?Required readings:

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

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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

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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

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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

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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)

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Microscopic-Macroscopic Structure of Skeletal
Muscle
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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

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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

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• 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.

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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

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Size Principle

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

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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

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• 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)

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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)

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Motor Unit Twitch
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Shape of Graded Contraction
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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

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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

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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

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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)

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Isometric Contraction
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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

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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

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Muscle Model
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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

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Force Production Gradation of Contraction
(continued)

• Summation increase frequency of stimulation, to
  some limit, increases the force of contraction

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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

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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 Positive work muscle moment and angular velocity of joint in same direction
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
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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

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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)

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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

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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

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3-D Relationship of Force-Velocity-Length
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3-D Relationship of Force-Velocity-Length
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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)

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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
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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

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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
?
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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

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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 Fast
Primary source of ATP production Oxidative phosphorylation Oxidative phosphorylation Anaerobic glycolysis
Glycolytic enzyme activity Low Intermediate High
Capillaries Many Many Few
Myoglobin content High High Low
Glycogen content Low Intermediate High
Fiber diameter Small Intermediate Large
Rate of fatigue Slow Intermediate Fast
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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).

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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

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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

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Electrical Signals of Muscle Fibers

• At rest, action potential of muscle fiber ? -90
  mVcaused by concentrations of ions outside and
  inside fiber (resting state)
• With sufficient stimulation, potential inside
  cell raised to ? 30-40 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

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