Biomechanics of musculoskeletal system pp 420 436 Objectives

Biomechanics of musculoskeletal system (pp 420 -436) -Objectives n n n 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 n 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) n 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 n Elements of levers - axis (joint center) - rigid bar (long bone) - motive and resistance torques (muscle pull, gravity, inertia), or moments n n Concept of Net Torque Law of levers ( CW torques = CCW torques) - Force X Force Arm = Resistance X Resistance Arm - or Ff = Rr n 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 n 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) n 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 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 sin F=

Wheel and axle and mechanical advantage n n 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 n Pulleylike arrangements • Only simple pulleys found in the body to change direction of force application n General considerations of musculoskeletal machines • human body is built for speed & ROM - not force n 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?