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Skeletal Muscle - Tension

Skeletal Muscle - Tension. (2) active tension (muscle activation) Excitation-contraction coupling Ca 2+ , ATP Muscle size (cross-sectional area) Muscle length (stretch) Rate coding (frequency modulation) & motor unit recruitment Shortening velocity Temperature Fiber-type Reflexes.

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Skeletal Muscle - Tension

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  1. Skeletal Muscle - Tension • (2) active tension (muscle activation) • Excitation-contraction coupling • Ca2+, ATP • Muscle size (cross-sectional area) • Muscle length (stretch) • Rate coding (frequency modulation) & motor unit recruitment • Shortening velocity • Temperature • Fiber-type • Reflexes

  2. Force-velocity relationships: types of muscle contractions • Isometric - • muscle length remains constant • Static mechanics • SFx,y = 0; ST = 0 • Isotonic • Concentric (shortening) • Eccentric (lengthening)

  3. Force-velocity relationships: types of muscle contractions • Concentric Contraction - shortening contraction • The torque produced by the muscles at the joint overcomes resistance to movement • Inertia and weight of body segments/limb • Outside contact forces (impact, free weight, etc.)

  4. Force-velocity relationships: types of muscle contractions • Eccentric Contraction • The torque produced by muscles at the joint is less than the resistance to movement • 1. Muscle must produce some tension • 2. Usefulness • Braking against a powerful/rapid movement in order to protect joints/muscles (co-contraction) • Ex. Lifting a heavy weight • Braking against gravity (also protection) • Ex. Letting down a heavy weight • Precise movement towards a target • Ex. Catching a softball

  5. Force-Velocity Relationship Isometric Maximum Eccentric Force Concentric - + 0 Velocity lengthening shortening

  6. Torque-angle relationships maximal Eccentric contractions - Protective mechanism 2. Muscle’s ability to produce torque changes throughout ROM

  7. Anatomical Analysis of Movement • Identify the joints that are moving • Describe the type of movement (flexion, extension, abduction, etc.) • Identify the muscle that may be used during this movement type • Deduce whether these muscle are producing isometric, concentric, or eccentric contractions or whether the muscle is passive First/oldest type of analysis in Exercise Biomechanics

  8. Anatomical Analysis of Movement

  9. Mechanical Work • A force applied an object multiplied by the displacement of that object in the direction of the applied force • SYMBOL: W • FORMULA: W = F•d • UNITS: Metric - Joules (J) 1J = 1 N•m English – foot•lbs • Exs. Lifting a free weight, riding a cycle ergometer, pushing a sled (if frictional force measured - dynamometer)

  10. Mechanical Work • Problem: Calculate the work involved in lifting a 300 N weight a height of .6 m W = F•d W = (300N)(.6 m) W = 180 N•m = 180 J

  11. Negative Work – The direction of muscle force is opposite the direction of movement. An eccentric contraction. Positive Work – The direction of muscle force is the same as the direction of movement. A concentric contraction.

  12. Energy DEFINITION: the ability to produce work. It is manifested in various forms: Motion (kinetic) Position (potential) Strain (spring) Heat, light, sound UNITS: Joules (J), calories

  13. Energy Types:Formula: Motion (kinetic) Ek = 1/2 m•v2 Position (potential) Ep = m•ag•h Strain (spring) Es = 1/2k•x2 Heat, light, sound UNITS: Joules (J), calories

  14. Mechanical Gross Efficiency • Efficiency = mechanical work/energy • Most exercises (weightlifting, climbing stairs, cycling) ~ 20% efficient!

  15. Mechanical Efficiency • Walking and running >>20% efficient! • Why? • “Natural springs” (arch of foot, Achilles’ tendon, muscles)

  16. Strain Energy – ApplicationNatural Springs • For a man with a mass of 70 kg running at a velocity of 4.5 m/s, the arch of the foot will store about 17 J of energy when the foot is at maximum compression. • Each Achilles’ tendon + plantar flexors stores about 35 J of energy. • Other muscles (ex. quads) store >20 J of energy • This equals a return of over 50% of the energy expended while running (~110 J). • Energy Savings when recoil force used during pushoff

  17. Mechanical Efficiency • Walking and running >>20% efficient! • Why? • “Natural springs” (arch of foot, Achilles’ tendon, muscles) • Legs act as a pendulum in locomotion f = 1/(2)(ag/l) Where: ag = acceleration due to gravity l = distance from axis of rotation to center of mass (gravity) • most efficient running speed will match the dynamic pendulum frequency of the limbs • this is a dynamic frequency which changes as joint angles change in a multi-segmented limb • Aging, stroke, injury, crutches, neuromuscular disease decrease efficiency dramatically long pendulum Short pendulum

  18. Power • The amount of work performed in given time period (rate of work performed. • SYMBOL: P • FORMULA: P = W/Dt = F•d/Dt = F•v • strength x speed • UNITS: Metric – watts (W) 1W = 1J/sec • English – horsepower

  19. Power and Work Calculation GIVEN: A person weighing 580 N runs up a flight of 30 stairs, each with a height of 25 cm. The time for this effort is 15 seconds. FIND: The work and power done

  20. Power and Work Calculation F = weight = 580 N d = 25 cm * 30 = 750 cm = 7.5 m t = 15 s W = F•d = (580 N)(7.5 m) = 4350 J P = W/t = 4350 J/15s = 290 watts

  21. Force-Velocity Relationship Isometric Maximum Eccentric Force Concentric - + 0 Velocity

  22. Maximum. Isometric Force Force Velocity Concentric Force - Velocity Curve

  23. Peak Power Power 1/3 Velocity Power – Velocity Curve

  24. Maximum Isometric Force Peak Power Power Force 1/3 Velocity Power and Force – Velocity Curves P = W/∆t P = F• d/∆t P = F• v

  25. Muscle Dynamics • Strength • Dead lift • Bench press • Lifting a suitcase • Power • 100 m dash • High jump • Shot put • Speed • Badminton swing • Frisbee throw • Short shop stab for a line drive

  26. Muscle Dynamics Power athletes • 100 m dash • High jump • Shot put

  27. Energy • DEFINITION: The capacity to do work • UNITS: The same as the units for work (joules) • Two Forms of motion • Kinetic Energy • Potential Energy

  28. Kinetic Energy • DEFINITION: The energy of motion. • SYMBOL: KE • FORMULA: KE = 1/2(mv2) (Kinetic energy equals one-half of an object’s mass times the square of its velocity.) • For a motionless body, v = 0, therefore KE = 0

  29. Kinetic Energy Calculation • GIVEN: A ball with a weight of 70 N is rolling with a velocity of 3 m/s. • FIND: The kinetic energy of the ball m = weight / g = 70 N / 9.81 m/s2 = 7.14 kg KE = 1/2(mv2) = 1.2(7.14 kg)(3 m/s)2 = 32.11 J

  30. Potential Energy • DEFINITION: The energy of position. • SYMBOL: PE • FORMULA: PE = m•ag•h (Potential energy equals the weight of an object times its height above a surface to which it could fall.) • Increasing an object’s height increases its potential energy.

  31. Potential Energy Calculation • GIVEN: A ball with a weight of 70 N is resting on a shelf that is 2 m above the floor • FIND: The potential energy of the ball PE = m•ag • h = 70 N • 2 m = 140 J

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