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Chapter 8: Skeletal Muscle

Chapter 8: Skeletal Muscle. EXERCISE PHYSIOLOGY Theory and Application to Fitness and Performance, 6 th edition Scott K. Powers & Edward T. Howley. Objectives. Draw & label the microstructure of skeletal muscle Outline the steps leading to muscle shortening

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Chapter 8: Skeletal Muscle

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  1. Chapter 8:Skeletal Muscle EXERCISE PHYSIOLOGY Theory and Application to Fitness and Performance, 6th edition Scott K. Powers & Edward T. Howley

  2. Objectives • Draw & label the microstructure of skeletal muscle • Outline the steps leading to muscle shortening • Define the concentric and isometric • Discuss: twitch, summation & tetanus • Discus the major biochemical and mechanical properties of skeletal muscle fiber types

  3. Objectives • Discuss the relationship between skeletal muscle fibers types and performance • List & discuss those factors that regulate the amount of force exerted during muscular contraction • Graph the relationship between movement velocity and the amount of force exerted during muscular contraction • Discuss structure & function of muscle spindle • Describe the function of a Golgi tendon organ

  4. Skeletal Muscle • Human body contains over 400 skeletal muscles • 40-50% of total body weight • Functions of skeletal muscle • Force production for locomotion and breathing • Force production for postural support • Heat production during cold stress

  5. Connective Tissue Covering Skeletal Muscle • Epimysium • Surrounds entire muscle • Perimysium • Surrounds bundles of muscle fibers • Fascicles • Endomysium • Surrounds individual muscle fibers

  6. Connective Tissue Covering Skeletal Muscle Fig 8.1

  7. Microstructure of Skeletal Muscle • Sarcolemma: Muscle cell membrane • Myofibrils Threadlike strands within muscle fibers • Actin (thin filament) • Myosin (thick filament) • Sarcomere • Z-line, M-line, H-zone, A-band & I-band

  8. Microstructure of Skeletal Muscle Fig 8.2

  9. Microstructure of Skeletal Muscle • Within the sarcoplasm • Sarcoplasmic reticulum • Storage sites for calcium • Transverse tubules • Terminal cisternae • Mitochondria

  10. Within the Sarcoplasm Fig 8.3

  11. The Neuromuscular Junction • Where motor neuron meets the muscle fiber • Motor end plate: pocket formed around motor neuron by sarcolemma • Neuromuscular cleft: short gap • Ach is released from the motor neuron • Causes an end-plate potential (EPP) • Depolarization of muscle fiber

  12. Neuromuscular Junction Fig 8.4

  13. Muscular Contraction • The sliding filament model • Muscle shortening occurs due to the movement of the actin filament over the myosin filament • Formation of cross-bridges between actin and myosin filaments “Power stroke” • 1 power stroke only shorten muscle 1% • Reduction in the distance between Z-lines of the sarcomere

  14. The Sliding Filament Model Fig 8.5

  15. Actin & Myosin Relationship • Actin • Actin-binding site • Troponin with calcium binding site • Tropomyosin • Myosin • Myosin head • Myosin tais

  16. Actin & Myosin Relationship Fig 8.6

  17. Energy for Muscle Contraction • ATP is required for muscle contraction • Myosin ATPase breaks down ATP as fiber contracts • Sources of ATP • Phosphocreatine (PC) • Glycolysis • Oxidative phosphorylation

  18. Sources of ATP for Muscle Contraction Fig 8.7

  19. Excitation-Contraction Coupling • Depolarization of motor end plate (excitation) is coupled to muscular contraction • Nerve impulse travels down T-tubules and causes release of Ca++ from SR • Ca++ binds to troponin and causes position change in tropomyosin, exposing active sites on actin • Permits strong binding state between actin and myosin and contraction occurs

  20. Excitation-Contraction Coupling Fig 8.9

  21. Steps Leading to Muscular Contraction Fig 8.10

  22. Properties of Muscle Fiber Types • Biochemical properties • Oxidative capacity • Type of ATPase • Contractile properties • Maximal force production • Speed of contraction • Muscle fiber efficiency

  23. Fast fibers Type IIx fibers Fast-twitch fibers Fast-glycolytic fibers Type IIa fibers Intermediate fibers Fast-oxidative glycolytic fibers Slow fibers Type I fibers Slow-twitch fibers Slow-oxidative fibers Individual Fiber Types

  24. Muscle Fiber Types Fast FibersSlow fibers Characteristic Type IIx Type IIa Type I Number of mitochondria Low High/mod High Resistance to fatigue Low High/mod High Predominant energy system Anaerobic Combination Aerobic ATPase Highest High Low Vmax (speed of shortening) Highest Intermediate Low Efficiency Low Moderate High Specific tension High High Moderate

  25. Type IIx Comparison of Maximal Shortening Velocities Between Fiber Types Fig 8.11

  26. Histochemical Staining of Fiber Type Type IIa Type IIx Type I Fig 8.12

  27. + 1 2 3 4 5 Type I Type IIA Type IIx 5 - Biceps 4 – Quads 3 – Gastroc 2 – Soleus 1 – Marker _ Fiber Typing • Gel electrophoresis: myosin isoforms • different weight move different distances Table 10.2

  28. Fiber Typing • Immunohistochemical: • Four serial slices of muscle tissue • antibody attach to certain myosin isoforms

  29. Muscle Tissue: Rat Diaphragm Type 1 fibers - dark antibody: BA-D5 Type 2b fiber - dark Antibody: BF-F3 Type 2x fibers - light/white antibody: BF-35 Type 2a fibers - dark antibody: SC-71

  30. Fiber Types and Performance • Power athletes • Sprinters • Possess high percentage of fast fibers • Endurance athletes • Distance runners • Have high percentage of slow fibers • Others • Weight lifters and nonathletes • Have about 50% slow and 50% fast fibers

  31. Alteration of Fiber Type by Training • Endurance and resistance training • Cannot change fast fibers to slow fibers • Can result in shift from Type IIx to IIa fibers • Toward more oxidative properties

  32. Training-Induced Changes in Muscle Fiber Type Fig 8.13

  33. Age-Related Changes in Skeletal Muscle • Aging is associated with a loss of muscle mass • Rate increases after 50 years of age • Regular exercise training can improve strength and endurance • Cannot completely eliminate the age-related loss in muscle mass

  34. Types of Muscle Contraction • Isometric • Muscle exerts force without changing length • Pulling against immovable object • Postural muscles • Isotonic (dynamic) • Concentric • Muscle shortens during force production • Eccentric • Muscle produces force but length increases

  35. Isotonic and Isometric Contractions Fig 8.14

  36. Speed of Muscle Contraction and Relaxation • Muscle twitch • Contraction as the result of a single stimulus • Latent period • Lasting only ~5 ms • Contraction • Tension is developed • 40 ms • Relaxation • 50 ms

  37. Muscle Twitch Fig 8.15

  38. Force Regulation in Muscle • Types and number of motor units recruited • More motor units = greater force • Fast motor units = greater force • Increasing stimulus strength recruits more & faster/stronger motor units • Initial muscle length • “Ideal” length for force generation • Nature of the motor units neural stimulation • Frequency of stimulation • Simple twitch, summation, and tetanus

  39. Relationship Between Stimulus Frequency and Force Generation Fig 8.16

  40. Length-Tension Relationship Fig 8.17

  41. Simple Twitch, Summation, and Tetanus Fig 8.18

  42. Force-Velocity Relationship • At any absolute force the speed of movement is greater in muscle with higher percent of fast-twitch fibers • The maximum velocity of shortening is greatest at the lowest force • True for both slow and fast-twitch fibers

  43. Force-Velocity Relationship Fig 8.19

  44. Force-Power Relationship • At any given velocity of movement the power generated is greater in a muscle with a higher percent of fast-twitch fibers • The peak power increases with velocity up to movement speed of 200-300 degrees•second-1 • Force decreases with increasing movement speed beyond this velocity

  45. Force-Power Relationship • At any given velocity of movement the power generated is greater in a muscle with a higher percent of fast-twitch fibers • The peak power increases with velocity up to movement speed of 200-300 degrees/sec • Force decreases with increasing movement speed beyond this velocity

  46. Force-Power Relationship Fig 8.20

  47. Receptors in Muscle • Muscle spindle • Changes in muscle length • Rate of change in muscle length • Intrafusal fiber contains actin & myosin, and therefore has the ability to shorten • Gamma motor neuron stimulate muscle spindle to shorten • Stretch reflex • Stretch on muscle causes reflex contraction

  48. Muscle Spindle Fig 8.21

  49. Receptors in Muscle • Golgi tendon organ (GTO) • Monitor tension developed in muscle • Prevents damage during excessive force generation • Stimulation results in reflex relaxation of muscle

  50. Golgi Tendon Organ Fig 8.22

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