems
• improve understanding
• even with gross simplifications and assumptions
• rigid links vs complex anatomy of segment
• SEM vs multiple muscles of the group
• If error in the model is too large
• improve our model parameters
• TBM in segment, CofM location
• Increase complexity
• individual muscles

3
Why Model?

• Biomechanical model allows to predict potentially
  hazardous loading conditions on NMS components
  without actual subject risk
• manipulate parameters (loading, geometry)
• Measure response
• compare model prediction to real world
• refine the model (limit interpretation)

4
Why Model?

• Biomechanical model allows to predict potentially
  hazardous loading conditions on NMS components
  without actual subject risk
• Provide understanding of Guidelines to improve
  efficiency and safety in the workplace

5
Our application

• estimate forces acting on different components of
  the body.
• F gt stress gt injury
• understanding risk
• compare force (stress) predicted from model to
  force (stress) known to cause fatigue and/or
  injury
• Identify dangerous situations and success of
  interventions.

6
Planar Static Biomechanical Models

• Single-body segment static model -
• used when no movement no linear or angular
  acceleration is present.
• standing with a load in the hands.
• moving at constant velocity (isokinetic)
• Planar analysis is limited to a 2D analysis.

7
Planar Static Biomechanical Models

• KEY identify the magnitude of the external
  forces acting on the stationary mass
• Always gt gravity
• W mg
• where
• weight measured in Newtons
• mass measured in kg
• g is the gravitational acceleration (-9.8 m.s-2)

8
Forces is a vector quantity, and has 4
characteristics

• 1. Magnitude.
• 2. Direction (-)
• 3. Line of action.
• 4. Point of application.

Vector
9
Planar Static Biomechanical Models

• Consider 10kg load in one-handed lift
• Since a0, ? forces on the body ???
• Some other external force must be acting to
  counter the weight
• Obviously 2nd force is provided by the hand
  pulling up the load

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Planar Static Biomechanical Models

• Since the object is not moving, it is defined to
  be in static equilibrium.
• This means that the additive effect of all
  external forces acting on a mass is zero

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Planar Static Biomechanical Models

• Since the object is not moving, it is defined to
  be in static equilibrium.
• This means that the additive effect of all
  external forces acting on a mass is zero

? F 0
dir
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Planar Static Biomechanical Models

• Since the object is not moving, it is defined to
  be in static equilibrium.
• This means that the additive effect of all
  external torques acting on a mass is zeroa

13
Planar Static Biomechanical Models

• Since the object is not moving, it is defined to
  be in static equilibrium.
• This means that the additive effect of all
  external torques acting on a mass is zero

14
Free body diagrams

• Force vectors are scaled in the drawing to
  indicate magnitude
• Vectors are orientated in the direction of the
  force (tip)
• Vectors are aligned on the body to indicate point
  of application and line of action.

15
Solve for force in EACH hand
?F 0
vert
Identify the forces to be summed
Weight 2 hands 0 2 hands - Weight Each hand
- Weight / 2 Each hand - (-98) / 2 Each hand
49 N
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Planar Static Biomechanical Models

• Determine force on each hand to hold a 10-kg
  mass in static equilibrium
• Extend the planar static analysis to estimate the
  elbow forces and moments (torques) with forearm
  horizontal
• Assumptions
• Average Anthro Inertia
• Load applied at CofM of hand
• Forearm/hand is a single segment

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Calculate Force at Elbow 1st condition of
equilibrium
W load W fh R elbow 0 R elbow -W load -
W fh R elbow - (-49) - (-15.8) R elbow
64.8 N
18
Joint Reaction Force

• R elbow 64.8 N
• The NET tensile force created by ligaments and
  muscles holding the joint together.
• MUST be present to give a 0 (no translational
  acceleration)

19
Forces acting on the system will cause torque

• If eccentric to an axis
• W fh W load
• Not if centric to an axis
• R elbow

20
Calculate torque at elbow 2nd condition of
equilibrium
T load T fh T elbow 0 T elbow - (T
load) - (T fh) T elbow - (-49 x .355) - (
-15.8 x .172) T elbow - (-17.4) - (-2.7)
20.1 Nm (CCW, flexion)
21
Net Joint Torque (net moment of force)

• T elbow 20.1 Nm (CCW, flexion)
• Present at EACH elbow
• The NET torque created at the elbow joint by
  muscles.
• Which muscles?
• Ignores co-contraction
• MUST be present to give a 0
• (no angular acceleration)

22
On own

• Calculate Rf at elbow and torque at elbow for the
  segment held at the horizontal without the hand
  held load.

23
Arm not at horizontal (20º below)

• Reaction force at elbow is the same because ...
• Muscle torque at elbow is decreased because .....

Trig? SOH CAH TOA
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Two-body segment static model

• Start at segment with known external force (or
  only one unknown force)
• FH W fh, R elbow, T fh, T elbow
• Horizontal position

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Two-body segment static model

• Start at segment with known external force (or
  only one unknown force)
• FH W fh, R elbow, T fh, T elbow
• Horizontal position
• Non-horizontal position
• Note elbow load from task posture

27
Extend model to nonparallel forces

• Preceding egs. have considered gravity as the
  only source of external forces (parallel force
  systems)
• What if person is pushing or pulling on a load???
  
• Resolve force to orthogonal components
• horizontal
• vertical

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Planar static analysis of internal forces

• Extend the model technique to estimate the force
  on various musculoskeletal tissues
• tension within a muscle (SEM) that creates the
  observed moment of force
• bone on bone force (not just JRF) that accounts
  for the tension in the muscle

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Planar static analysis of internal forces

• Needed the point of application and the line of
  action of muscle(s) tendons within the
  musculoskeletal structure
• Our simplification
• concept of Single Equivalent Muscle (SEM)
• only Biceps Brachii acting at the elbow joint,
  inserting 0.05 m from the axis

30
Solve for Biceps Muscle Force
Earlier T load T fh T elbow
0 Becomes T load T fh T biceps
0 Isolate T biceps - (T load) - (T fh) Expand
to F bi x MA bi - (T load) - (T fh) F bi
(- (T load) - (T fh)) / MA bi Substitute F bi
(- (-49 x .355) - ( -15.8 x .172)) /
0.05 Solve F bi 20.1 Nm / 0.05 m F bi 402 N
31
Solve for Biceps Muscle Force
Earlier T load T fh T elbow
0 Becomes T load T fh T biceps
0 Isolate T biceps - (T load) - (T fh) Expand
to F bi x MA bi - (T load) - (T fh) F bi
(- (T load) - (T fh)) / MA bi Substitute F bi
(- (-49 x .355) - ( -15.8 x .172)) /
0.05 Solve F bi 20.1 Nm / 0.05 m F bi 402 N
gt 8x gt HHW
32
Solve for Joint Reaction Force
Earlier W load W fh R elbow 0 Becomes W
load W fh F bi R elbow 0 R elbow -W
load - W fh - F bi R elbow - (-49) - (-15.8) -
(402) R elbow 64.8 - 402 N R elbow -
337.2 N
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Joint Reaction Force

• Previous R elbow 64.8 N (no Biceps)
• Now R elbow - 337.2 N
• The NET compressive force pushing DOWN on the
  forearm from the humerus (created by muscle
  squeezing the joint together)
• Minimum MUST be present to give a 0 (no
  translational acceleration)
• ignores potential co-contraction

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Planar static analysis of internal forces

• Technique more complicated if
• consider gt 1 muscle

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Planar static analysis of internal forces

• Technique more complicated if
• consider gt 1 muscle
• Overhead Scott Winter, MSSE, 1991
• determine each muscle contribution
• move through the ROM (L/T)

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Models used to calculate Forces
38
Results
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Planar static analysis of internal forces

• Compare relative lengths of MAs
• Hand held load gt muscle
• To generate equal but opposite torque, force in
  muscle must be greater
• mechanical disadvantage

40
Multiple-link coplanar static modeling

• Posture has no effect on calculated JRF, but has
  a very large effect on calculated JMF
• note JRF constant at 549 N while moment increase
  is approximately 10x
• Note has not considered the increase in JRF from
  muscle tension to provide the moment (very
  complex musculature)
• What muscle group active in a), b), c)??
• What happens to alignment of vertebrae?
• What happens if load added to hands?
• What happens if arm/ab used for support?

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Qualitative Low Back Load
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Importance

• Since skeletal muscle responds to the load
  moments (creates the calculated net JMF)...
• simple static models give insight into what
  postures require
• specific muscle groups to be active
• to what relative magnitude each specific muscle
  group must be active

43
Dynamic Biomechanical ModelsHPR 482 Advanced
Biomechanics

• The introduction of motion into biomechanical
  models introduces two types of complexity
• kinematics must be quantified
• position, velocity, acceleration
• linear and angular
• must account for inertial force and inertial
  torque in calculations
• F ma when a ltgt 0
• T I (alpha) when (alpha) ltgt 0

44
Dynamic biomechanical model

• Analysis indicates the increased hazard of
  performing dynamic movements.
• greater force for linear acceleration
• speed up or slow down
• greater torque for angular acceleration
• speed up or slow down
• Musculoskeletal load increases as speed of
  movement increases
• greater accelerations
• Add additional mass ???
• Additional segments????

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Summary of dynamic biomechanical models

• Prudent to encourage workers to develop smooth
  movements that reduce accelerations and
  decelerations, especially if heavy loads are
  being manipulated.

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Coplanar Biomechanical Models of Foot Slip
Potential While Pushing a Cart.

• Common activity in workplace
• mailroom, scrap, TVs, luggage
• Load may approach max strength
• Common time to slip
• What causes slipping???

48
Coplanar Biomechanical Models of Foot Slip
Potential While Pushing a Cart.

• Common activity in workplace
• mailroom, scrap
• Load may approach max strength
• Common time to slip
• What causes slipping???
• Low Friction between sole and surface
• What prevents slipping???

49
Coplanar Biomechanical Models of Foot Slip
Potential While Pushing a Cart.

• Common activity in workplace
• mailroom, scrap
• Load may approach max strength
• Common time to slip
• What causes slipping???
• Low Friction between sole and surface
• What prevents slipping???
• Adequate friction between sole and surface

50
Coplanar Biomechanical Models of Foot Slip
Potential While Pushing a Cart.

• What is friction??
• Force that tends to resist slipping
• Reflects nature of TWO surfaces

Friction u N
mu
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Coefficient of Friction
Max Limiting Friction
u
Normal Reaction Force

• property of two materials placed in contact.
• 0.2 smooth wet 0.9 rough dry

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Coefficient of Friction
Peak Shear Force
Max Limiting Friction
u
Normal Reaction Force
Peak Normal Force
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Coplanar Biomechanical Models of Foot Slip
Potential While Pushing a Cart.

• To prevent slipping
• What are the peak normal and shearing forces
  expected at the foot/floor contact point?
• Design footwear and floor surfaces that will
  provide friction under these circumstances.
• ie provide greater coefficient of friction

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Materials in Contact
from http//www.fearofphysics.com/Friction/frintr
o.html
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Typical Coefficients
Sticky
Slippery
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Extent of problem of falls
Pedestrian-fall accidents have been the second
largest generator of unintentional workplace
fatalities, accounting for nearly 11 and 20,
respectively, of all fatal and non-fatal
occupational injuries in the USA. Redfern et al
(2001). Biomechanics of slips. Ergonomics,
44131138-1166.
58
Workplace falls
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Friction Manipulation
Occupational Health Safety E-News
03-17-03   Our Safety Tip of the Week is courtesy
of Manuel (Mel) Rosas, a safety consultant for
Carolinas Associated General Contractors. "I
consistently find employers do not have
procedures in place to inspect the soles of the
shoes their employees wear to work. During
walk-around inspections I ask employees to lift
up and show me the bottom of the work shoe
(boot), and I find many with worn or nearly slick
soles. Employers should address this issue to
reduce the risk of injury due to a worn shoe or
boot."
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Friction Manipulation
floor design
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How does employee alter pushing action as the
load gets heavier? What does this do to
friction requirements?
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Special Purpose Biomechanical Models of
Occupational Tasks

• Model specific areas that are prone to overuse
  and/or traumatic injury
• low back
• wrist/hand
• knee
• shoulder

63
Low-back biomechanical models

• NIOSH suggests using the load moment about the
  lumbosacral disc (L5/S1) as the basis for limits
  when
• lifting
• carrying loads
• Why L5/S1
• 85 to 95 of all disc herniations
• loads in hands have the largest moment arms
  relative to this axis

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Earlier, showed

• Large increase in Net moment at low back with
  lifting
• What will happen with load in hands?
• What about anatomical moment??
• Muscle force erector spinae
• Moment arm 0.05 m
• Abdominal pressure
• pushes torso into extension
• Role of abdominal muscles??

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Resultant effect on vertebral column

• HUGH compressive forces
• some from the load itself (posture)
• greatest from the muscle force
• STRESS on vertebral disk Force / area
• effect of posture
• increase force
• decrease area

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Low-back biomechanical models

• Initial calcs used simple back model
• Cadavers compression forces created
  micro-fractures on intervertebral disks
• weak spot for potential herniation?
• Recent models incorporate
• more muscle groups
• corrected moment arms
• relative contribution of each muscle group
• effect on compressive force?????

67
Low-back biomechanical models

• NIOSH (1981)
• recommended that predicted L5/S1 compression
  values
• above 3400 N be considered potentially hazardous
  for some workers.
• above 6400 N be considered hazardous for most
  workers.
• Basis repeated, large compression force may
  increase risk of disc degeneration chronic
  low-back symptoms.
• NIOSH (1993) to be discussed

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Summary

• Models vary in complexity
• All based on Newton II
• Require adeptness with Tables
• Require logical thinking
• CM locations, Moment Arm length
• Provide insight to joint muscle loading
• Underly postural load guidelines