Biomechanics of musculoskeletal
system (pp 420-436) -Objectives
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Identify the musculoskeletal machines and machine
functions found in the human body
Define torque, quantify resultant torques, and identify
the factors that affect resultant joint torques
Describe the concept of net torques and it’s use in
estimating muscle force
Describe the elements of the wheel and axle and give
examples in the musculoskeletal system of humans
Describe the concept of mechanical advantage
associated with the different classes of levers and
wheel and axle
Musculoskeletal machine functions
and machines
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Most important machine functions found in the
human body
- provide advantage for ROM and speed (levers
and wheel & axle)
- change direction of applied force (pulley)
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Three machines found in the body:
- levers (ex. biceps brachii pulling on radius)
- wheel and axle (rotator cuff muscles pulling on
humerus)
- pulley (patella, lateral malleolus of fibula)
Levers in the Human Body
(all amplify movement at expense of force)
Class III:
Class :I
Wheel
&
Axle:
Another movement amplifier!
Simple pulley: Changes
direction of force application
Musculoskeletal Levers
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Elements of levers
- axis (joint center)
- rigid bar (long bone)
- motive and resistance torques (muscle pull, gravity,
inertia), or moments
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Concept of Net Torque
Law of levers (CW torques = CCW torques)
- Force X Force Arm = Resistance X Resistance Arm
- or Ff = Rr
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Analysis of musculoskeletal lever system
- Turning, or rotary component (Fd sin )
- Stabilizing and dislocating component (Fd cos )
Concept of Net Torque, concentric
and eccentric contraction
Muscle force acting on lever
systems
Sample Problem #2, p 433
Analysis of elbow flexors:
Note how angle of pull changes
Classes of levers & mechanical
advantage
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Classes of levers
- Class I: Axis between force and resistance application
(ex. triceps tendon in elbow extension)
- Class II: Resistance application between axis & force
application (ex: wheelbarrow, obliques and spinal rotation)
- Class III: Force application between axis & resistance
application (ex: biceps tendon in elbow flexion)
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Calculating mechanical advantage
- MA = force out (resistance)/ force in (motive force)
- MA = motive force arm/resistance force arm
Levers in the Human Body
(all amplify movement at expense of force)
Class III:
Class :I
Class I
Class II
Class III
Mechanical Advantage:
MA = f/r = R/F
This lever is a movement amplifier (MA less than 1)
If f = 1 cm and r = 20 cm, what would R, the
resistance be?
f = d = d sin
F=
Wheel and axle and
mechanical advantage
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Wheel and axle arrangements
Very similar to lever arrangements with radius of axle
and radius of wheel equivalent to force and resistance
arms
- Mechanical advantage
• Force applied to wheel (ex: obliques in spinal rotation)
– MA = radius of wheel/radius of axle
• Force applied to axle (ex: rotator cuff muscles in humeral
rotation)
– MA = radius of axle/radius of wheel
Wheel and Axle
Mechanical Advantage is f/r, force arm ÷ resistance arm
Force applied to wheel:
Force applied to axle:
MA greater than 1
(force amp)
MA less than 1 (movement amp)
Musculoskeletal pulleys, study
questions
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Pulleylike arrangements
• Only simple pulleys found in the body to change
direction of force application
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General considerations of musculoskeletal
machines
• human body is built for speed & ROM - not force
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Study questions
- Introductory problems, p 445 - # 1,2,3,4,6,7,8
- Additional problems, p 446 - #3, 8
Ex equipment problem –
Assume force is applied
perpendicular to the bar:
a. What class lever is this?
b. In which position will the
exercise be easier?
c. If it takes 100 N to move
the system at position 2,
what will it take to lift it at
positions 1 and 3?