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

- Elastic and contractile components

Force-velocity

relationship:

Active insufficiency:

length-tension

relationship:

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

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

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)

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 )

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 equalsForce 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

Mechanical advantage is the output force (F) divided by input force (R)

Lever arrangements• MA = output force (R) divided by input force (F) = force arm (f)

divided by resistance arm (r)

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

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 Self-study questions

- Introductory problems, p 445 - # 1,2,3,6,7

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

Homework # 7 – Musculoskeletal machines: (due Monday, 3/15/04)Introductory problems, p 445 - # 4, 8Additional problems, p 446 - #3Exercise equipment problem illustrated below: