Transcript 11__biomechanics-physiolo gy

11. Occupational
Biomechanics & Physiology
TI 2111 Work System Design and Ergonomics
Biomechanics
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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).
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Application:
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Ergonomics
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Orthopedics
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Sports science
TI 2111 Work System Design and Ergonomics
Occupational Biomechanics
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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.
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Motivation:
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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
TI 2111 Work System Design and Ergonomics
Problems (example)
TI 2111 Work System Design and Ergonomics
Free-Body Diagrams
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Free-body diagrams are schematic representations
of a system identifying all forces and all moments
acting on the components of the system.
TI 2111 Work System Design and Ergonomics
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
TI 2111 Work System Design and Ergonomics
2-D Model of the Elbow
TI 2111
Work System
Design Fig
and6.7
Ergonomics
From Chaffin, DB and Andersson, GBJ (1991)
Occupational
Biomechanics.
Biomechanics Example
ELBOW
FB?
5 cm
17.0 cm
COM
HAND
10 N
180 N
35.0 cm
 Unknown
values:
Free-body
Diagram:
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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
TI 2111 Work System Design and Ergonomics
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)
TI 2111 Work System Design and Ergonomics
Example : Solution
+Y
FJT=??
FB=??
FBD:
E
H
WLA=mLAg
=10N
ME=??
_
External moment
is due to external
forces
+X
+Z
FH=mHg=
180N
_
=M
MEE++MM
=E-ME
• M
MEE ==00
->
MEM
=E-M
E E
• ME M
= M=LAM+ M+HM= (W
x ma LA) +) (F
x ma H) =)
LA LA x ma
H H x ma
E
LA
H = (W
LA + (F
H
•M
(-10= x(-10
0.17)
+ (-180 x 0.35) =
x 0.17) + (-180 x 0.35) = -1.7 - 63
E
• -1.7 - 63 = -64.7 Nm, or 64.7Nm (CW)
Internal moment
is due to internal
forces
_ ME = -64.7
_ Nm (or 64.4 Nm CW)
• ME = -ME  64.7 = FB x ma B = FB x 0.05 
ME =_-ME -> ME = 64.7
• FB = 1294N ()
ME = (FJT x maJT) + (FB x maB) = FB x 0.05
TI 2111 Work System Design and Ergonomics
FB = 1294 N (up)
Example 1: Solution
_
_
FE = 0 = FE + FE -> FE = -FE
FE = WLA + FH = -10 + (-180)
_ FE = -190
_ N (or 190 N down)
FE =_- FE -> FE = 190
FE = FJT + FB
FJT = 190 - 1294 = -1104 N (down)
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Thus, an 18 kg mass (~40#) requires 1300N
(~290#) of muscle force and causes 1100N
(250#) of joint contact force.
TI 2111 Work System Design and Ergonomics
Assumptions Made in 2-D Static
Analysis
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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
TI 2111 Work System Design and Ergonomics
3-D Biomechanical Models
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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.
TI 2111 Work System Design and Ergonomics
From Biomechanics to Task
Evaluation
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Biomechanical analysis yields external moments
at selected joints
Compare external moments with joint strength
(maximum internal moment)
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Typically use static data, since dynamic strength data
are limited
Use appropriate strength data (i.e. same posture)
Two Options:
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Compare moments with an individuals joint strength
Compare moments with population distributions to
obtain percentiles (more common)
TI 2111 Work System Design and Ergonomics
Example use of z-score
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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 TI 2111 Work System Design and Ergonomics
Task Evaluation and Ergonomic Controls
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Demand (moments) < Capacity (strength)
Are the demands excessive?
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Is the percentage capable too small?
What is an appropriate percentage? [95% or 99% capable
commonly used]
Strategies to Improve the Task:
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Decrease D
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Forces: masses, accelerations (increase or decrease,
depending on the specific task)
Moment arms: distances, postures, work layout
Increase C
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Design task to avoid loading of relatively weak joints
Maximize joint strength (typically in middle of ROM)
Use only strong workers
TI 2111 Work System Design and Ergonomics
UM 2-D Static Strength Model
TI 2111 Work System Design and Ergonomics
Work Physiology
Food
Oxygen
Aerobic
Anaerobic
Metabolism
Metabolism
Lactic Acid
HEAT
WORK
Carbon
Dioxide
TI 2111 Work System Design and Ergonomics
Aerobic vs. Anaerobic Metabolism
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Aerobic
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Anaerobic
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Use of O2, efficient, high capacity
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)
TI 2111 Work System Design and Ergonomics
Oxygen Consumption and Exercise
Max. Aerobic Capacity
Job Demands
steady state
Oxygen
Uptake
or
Heart Rate
Oxygen Debt
Recovery
Oxygen Deficit
Basal
Rate
End Work
Start Work
Time
TI 2111 Work System Design and Ergonomics
Oxygen Uptake and Energy Production
Atmosphere
Oxygen
Available
Respiratory
System
Tidal Volume
Respiratory
Rate
Oxygen
Uptake (VO2)
Circulatory
System
Blood
Muscle
Capillary
System
Heart Rate
Stroke
Volume
Energy
Production (E)
TI 2111 Work System Design and Ergonomics
Changes with Endurance Training
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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
TI 2111 Work System Design and Ergonomics
Problems with Excessive Work Load
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Elevated HR
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Elevated Respiratory Rate
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cannot maintain energy equilibrium
insufficient blood supply to heart may increase risk of heart
attack in at-risk individuals
chest pain in at-risk individuals
loss of fine control
General and Localized Muscle Fatigue
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insufficient oxygen -> anaerobic metabolism -> lactic acid ->
pain, cramping
A fatigued worker is less satisfied, less productive, less
efficient, and more prone to errors
TI 2111 Work System Design and Ergonomics
Evaluating Task Demands:
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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:
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Tabular Values
Subjective Evaluation
Estimate from HR
Job Task Analysis
More Complex
More Accurate
TI 2111 Work System Design and Ergonomics