Transcript Biomechanics of Musculoskeletal System

Factors Influencing Production of
Muscular Tension and Applied
Force
Force-velocity relationship - Fig 6.17, p 162
 Length - tension relationship - Fig 6.18, p 164
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- Elastic and contractile components
Forcevelocity
relationship:
Active
insufficiency:
lengthtension
relationship:
Biomechanics of musculoskeletal
system (pp 420-436) -Objectives
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Identify the musculoskeletal machines and machine
functions found in the human body
Identify elements of the lever
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 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)
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 )
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
Concept of Net Torque, concentric
and eccentric contraction
Sample Problem #2, p 433
Analysis of elbow flexors:
Note how angle of pull changes
Angle of Pull of Muscle &
degree of force application
Turning component equals
Force times sin θ
Mechanical Advantage of Elbow
Flexors
Length of Elbow Flexors as
Joint Angle Changes
Additional problem #2, p 173: Length-tension,
angle of pull combined
Sine of
Mechanical advantage
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Mechanical advantage is the output force (F)
divided by input force (R)
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Lever arrangements
• MA = output force (R) divided by input force (F) = force arm (f)
divided by resistance arm (r)
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Wheel and axle arrangements
• Very similar to lever arrangements with force applied to the axle,
which is usually the case (e.g., shoulder joint in throwing) with radius
of axle and radius of wheel equivalent to force arm (f) and resistance
arms (r), respectively
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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Self-study questions
- Introductory problems, p 445 - # 1,2,3,6,7
Homework # 7 – Musculoskeletal machines: (due Monday, 3/15/04)
Introductory problems, p 445 - # 4, 8
Additional problems, p 446 - #3
Exercise equipment problem illustrated below:
Assume force is applied
perpendicular to the bar:
a. In which position will the
exercise be easier?
b. If it takes 100 N to move
the system at position 2,
what will it take to lift it at
positions 1 and 3?
Hint: use the law of levers: Ff = Rr
and solve for F