1. 11. Occupational Biomechanics & Physiology

2. 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

3. 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

4. Problems (example)

5. 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.

6. 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

7. 2-D Model of the Elbow
From Chaffin, DB and Andersson, GBJ (1991) Occupational Biomechanics. Fig 6.7

8. Biomechanics Example Unknown values: Lower arm selected as free body
ELBOW
COM
HAND
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

9. 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)

10. Example : Solution SME = 0 = ME + ME -> ME = -ME_ _
SME = 0 = ME + ME -> ME = -ME
ME = MLA + MH = (WLA x maLA) + (FH x maH)
ME = (-10 x 0.17) + (-180 x 0.35) =
ME = Nm (or 64.4 Nm CW)
ME = -ME -> 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

11. Example 1: Solution SFE = 0 = FE + FE -> FE = -FE_ _
SFE = 0 = FE + FE -> FE = -FE
FE = WLA + FH = (-180)
FE = -190 N (or 190 N down)
FE = - FE -> FE = 190
FE = FJT + FB
FJT = = N (down) _ _ _
Thus, an 18 kg mass (~40#) requires 1300N (~290#) of muscle force and causes 1100N (250#) of joint contact force.

12. 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

13. 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.

14. 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)

15. 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 = (std dev below the mean)
From table, the area A corresponding to z = is 0.95
Thus, 95% of the population has strength ≥ 15.4 Nm

16. Task Evaluation and Ergonomic Controls
Demand (moments) < 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

17. UM 2-D Static Strength Model

18. Work Physiology

19. Aerobic vs. Anaerobic Metabolism
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 -> lactic acid (pain, cramps, tremors)
D < C (energy demands < energy generation capacity)
Metabolism is the process of releasing energy stored in chemical bonds

20. Oxygen Consumption and Exercise

21. Oxygen Uptake and Energy Production
Respiratory
Circulatory
Atmosphere
Muscle
System
System
Oxygen
Tidal Volume
Blood
Capillary
Available
System
Heart Rate
Respiratory
Rate
Stroke
Volume
Oxygen Uptake (VO2)
Energy Production (E)

22. Changes with Endurance Training
Low force, high repetition training
increased SVmax => increased COmax
incr. efficiency of gas exchange in lungs (more O2)
incr. in O2 carrying molecule (hemoglobin)
increase in #capillaries in muscle

23. 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 -> anaerobic metabolism -> lactic acid -> pain, cramping
A fatigued worker is less satisfied, less productive, less efficient, and more prone to errors

24. 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