11. Occupational Biomechanics

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11. Occupational Biomechanics Physiology
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Biomechanics

• Biomechanics uses the laws of physics and
  engineering mechanics to describe the motions of
  various body segments (kinematics) and understand
  the effects of forces and moments acting on the
  body (kinetics).
• Application
• Ergonomics
• Orthopedics
• Sports science

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Occupational Biomechanics

• Occupational Biomechanics is a sub-discipline
  within the general field of biomechanics which
  studies the physical interaction of workers with
  their tools, machines and materials so as to
  enhance the workers performance while minimizing
  the risk of musculoskeletal injury.
• Motivation
• About 1/3 of U.S. workers perform tasks that
  require high strength demands
• Costs due to overexertion injuries - LIFTING
• Large variations in population strength
• Basis for understanding and preventing
  overexertion injuries

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Problems (example)
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Free-Body Diagrams

• Free-body diagrams are schematic representations
  of a system identifying all forces and all
  moments acting on the components of the system.

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2-D Model of the Elbow
Unknown Elbow force and moment
17.0 cm
10 N
35.0 cm
180 N
From Chaffin, DB and Andersson, GBJ (1991)
Occupational Biomechanics. Fig 6.2
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2-D Model of the Elbow
From Chaffin, DB and Andersson, GBJ (1991)
Occupational Biomechanics. Fig 6.7
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Biomechanics Example
HAND
COM
ELBOW

• Unknown values
• Biceps and external elbow force (FB and FE), and
  any joint contact force between upper and lower
  arms (FJT)
• External elbow moment (ME)
• Lower arm selected as free body

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General Approach

• 1. Establish coordinate system (sign convention)
• 2. Draw Free Body Diagram, including known and
  unknown forces/moments
• 3. Solve for external moment(s) at joint
• 4. Determine net internal moment(s), and solve
  for unknown internal force(s)
• 5. Solve for external force(s) at joint can also
  be done earlier
• 6. Determine net internal force(s), and solve for
  remaining unknown internal force(s)

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Example Solution
_
_
SME 0 ME ME -gt ME -ME ME MLA MH
(WLA x maLA) (FH x maH) ME (-10 x 0.17)
(-180 x 0.35) -1.7 - 63 ME -64.7 Nm (or 64.4
Nm CW) ME -ME -gt ME 64.7 ME (FJT x maJT)
(FB x maB) FB x 0.05 FB 1294 N (up)
External moment is due to external forces
_
_
_
Internal moment is due to internal forces
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Example 1 Solution
_
_
SFE 0 FE FE -gt FE -FE FE WLA FH
-10 (-180) FE -190 N (or 190 N down) FE -
FE -gt FE 190 FE FJT FB FJT 190 - 1294
-1104 N (down)
_
_
_

• Thus, an 18 kg mass (40) requires 1300N (290)
  of muscle force and causes 1100N (250) of joint
  contact force.

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Assumptions Made in 2-D Static Analysis

• Joints are frictionless
• No motion
• No out-of-plane forces (Flatland)
• Known anthropometry (segment sizes and weights)
• Known forces and directions
• Known postures
• 1 muscle
• Known muscle geometry
• No muscle antagonism (e.g. triceps)
• Others

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3-D Biomechanical Models

• These models are difficult to build due to the
  increased complexity of calculations and
  difficulties posed by muscle geometry and
  indeterminacy.
• Additional problems introduced by indeterminacy
  there are fewer equations (of equilibrium) than
  unknowns (muscle forces)
• While 3-D models are difficult to construct and
  validate, 3-D components of lifting, especially
  lateral bending, appear to significantly increase
  risk of injury.

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From Biomechanics to Task Evaluation

• Biomechanical analysis yields external moments at
  selected joints
• Compare external moments with joint strength
  (maximum internal moment)
• Typically use static data, since dynamic strength
  data are limited
• Use appropriate strength data (i.e. same posture)
• Two Options
• Compare moments with an individuals joint
  strength
• Compare moments with population distributions to
  obtain percentiles (more common)

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Example use of z-score

• If ME 15.4 Nm, what of the population has
  sufficient strength to perform the task (at least
  for a short time)?

m 40 Nm s 15 Nm (from strength table) z
(15.4 - 40)/15 -1.64 (std dev below the
mean) From table, the area A corresponding to z
-1.64 is 0.95 Thus, 95 of the population has
strength 15.4 Nm

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Task Evaluation and Ergonomic Controls

• Demand (moments) lt Capacity (strength)
• Are the demands excessive?
• Is the percentage capable too small?
• What is an appropriate percentage? 95 or 99
  capable commonly used
• Strategies to Improve the Task
• Decrease D
• Forces masses, accelerations (increase or
  decrease, depending on the specific task)
• Moment arms distances, postures, work layout
• Increase C
• Design task to avoid loading of relatively weak
  joints
• Maximize joint strength (typically in middle of
  ROM)
• Use only strong workers

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UM 2-D Static Strength Model
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Work Physiology
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Aerobic vs. Anaerobic Metabolism

• Aerobic
• Use of O2, efficient, high capacity
• Anaerobic
• No O2, inefficient, low capacity
• Aerobic used during normal work (exercise)
  levels, anaerobic added during extreme demands
• Anaerobic metabolism -gt lactic acid (pain,
  cramps, tremors)
• D lt C (energy demands lt energy generation
  capacity)

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Oxygen Consumption and Exercise
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Oxygen Uptake and Energy Production
Respiratory
Circulatory
Atmosphere
Muscle
System
System
Blood
Capillary
Oxygen

Tidal Volume
System
Available
Heart Rate
Respiratory
Rate
Stroke
Volume
Energy Production (E)
Oxygen Uptake (VO2)
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Changes with Endurance Training

• Low force, high repetition training
• increased SVmax gt increased COmax
• incr. efficiency of gas exchange in lungs (more
  O2)
• incr. in O2 carrying molecule (hemoglobin)
• increase in capillaries in muscle

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Problems with Excessive Work Load

• Elevated HR
• cannot maintain energy equilibrium
• insufficient blood supply to heart may increase
  risk of heart attack in at-risk individuals
• Elevated Respiratory Rate
• chest pain in at-risk individuals
• loss of fine control
• General and Localized Muscle Fatigue
• insufficient oxygen -gt anaerobic metabolism -gt
  lactic acid -gt pain, cramping
• A fatigued worker is less satisfied, less
  productive, less efficient, and more prone to
  errors

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Evaluating Task Demands

• Task demands can be evaluated the same way that
  maximum aerobic capacity is evaluated by direct
  measurement of the oxygen uptake of a person
  performing the task.
• Indirect methods for estimating task demands
• Tabular Values
• Subjective Evaluation
• Estimate from HR
• Job Task Analysis

More Complex More Accurate