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

2. Next Class Reading assignment Exam on anthropometry
Biomechanics of Skeletal Muscle by T. Lorenz and M. Campello (adapted from M. I. Pitman and L. Peterson; pp
EMG by W. Herzog, A. C. S. Guimaraes, and Y. T. Zhang; pp
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: 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 ( 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
Type of Contraction
Definition
Work
Concentric
Force of muscle contraction  resistance
Positive work; muscle moment and angular velocity of joint in same direction
Eccentric
Force of muscle contraction  resistance
Negative work; muscle moment and angular velocity of joint in opposite direction
Isokinetic
Force of muscle contraction = resistance; constant angular velocity; special case is isometric contraction
Isometric
Force of muscle contraction  resistance; series elastic component stretch = shortening of contractile element (few to 7% of resting length of muscle)
No mechanical work; physiological work

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

51. 3-D Relationship of Force-Velocity-Length

52. Effect of Muscle Architecture on Contraction
Fusiform muscle
Fibers parallel to long axis of muscle
Many sarcomeres make up long myofibrils
Advantage for length of contraction
Example: sartorius muscle
Force of contraction along long axis of muscle   of force of contraction of all muscle fibers
Tends to have smaller physiological cross sectional area
(see figure)

53. Fusiform Fiber Arrangement
Fa = force of contraction of muscle fiber parallel to longitudinal axis of muscle
Fa = sum of all muscle fiber contractions parallel to long axis of muscle Fa

54. Effect of Muscle Architecture on Contraction (continued)
Pennate muscle
Fibers arranged obliquely to long axis of muscle (pennation angle)
Uni-, bi-, and multi-pennate
Advantage for force of contraction
Example: rectus femoris (bi-pennate)
Tends to have larger physiological cross sectional area

55. Pennate Fiber Arrangement
Fa = force of contraction of muscle fiber parallel to longitudinal axis of muscle
Fm = force of contraction of muscle fiber
 = pennation angle
Fa = (cos )(Fm)
Fa = sum of all muscle fiber contractions parallel to long axis of muscle Fa Fm 

58. Effect of Muscle Architecture on Contraction (continued)
Force of muscle contraction proportional to physiological cross sectional area (PCSA); sum of the cross sectional area of myofibrils
Velocity and excursion (working range or amplitude) of muscle is proportional to length of myofiblril

60. Muscle Fiber Types Type I Slow-Twitch Oxidative (SO) Type IIA
Fast-Twitch Oxidative-Glycolytic (FOG)
Type IIB
Fast-Twitch Glycolytic (FG)
Speed of contraction
Slow
Fast
Primary source of ATP production
Oxidative phosphorylation
Anaerobic glycolysis
Glycolytic enzyme activity
Low
Intermediate
High
Capillaries
Many
Few
Myoglobin content
Glycogen content
Fiber diameter
Small
Large
Rate of fatigue

61. Muscle Fiber Types (continued)
Smaller slow twitch motor units are characterized as tonic units, red in appearance, smaller muscle fibers, fibers rich in mitochondria, highly capillarized, high capacity for aerobic metabolism, and produce low peak tension in a long time to peak (60-120ms). 
Larger fast twitch motor units are characterized as phasic units, white in appearance, larger muscle fibers, less mitochondria, poorly capillarized, rely on anaerobic metabolism, and produce large peak tensions in shorter periods of time (10-50ms).

62. Muscle Fiber Types (continued)
Nerve innervating muscle fiber determines its type; possible to change fiber type by changing innervations of fiber
All fibers of motor unit are of same type
Fiber type distribution in muscle genetically determined
Average population distribution:
50-55% type I
30-35% type IIA
15% type IIB

63. Muscle Fiber Types (continued)
Fiber composition of muscle relates to function (e.g., soleus – posture muscle, high percentage type I)
Muscles mixed in fiber type composition
Natural selection of athletes at top levels of competition

64. Electrical Signals of Muscle Fibers
At rest, action potential of muscle fiber  -90 mV;caused by concentrations of ions outside and inside fiber (resting state)
With sufficient stimulation, potential inside cell raised to  mV (depolarization); associated with transverse tubular system and sarcoplasmic reticulum; causes contraction of fiber
Return to resting state (repolarization)
Electrical signals from the motor units (motor unit action potential, muap) can be recorded (EMG) via electrodes