IE 327
Introduction to Work Design
Dr. Andris Freivalds
Class #4
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Ch. 4 – MUSCULOSKELETAL
SYSTEM (and Work Design)
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MUSCLE STRUCTURE
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Muscle and tendon
Bundle of fibers
Blood vessels
Muscle fiber
Myofibril
– Thick filament = myosin
– Thin filament = actin
• Sliding filament theory
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FORCE-LENGTH RELATIONSHIP
(Max strength at midrange)
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Achieve max muscle strength at
midrange of motion
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Where is midrange?
Relaxed posture
No muscle force
Weightless astronaut
in space
• Avoid extremes!!
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FORCE-VELOCITY RELATIONSHIP
• Max muscle force
with slow motion
• Use momentum to
assist work
• Minimize fighting
gravity
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OPTIMIZE HUMAN STRENGTH
• Use optimum
postures
• Use large muscles
for power
• Avoid using back
• Design tasks so
that most can do!
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MUSCLE ENDURANCE
• Static hold
• Occlusion of
blood flow
• Rapid fatigue
• Stay < 15% max
• Short, frequent
work-rest cycles
• Dynamic work
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MOTOR UNIT RECRUITMENT
• Use small muscles for precision
• Use large muscles for power
• Power vs. pinch grip!!
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CONTROL OF MOVEMENTS #1
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Pivot on elbow
Curved motions
Use both hands
Begin and end motion
simultaneously
• Move simultaneously
to and from body
• Use natural rhythms
– Whole body = 17/min
– Whole arm = 35/min
– Forearm = 60/min
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CONTROL OF MOVEMENTS #2
• Use lowest
classification of
movement
• Decrease difficulty
of task
– ↓ distance
– ↑ target width
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MINIMIZE EYE MOTION
(Within normal line of sight)
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KEY POINTS:
• Design work according to human
capabilities and limitations
• For manipulative tasks:
– Dynamic motion rather than static holds
– Strength requirements < 15% max
– Avoid extreme ranges of motion
– Small muscles for speed and precision
– Large muscles for strength
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