Chapter 6: The Biomechanics of Human Skeletal Muscle

1. Chapter 6:The Biomechanics of Human Skeletal Muscle Basic Biomechanics, 4th edition Susan J. Hall Presentation Created by TK Koesterer, Ph.D., ATC Humboldt State University

2. Objectives • Identify the basic behavioral properties of the musculotendinous unit. • Explain the relationships of fiber types and fiber architecture to muscle function. • Explain how skeletal muscles function to produce coordinated movement of the human body. • Discuss the effects of the force-velocity and length-tension relationships and electromechanical delay on muscle function. • Discuss the concepts of strength, power, and endurance from a biomechanical perspective.

3. Behavioral Properties of the Musculotendinous Unit • Behavioral properties of muscle tissue: • Extensibility • Elasticity • Irritability • Ability to develop tension • Behavioral properties common to all muscle: • Cardiac, smooth, skeletal

4. Extensibility and Elasticity • Extensibility • Elasticity • Two components: • Parallel elastic component (PEC) • Series elastic component (SEC) • Contractile component • Visoelastic

5. Irritability and the Ability to Develop Tension • Irritability • The ability to respond to electrical or mechanical stimulus. • Response is the development of tension. • Not necessarily a contraction

6. Structural Organization of Skeletal Muscle • Human body has approx. 434 muscles • 40-45% of total body weight in adults • 75 muscle pairs responsible for bodily movements and posture • Muscle Fibers • Motor Units • Fiber Types • Fiber Architecture

7. Contain: sarcolemma sarcoplasm nuclei mitochondria myofibrils myofilaments Sarcomere Z lines M line A band myosin filaments I band actin filaments H zone Muscle Fibers

8. Muscle Fibers • During contraction, cross-bridges form • Sarcoplasmic Reticulum • Transverse Tubules • Endomysium • Perimysium • Fascicles • Epimysium • Variation of length and diameter within muscles seen in adults.

9. Motor Units • Motor unit: • Axon • Motor end plate • Twitch Type • Tonic Type • Summation • Tetanus

10. Fiber Types • Fast Twitch (FT) • Type IIa • Type IIb • Slow Twitch (ST) • Type I • Peak tension reached in FT in 1/7 time of ST • ST and FT compose skeletal muscles • Percentages of each range from muscle to muscle and individual to individual.

11. Fiber Types • Effects of training: • Endurance training can increase ST contraction velocity by 20% • Resistance training can convert FT fibers from Type IIb to Type IIa • Elite athlete fiber type distribution does not significantly differ from untrained individuals • Affected by: • Age and Obesity

12. Fiber Architecture • Parallel fiber arrangement • Resultant tension from shortening of muscle fibers • Shortens the muscle • Pennate fiber arrangement • Resultant tension from shortening of muscle fibers • Increases the angle of pennation (attachment) to a tendon.

13. Skeletal Muscle Function • Recruitment of motor units • Change in length with tension development • Roles assumed by muscles • Two-joint and multijoint muscles

14. Recruitment of Motor Units • CNS enables matching of speed and magnitude of muscle contraction to requirement of movement. • Threshold activation • ST activated first (low threshold) • With an increase in speed, force, and/or duration requirement, higher threshold motor units are activated (FT fibers)

15. Change in Muscle Length with Tension Development • Concentric • Bicep shortening with the bicep curl (flexion) • Isometric • Body builders develop isometric contraction in competition • Eccentric • Acts as a breaking mechanism to control movement

16. Roles Assumed by Muscles • Agonist • Primary & Secondary • Antagonist • Stabilizer • Neutralizer • Agonists and Antagonists are typically positioned on opposite sides of a joint.

17. Two-joint and Multijoint Muscles • Movement effectiveness depends on: • Location and orientation of muscle’s attachment relative to the joint • Tightness or laxity of musculotendinous unit • Actions of other muscles crossing the joint • Disadvantages: • Active insufficiency • Passive insufficiency

18. Factors Affecting Muscular Force Generation • Force-Velocity Relationship • Length-Tension Relationship • Electromechanical Delay • Stretch-Shortening Cycle

19. Force-Velocity Relationship • Maximal force developed by muscle governed by velocity of muscle’s shortening or lengthening. • Holds true for all muscle types • Does not imply: • It’s impossible to move heavy resistance at a fast speed. • It’s impossible to move light loads at low speeds

20. Force-Velocity Relationship • Maximum isometric tension • Eccentric conditions • Volitionally • Represents contribution of the elastic components of muscle • Eccentric Strength Training • More effective than concentric training in increasing muscle size and strength.

21. Length-Tension Relationship • In human body, force generation increases when muscle is slightly stretched. • Parallel fibers at max just over resting length • Pennate fibers at max with 120%-130% resting length. • Due to contribution of elastic components of muscle (primarily the SEC)

22. Electromechanical Delay • Electromechanical Delay (EMD) • Varies among human muscles (20-100 msec) • Short EMDs produced by muscles with high percentage of FT fibers • Associated with development of higher contraction forces • Not effected by muscle length, contraction type, contraction velocity, or fatigue

23. Stretch-Shortening Cycle • Stretch-Shortening Cycle (SSC) • Elastic Recoil • Stretch Reflex Activation • Muscle can perform more work with active stretch prior to shortening contraction • Less metabolic costs when SSC utilized. • Eccentric training increases ability of musculotendinous unit to store and produce more elastic energy.

24. Muscular Strength, Power, and Endurance • Muscular Strength • Muscular Power • Muscular Endurance • Muscular Fatigue • Effect of Muscle Temperature

25. Muscular Strength • The ability of a given muscle group to generate torque at a particular joint. • Two orthogonal components: • 1) Rotary Component • 2) Parallel to bone • Derived from: • amount of tension the muscles can generate • moment arms of contributing muscles with respect to joint center.

26. Muscular Strength • Tension-generating capability of a muscle affected by: • Cross-sectional area • Training state • Moment arm of a muscle affected by: • Distance between the muscle’s anatomical attachment to bone and the axis of rotation at the joint center • Angle of muscle’s attachment to bone.

27. Muscular Power • The product of muscular force and the velocity of muscular shortening. • The rate of torque production at a joint • Max. power occurs at: • approx. 1/3 max. velocity, and • approx. 1/3 max concentric force • Affected by muscular strength and movement speed

28. Muscular Endurance • The ability to exert tension over a period of time. • Constant: gymnast in iron cross • Vary: rowing, running, cycling • Length of time dramatically effected by force and speed requirements of activity. • Training involves many repetitions with light resistance.

29. Muscular Fatigue • Opposite of endurance • Characteristics: • Reduction in force production • Reduction in shortening velocity • Prolonged relaxation of motor units between recruitment • Absolute Fatigue • Resistance: • SO > FOG > FG • Causes

30. Effect of Muscle Temperature • Increased body temperature, increases speed of nerve and muscle function • Fewer motor units needed to sustain given load • Metabolic processes quicken • Benefits of increased muscular strength, power and endurance • Key point: Be sure to warm-up!

31. Common Muscle Injuries • Strains • Mild, moderate or severe • Contusions • Myositis ossificans • Cramps • Delayed-Onset Muscle Soreness (DOMS) • Compartment Syndrome

32. Summary • Muscle is the only biological tissue capable of developing tension. • Resulting actions can be concentric, eccentric, isometric for muscle shortening, lengthening or remaining unchanged in length • Force production the the combination of many relationships (ex: force-velocity) • Specific activity performance is related power, endurance, and strength