Introduction to Skeletal Muscle Structure and Function

1. Required readings: Biomechanics and Motor Control of Human Movement (class text) by D.A. Winter, pp. 165-212

2. Next Class • Reading assignment • Biomechanics of Skeletal Muscle by T. Lorenz and M. Campello (adapted from M. I. Pitman and L. Peterson; pp. 149-171 • EMG by W. Herzog, A. C. S. Guimaraes, and Y. T. Zhang; pp. 308-336 • http://www.delsys.com/library/tutorials.htm • Surface Electromyography: Detecting and Recording • The Use of Surface Electromyography in Biomechanics • Exam on anthropometry • Turn in EMG abstract • Prepare short presentation on EMG research article • Laboratory experiment on EMG • Hour assigned

3. Advanced Biomechanics of Physical Activity (KIN 831) • Muscle – Structure, Function, and Electromechanical Characteristics • Material included in this presentation is derived primarily from two sources: • Jensen, C. R., Schultz, G. W., Bangerter, B. L. (1983). Applied kinesiology and biomechanics. New York: McGraw-Hill • Nigg, B. M. & Herzog, W. (1994). Biomechanics of the musculo-skeletal system. New York: Wiley & Sons • Nordin, M. & Frankel, V. H. (1989). Basic Biomechanics of the Musculoskeletal System. (2nd ed.). Philadelphia: Lea • & Febiger • Winter, D.A. (1990). Biomechanical and motor control of human movement. (2nd ed.). New York: Wiley & Sons

4. Introduction • Muscular system consists of three muscle types: cardiac, smooth, and skeletal • Skeletal muscle most abundant tissue in the human body (40-45% of total body weight) • Human body has more than 430 pairs of skeletal muscle; most vigorous movement produced by 80 pairs

5. Introduction (continued) • Skeletal muscles provide strength and protection for the skeleton, enable bones to move, provide the maintenance of body posture against gravity • Skeletal muscles perform both dynamic and static work

6. Muscle Structure • Structural unit of skeletal muscle is the multinucleated muscle cell or fiber (thickness: 10-100 m, length: 1-30 cm • Muscle fibers consist of myofibrils (sarcomeres in series: basic contractile unit of muscle) • Myofibrils consist of myofilaments (actin and myosin)

7. Microscopic-Macroscopic Structure of Skeletal Muscle

8. Muscle Structure (continued) • Composition of sarcomere • Z line to Z line ( 1.27-3.6 m in length) • Thin filaments (actin: 5 nm in diameter) • Thick filaments (myosin: 15 nm in diameter) • Myofilaments in parallel with sarcomere • Sarcomeres in series within myofibrils

10. Muscle Structure (continued) • Motor unit • Functional unit of muscle contraction • Composed of motor neuron and all muscle cells (fibers) innervated by motor neuron • Follows “all-or-none” principle – impulse from motor neuron will cause contraction in all muscle fibers it innervates or none

12. Smallest MU recruited at lowest stimulation frequency • As frequency of stimulation of smallest MU increases, force of its contraction increases • As frequency of stimulation continues to increase, but not before maximum contraction of smallest MU, another MU will be recruited • Etc.

13. Size Principle • Smallest motor units recruited first • Smallest motor units recruited with lower stimulation frequencies • Smallest motor units with relatively low levels of tension provide for finer control of movement • Larger motor units recruited later with increased frequency of stimulation and increased need for greater tension

14. Size Principle • Tension is reduced by the reverse process • Successive reduction of firing rates • Dropping out of larger units first

15. Muscle Structure (continued) • Motor unit • Vary in ratio of muscle fibers/motor neuron • Fine control – few fibers (e.g., muscles of eye and fingers, as few as 3-6/motor neuron), tetanize at higher frequencies • Gross control – many fibers (e.g., gastrocnemius,  2000/motor neuron), tetanize at lower frequencies • Fibers of motor unit dispersed throughout muscle

17. Motor Unit • Tonic units – smaller, slow twitch, rich in mitochondria, highly capillarized, high aerobic metabolism, low peak tension, long time to peak (60-120ms) • Phasic units – larger, fast twitch, poorly capillarized, rely on anaerobic metabolism, high peak tension, short time to peak (10-50ms)

18. Muscle Structure (continued) • Motor unit (continued) • Weakest voluntary contraction is a twitch (single contraction of a motor unit) • Twitch times for tension to reach maximum varies by muscle and person • Twitch times for maximum tension are shorter in the upper extremity muscles (≈40-50ms) than in the lower extremity muscles (≈70-80ms)

19. Motor Unit Twitch

20. Shape of Graded Contraction

21. Shape of Graded Contraction • Shape and time period of voluntary tension curve in building up maximum tension • Due to delay between each MU action potential and maximum twitch tension • Related to the size principle of recruitment of motor units • Turn-on times ≈ 200ms • Shape and time period of voluntary relaxation curve in reducing tension • Related to shape of individual muscle twitches • Related to the size principle in reverse • Due to stored elastic energy of muscle • Turn-off times ≈ 300ms

22. Force Production – Length-Tension Relationship • Force of contraction in a single fiber determined by overlap of actin and myosin (i.e., structural alterations in sarcomere) (see figure) • Force of contraction for whole muscle must account for active (contractile) and passive (series and parallel elastic elements) components

25. Parallel Connective Tissue • Parallel elastic component • Tissues surrounding contractile elements • Acts like elastic band • Slack when muscle at resting length of less • Non-linear force length curve • Sarcolemma, endomysium, perimysium, and epimysium forms parallel elastic element of skeletal muscle

29. Series Elastic Tissue • Tissues in series with contractile component • Tendon forms series elastic element of skeletal muscle • Endomysium, perimysium, and epimysium continuous with connective tissue of tendon • Lengthen slightly under isometric contraction (≈ 3-7% of muscle length) • Potential mechanism for stored elastic energy (i.e., function in prestretch of muscle prior to explosive concentric contraction)

30. Isometric Contraction

31. Musculotendinous Unit • Tendon and connective tissues in muscle (sarcolemma, endomysium, perimysium, and epimysium) are viscoelastic • Viscoelastic structures help determine mechanical characteristics of muscles during contraction and passive extension

32. Musculotendinous Unit (continued) • Functions of elastic elements of muscle • Keep “ready” state for muscle contraction • Contribute to smooth contraction • Reduce force buildup on muscle and may prevent or reduce muscle injury • Viscoelastic property may help muscle absorb, store, and return energy

33. Muscle Model

34. Force Production – Gradation of Contraction • Synchronization (number of motor units active at one time) – more  force potential • Size of motor units – motor units with larger number of fibers have greater force potential • Type of motor units – type IIA and IIB  force potential, type I  force potential

36. Force Production – Gradation of Contraction (continued) • Summation – increase frequency of stimulation, to some limit, increases the force of contraction

40. Force Production – Gradation of Contraction (continued) • Size principle – tension increase • Smallest motor units recruited first and largest last • Increased frequency of stimulation  force of contraction of motor unit • Low tension movements can be achieved in finely graded steps • Increases frequency of stimulation  recruitment of additional and larger motor units • Movements requiring large forces are accomplished by recruiting larger and more forceful motor units • Size principle – tension decrease • Last recruited motor units drop out first

41. Types of Muscle Contraction

42. Force Production – Length-Tension Relationship • Difficult to study length-tension relationship • Difficult to isolate single agonist • Moment arm of muscle changes as joint angle changes • Modeling may facilitate this type of study

43. Force Production – Load-Velocity Relationship • Concentric contraction (muscle shortening) occurs when the force of contraction is greater than the resistance (positive work) • Velocity of concentric contraction inversely related to difference between force of contraction and external load • Zero velocity occurs (no change in muscle length) when force of contraction equals resistance (no mechanical work)

45. Force Production – Load-Velocity Relationship • Eccentric contraction (muscle lengthening) occurs when the force of contraction is less than the resistance (negative work) • Velocity of eccentric contraction is directly related to the difference between force of contraction and external load

48. Force Production – Force-Time Relationship • In isometric contractions, greater force can be developed to maximum contractile force, with greater time • Increased time permits greater force generation and transmission through the parallel elastic elements to the series elastic elements (tendon) • Maximum contractile force may be generated in the contractile component of muscle in 10 msec; transmission to the tendon may take 300msec

50. 3-D Relationship of Force-Velocity-Length