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

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

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

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  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 • 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) • 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 445 - # 1,2,3,4,6,7,8 • Additional problems, p 446 - #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?

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