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Chapter 6 – The Biomechanics of Skeletal Muscle

Chapter 6 – The Biomechanics of Skeletal Muscle. 1. Principal characteristics of skeletal muscle 2. Structural organization of skeletal muscle 3. Fast versus slow twitch motor units 4. Roles assumed by muscles 5. Types of muscular contraction 6. Factors affecting force production.

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Chapter 6 – The Biomechanics of Skeletal Muscle

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  1. Chapter 6 – The Biomechanics of Skeletal Muscle 1.Principal characteristics of skeletal muscle 2.Structural organization of skeletal muscle 3. Fast versus slow twitch motor units 4. Roles assumed by muscles 5. Types of muscular contraction 6. Factors affecting force production

  2. Chapter 6 – The Biomechanics of Skeletal Muscle • Four principal characteristics: • Excitability – ability to receive and respond to a stimulus • Contractilty (irritability) – ability of a muscle to contract and produce a force • Extensibility – ability of a muscle to be stretched without tissue damage • Elasticity – ability of a muscle to return to its original shape after shortening or extension

  3. Structural organization of skeletal muscle From Principles of Human Anatomy (7th edition), 1995 by Gerard J. Tortora, Fig 9.5, p 213

  4. From Basic Biomechanics by Susan Hall (3rd edition), Fig 6.6, page 153 6-6

  5. From Skeletal Muscle: Form and Function (2nd ed) by MacIntosh, Gardiner, and McComas. Fig 1.4, p. 8.

  6. From Basic Biomechanics by Susan Hall (3rd edition), Fig 6.5, page 152 6-5

  7. Structural organization of skeletal muscle From Principles of Human Anatomy (7th edition), 1995 by Gerard J. Tortora, Fig 9.5, p 213

  8. From Basic Biomechanics by Susan Hall (3rd edition), Fig 6.3, page 150 6-3

  9. From Exercise Physiology: Theory and Application to Fitness and Performance (6th Edition) by Scott K. Powers and Edward T. Howley. Fig 8.6 P. 147

  10. A motor unit: single motor neuron and all the muscle fibers it innervates From Basic Biomechanics Instructors manual by Susan Hall (2nd edition, 1995), Fig TM 31

  11. From Basic Biomechanics by Susan Hall (3rd edition), Fig 6.7, page 154 6-7

  12. From Basic Biomechanics by Susan Hall (3rd edition), Fig 6.8, page 154 6-8

  13. Types of muscle fiber: Fast twitch vs Slow Twitch Type I Type IIa Type IIb ST OxidativeFT Oxidative - FT Glycolytic (S0)Glycolytic (FOG) (FG) • Contraction speed slow fast (2xI) fast (4xI) • Time to peak force slow fast fast

  14. FT ST Twitch Tension Time Fast twitch (FT) fibers both reach peak tension and relax more quickly than slow twitch (ST) fibers. (Peak tension is typically greater for FT than for ST fibers.)

  15. Types of muscle fiber: Fast twitch vs Slow Twitch Type I Type IIa Type IIb ST OxidativeFT Oxidative - FT Glycolytic (S0)Glycolytic (FOG) (FG) • Contraction speed slow fast (2xI) fast (4xI) • Time to peak force slow fast fast • Fatigue rate slow inter. fast • Fiber diam. small inter. large • Aerobic capacity high inter. low • Mitochondrial conc. high inter. low • Anaerobic capacity low inter. High Sedentary people – 50% slow/50% fast, whereas elite athletes may differ e.g., cross country skiers – 75% slow 25% fast sprinters - 40% slow 60% fast

  16. Roles assumed by muscles • Agonist: acts to cause a movement • Antagonist: acts to slow or stop a • movement • Stabilizer: acts to stabilize a body part • against some other force • Neutralizer: acts to eliminate an • unwanted action produced by an agonist • Synergist: acts to perform the same action • as another muscle

  17. Types of muscular contraction • Concentric: fibers shorten • Eccentric: fibers lengthen • Isometric: no length change

  18. Factors affecting force Production 1. Cross-sectional area 2. Frequency of stimulation 3. Spatial recruitment 4. Velocity of shortening 5. Muscle length 6. Action of the series elastic component 7. Muscle architecture 8. Electromechanical delay 9. Muscle temperature

  19. 1. Cross-sectional area Hypertrophy: increase in the # of myofibrils and myofilaments Hyperplasia: increase in the number of fibers??? Factors affecting force Production 1. Cross sectional area

  20. Parallel vs serially arranged sarcomeres Optimal for force production Optimal for velocity of shortening and range of motion In series In parallel From Exercise Physiology: Human Bioenergetics and its applications (2nd edition) by Brooks, Fahey, and White (1996) Fig 17-20, P. 318

  21. 2. Rate Coding – frequency of stimulation From Basic Biomechanics by Susan Hall (3rd edition), Fig 6.9, page 155

  22. 3. Spatial recruitment • Increase # of active motor units (MUs) • Order of recruitment I ---> IIa -----> IIb • Henneman's size principle: MUs are recruited in order of their size, from small to large • Relative contributions of rate coding and spatial recruitment. • Small muscles - all MUs recruited at approximately 50% max. force; thereafter, rate coding is responsible for force increase up to max • Large muscles - all MUs recruited at approximately 80% max. force.

  23. (Low resistance, high contraction velocity) Force Velocity 4.Velocity of shortening: Force inversely related to shortening velocity The force-velocity relationship for muscle tissue: When resistance (force) is negligible, muscle contracts with maximal velocity.

  24. isometric maximum Force Velocity The force-velocity relationship for muscle tissue: As the load increases, concentric contraction velocity slows to zero at isometric maximum.

  25. Force-Velocity Relationship in different muscle fiber types Type II fiber Type I fiber

  26. Force -Velocity Relationship (Effect of strength-Training)

  27. Force/Velocity/Power Relationship Force/velocity curve Power/velocity curve Force Power 30% From Basic Biomechanics by Susan Hall (3rd edition), Fig 6.25, page 175 30% Velocity

  28. Effect of Muscle Fiber Types on Power-Velocity Relationship

  29. Force-velocity Relationship During Eccentric Muscular Contractions

  30. From Skeletal muscle structure, function, and plasticity (2nd Edition) by R.L. Leiber, P 312

  31. 5. Muscle length From Skeletal muscle structure, function, and plasticity by R.L. Leiber, P. 55

  32. From Exercise Physiology: Human Bioenergetics and its applications (2nd edition) by Brooks, Fahey, and White (1996) P. 306

  33. The length-tension relationship: Tension present in a stretched muscle is the sum of the active tension provided by the muscle fibers and the passive tension provided by the tendons fascia, and titin

  34. The stretch-shortening phenomenon The effectiveness and efficiency of human movement may be enhanced if the muscles primarily responsible for the movement are actively stretched prior to contracting concentrically. Mechanism: storage and release of elastic strain energy. 6.Action of the series elastic component

  35. Pennate fiber arrangements Parallel fiber arrangements 7. Muscle Architecture Fibers are roughly parallel to the longitudinal axis of the muscle Short fibers attach at an angle to one or more tendons within the muscle From Basic Biomechanics by Susan Hall (3rd edition), Fig 6.11, page 159

  36. From Basic Biomechanics by Susan Hall (3rd edition), Fig 6.13, page 161

  37. 8. Electromechanical delay 20-100 ms Time between arrival of a neural stimulus and tension development by the muscle From Basic Biomechanics by Susan Hall (3rd edition), Fig 6.20, page 171

  38. 9. Temperature: Effect on the Force-Velocity Relationship (22oC, 25oC, 31Co, and 37oC)

  39. Two- joint Muscles • Advantages • Actions at two joints for the price of one muscle. Possible metabolic saving if coordinated optimally • Shortening velocity of a two-joint muscle is less than that of its single-joint synergists Results in a more favorable position on the force velocity curve. • Act to redistribute muscle torque and joint power throughout a limb.

  40. Two- joint Muscles • Disadvantages: • Active insufficiency: unable to actively shorten sufficiently to produce a full range of motion at each joint crossed simultaneously • Passive insufficiency: unable to passively lengthen sufficiently to produce a full range of motion at each joint crossed simultaneously

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