TI 2111 Work System Design and Ergonomics 11. Occupational Biomechanics & Physiology.
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Transcript of TI 2111 Work System Design and Ergonomics 11. Occupational Biomechanics & Physiology.
TI 2111 Work System Design and Ergonomics
11. Occupational Biomechanics & Physiology
TI 2111 Work System Design and Ergonomics
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
TI 2111 Work System Design and Ergonomics
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
TI 2111 Work System Design and Ergonomics
Problems (example)
TI 2111 Work System Design and Ergonomics
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.
TI 2111 Work System Design and Ergonomics
2-D Model of the Elbow:
From Chaffin, DB and Andersson, GBJ (1991) Occupational Biomechanics. Fig 6.2
17.0 cm
35.0 cm
180 N
10 N
Unknown Elbow force and moment
TI 2111 Work System Design and Ergonomics
2-D Model of the Elbow
From Chaffin, DB and Andersson, GBJ (1991) Occupational Biomechanics. Fig 6.7
TI 2111 Work System Design and Ergonomics
Biomechanics Example
10 N180 N
FB?
35.0 cm17.0 cm
5 cm
Free-body Diagram: 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
HANDCOMELBOW
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
+X
+Z
FBD:
E HW
LA=m
LAg
=10NF
H=m
Hg=
180N
FB=??
FJT
=??
ME=??
• ME = 0 M
E + M
E M
E = -M
E
• ME = M
LA + M
H = (W
LA x ma
LA) + (F
H x ma
H) =
• (-10 x 0.17) + (-180 x 0.35) =
• -1.7 - 63 = -64.7 Nm, or 64.7Nm (CW)
• ME = -M
E 64.7 = F
B x ma
B = F
B x 0.05
• FB = 1294N ( )
ME = 0 = ME + ME -> 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 -> 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
_
TI 2111 Work System Design and Ergonomics
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)
_ _
_ _
_
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
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
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
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)
TI 2111 Work System Design and Ergonomics
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)?
= 40 Nm; = 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 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
TI 2111 Work System Design and Ergonomics
UM 2-D Static Strength Model
TI 2111 Work System Design and Ergonomics
Work Physiology
Aerobic Metabolism
Anaerobic Metabolism
Oxygen Food
Lactic Acid WORKHEAT Carbon Dioxide
TI 2111 Work System Design and Ergonomics
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 -> lactic acid (pain, cramps, tremors)
D < C (energy demands < energy generation capacity)
TI 2111 Work System Design and Ergonomics
Oxygen Consumption and Exercise
Oxygen Uptake
or Heart Rate
Max. Aerobic Capacity
Time
Start Work End Work
Oxygen Debt Recovery
Job Demands
Oxygen Deficit
Basal Rate
steady state
TI 2111 Work System Design and Ergonomics
Oxygen Uptake and Energy Production
RespiratorySystem
CirculatorySystem
Muscle
Oxygen Available
Tidal Volume
RespiratoryRate
Blood
StrokeVolume
Heart Rate
CapillarySystem
Atmosphere
Oxygen Uptake (VO2)
Energy Production (E)
TI 2111 Work System Design and Ergonomics
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
TI 2111 Work System Design and Ergonomics
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
TI 2111 Work System Design and Ergonomics
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 ComplexMore Accurate