1. Biomechanics of musculoskeletal system (pp 420-436) -Objectives
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

2. Musculoskeletal machine functions and machines
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)
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)

3. Levers in the Human Body (all amplify movement at expense of force)
Class III:
Class :I

4. Wheel& Axle:
Another movement amplifier!

5. Simple pulley: Changes direction of force application

6. Musculoskeletal Levers
Elements of levers
axis (joint center)
rigid bar (long bone)
motive and resistance torques (muscle pull, gravity, inertia), or moments
Concept of Net Torque
Law of levers (CW torques = CCW torques)
Force X Force Arm = Resistance X Resistance Arm
or Ff = Rr
Analysis of musculoskeletal lever system
Turning, or rotary component (Fd sin )
Stabilizing and dislocating component (Fd cos )

7. Concept of Net Torque, concentric and eccentric contraction

8. Muscle force acting on lever systems

9. Sample Problem #2, p 433

10. Analysis of elbow flexors:
Note how angle of pull changes

11. Classes of levers & mechanical advantage
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)
Calculating mechanical advantage
MA = force out (resistance)/ force in (motive force)
MA = motive force arm/resistance force arm

12. Levers in the Human Body (all amplify movement at expense of force)
Class III:
Class :I

13. Class I
Class II
Class III

14. 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 =

15. Wheel and axle and mechanical advantage
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

16. Wheel and Axle Mechanical Advantage is f/r, force arm ÷ resistance arm
Force applied to wheel:
Force applied to axle:
MA less than 1 (movement amp)
MA greater than 1
(force amp)

17. Musculoskeletal pulleys, study questions
Pulleylike arrangements
Only simple pulleys found in the body to change direction of force application
General considerations of musculoskeletal machines
human body is built for speed & ROM - not force
Study questions
Introductory problems, p # 1,2,3,4,6,7,8
Additional problems, p #3, 8

18. 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?