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MUSCLE ENERGETICS AND FATIGUE

This summary provides an overview of muscle mechanics, including muscle recruitment, force grading, muscle fibers, contraction mechanisms, excitation-contraction coupling, energy production, and muscle hypertrophy. Learn about the factors influencing muscle force, velocity, power, and fatigue.

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MUSCLE ENERGETICS AND FATIGUE

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  1. MUSCLE ENERGETICS AND FATIGUE

  2. Summary of Mechanics • 1 Muscles pull; they don’t push • 2 Muscle lengthen by being yanked on by antagonists or gravity • 3 Muscle force can be graded by recruitment of motor units • 4 You activate small motor units first: the size principle • 5 Muscle force can be graded by repetitive stimulation • 6 Muscle force can be graded by changing length, but who cares • 7 Muscle velocity is inversely related to muscle force: you can • be strong or fast but not both at the same time • 8 Muscle power peaks at 1/3 maximum force • 9 Pinnate muscle fibers develop more force at lower velocity • because of the angle • 10 Muscles fatigue: they drop force on continued use • 11 Muscles are heterogeneous based on contractile properties • Slow twitch (S) • Fast fatigue resistant (FR) • Fast Intermediate (FI) • Fast fatiguable (FF)

  3. Summary of contractile mechanisms 1 Muscle cells are highly organized 2 Myofibrils consists of interdigitating hexagonal arrays of filaments 3 Thick filaments are mainly myosin: A bands 4 Thin filaments are actin + tropomyosin + TnI + TnC + TnT: I bands 5 Sliding filaments explains the length-tension curve 6 Cross-Bridge cycling couples ATP hydrolysis to force or shortening 7 Myosin isoforms have different turnover numbers 8 Costameres may transmit force from myofilaments to muscle exterior through the cytoskeleton

  4. Summary of Excitation-Contraction Coupling 1 Contraction begins with neuromuscular transmission 2 Muscle action potential depolarizes the T-tubule 3 T-tubule depolarization tickles the DHPR 4 DHPR tickles RyR1 5 RyR1 release loads of Ca, but SR is not empty 6 Ca saturates TnC 7 TnC-Ca disinhibits Acto-myosin interaction 8 Acto-myosin cycles the cross bridge 9 SR re-uptake shuts off contraction 10 Series-elastic elements explain twitch time course 11 Prolonged Ca transient with series-elastic elements explains tetany

  5. ATP is the energy currency of the cell; its free energy of hydrolysisdrives muscle contraction and energy output

  6. Acto-myosin cross bridge cycle: ATP hydrolysis is linked to force generation or shortening

  7. Muscles are activated by brief trains of impulses followed by rest periods

  8. Lactic acid levels increases with exercise intensity

  9. Lactic acid is oxidized in mitochondria and carries in cytoplasmic NADH equivalents

  10. Lactic acid shuttles: 1. to the mitochondria2. to oxidative fibers3. to liver

  11. Exercise increases GLUT4 population in muscle membranes and increases glucose uptake independent of insulin

  12. Muscle glycogenoses

  13. Signals for muscle hypertrophy

  14. Nuclear domains in muscle

  15. Fiber Type Switching

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