SKELETAL MUSCLE PHYSIOLOGY

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SKELETAL MUSCLE PHYSIOLOGY

• Abraham D. Lee, Ph.D.,P.T.
• Department of Physical Therapy
• Office Collier Building 4206
• Phone [phone]
• Email abraham.lee2_at_utoledo.edu

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Contents
1. Muscle structure organization 2. Muscle
fiber type 3. Muscle action 4. Muscle
mechanics 5. Motor unit and its recruitment 6.
Local muscle control 7. Muscle plasticity 8.
Summary
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Muscle organization

• Epimysium wraps an entire muscle
• Perimysium wraps a bundle of muscle fibers. This
  bundle is called fascicle or fasciculus
• Endomysium wraps an individual muscle fiber
• Sarcolemma muscle membrane
• Myofibrils contractile filaments

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Myofibrils

• Thin filament
• Actin filaments
• Troponin
• Tropomyosin
• Thick filament
• Myosin 4 light chains and 2 heavy chains
• Heavy chains
• Myosin head region heavy meromyosin
• Myosin tail region light memromysin

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Muscle pennation

• Longitudinal (non-pennated) architecture muscle
  fibers in parallel to the muscle force generating
  axis
• Example biceps brachii, sartorius muscle
• In these muscles fibers are said to be fusiform
  or spindle shaped.
• Pennate architecture muscle fibers are
  oriented at an angle or multiple angles relative
  to force-generating axis.
• Unipennate soleus-25 degree vastus medialis-5
  degree
• Bipennate gastrocnemius, rectus femoris
• Multipennate deltoid

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Effect of pennation

• Force loss

Space saving
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Comparison b/n non-pennated pennated muscle

• Contraction
• Fiber packing
• w/ given volume
• Force loss due
• to pennation
• fiber
• Muscle force
• Production
• CSA

Non-pennated Pennated
Fast Slow Less More No Yes Less Mor
e Less Greater Less Greater
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Muscle Fiber Type
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Muscle fiber type

• Type I,
• Slow-oxidative (SO) fibers
• Type IIa,
• Fast-oxidative-glycolytic (FOG) fibers
• Type IIb,
• Fast-glycolytic (FG) fibers

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Characteristics of different fibers

• Type I Type IIa Type IIb
• H H/M L
• H H/M L
• A AAN AN
• L H HH
• L H HH
• H M L

Mitochondria Resistance to fatigue Energy ATPase
activity Vmax Efficiency
L low H high M moderate A aerobic AN
anaerobic HH highest
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Muscle composition in athletes

• Type I Type IIa IIb
• 70-80 20-30
• 25-30 70-75
• 45-55 45-55
• 47-53 47-53

Distance runners Track sprinters Weight
lifters Non-athletes
Will fiber type change with training?
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Muscle Action

• Excitation-contraction coupling
• Type of muscle action

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Excitation-Contraction Coupling

• Nerve impulse generation and propagation
• Neuromuscular junction transmission
• Muscle action potential propagation
• Ca2 release from SR
• Ca2 binding to troponin
• Interaction of myosin head and actin
• Cross bridge moves tension development
• Ca2 taken up to SR
• Ca2 removal from troponin
• Relaxation

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E-C coupling
DHPR dihydropyridine receptors RyR ryanodine
receptor
Other possible mechanism Inositol
1,4,5-triphosphate (InsP3) InsP3 receptor
activation Ca2 release from SR May play
a role in slow twitch muscle in developmental
stage (Talon et al., Am. J. Physiol 282
R1164-R1173, 2002)
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E-C coupling
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Sliding Filament Theory
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Changes during shortening muscle action

• Sarcomere length (distance between two adjacent Z
  lines) shortens
• A band no change
• I band shortens
• H zone shortens

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Different type of muscle action (contraction)

• Isometric action
• Isotonic action (dynamic action)
• Concentric action
• Eccentric action

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Muscle mechanics

• It deals with how muscle force is generated and
  regulated.

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Factors that affect muscle force generation

• Rate of muscle stimulation
• Muscle length
• Joint angle
• Speed of action (speed of contraction)
• Muscle fiber type
• of MU recruitment

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Rate of Muscle Stimulation

• Twitch
• Tetanus

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Muscle twitch
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Muscle tetanus
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Effect of Muscle Length
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Force-Length Relationship

• Isolated muscle
• In vivo human muscles

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Force-Length Relationship

• Isolated muscle

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Force-Length Relationship

• In vivo human muscles
• Two things are considered muscle length and
  joint angle
• In general, a group of muscles produces more
  force (torque) when muscles are lengthened before
  contraction. But some muscles do not follow this
  rule.

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Shoulder muscles
Shoulder flexors (anterior deltoid) causes to
flex shoulder joint Shoulder extensors(posterior
deltoid) causes to extend shoulder joint
180
135
90
45
40
0
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Knee flexors

• A person is lying on the stomach (prone position)

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120
45
0
Trunk
Thigh
Lower leg
Knee joint
Hip joint
Knee flexors (hamstring muscles) causes to flex
knee joint
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Hip flexors

• A person is lying on the back (supine position)

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120
45
0
Lower leg
Trunk
Thigh
hip joint
Knee joint
Hip flexors (iliopsoas, sartorius) causes to
flex hip joint
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Knee extensors

• A person is sitting on the bench

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45
120
90
Knee extensors (quadriceps muscles) causes to
extend knee joint
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Elbow flexors
Elbow flexors (biceps brachii) causes to flex
elbow joint
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Force arm distance
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Effect of Velocity (Speed of Action)
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Force-velocity curve
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Effect of Muscle Fiber
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Effect of muscle fiber type on force
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Muscle Power

• Need to consider two factors
• 1. Muscle force
• 2. Speed of action

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Power-Velocity Relationship

• Power work/time (force x distance)/time
  force x speed

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Effect of muscle fiber type on power
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Factors that affect muscle force/power generation

• Rate of muscle stimulation
• Muscle length
• Joint angle
• Speed of action
• Muscle fiber type
• of MU recruitment

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Motor Unit
How does an individual generate appropriate
force for a given task?
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Motor Unit (MU)
Functional unit of movement
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Motor Unit (MU)

• MU consists of
• Single ?-motor neuron
• Muscle fibers innervated by
• the ?-motor neuron

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Motor Unit (MU)

• Fast fatigable MU (FF)
• High twitch tension
• High fatigue index
• Fast fatigue resistant MU (FR)
• Intermediate twitch tension
• Intermediate fatigue index
• Slow MU (S)
• Low twitch tension
• Low fatigue index

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Reasons for different twitch tension in different
MU

• Depends on number of muscle fibers and fiber size
  
• muscle fiber FFgtFRgt S
• Size of fiber FFgtFRgtS

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Relationship b/n MU Fiber type

• MU Fiber type
• FF Fast glycolytic
• FR Fast oxidative
• S Slow oxidative

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Motor Unit

• Muscle neuron fibers/MU
• Biceps brachii 774 750
• Gastrocnemius 580 1720
• First lumbrical 98 110

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Motor Unit Recruitment

• Follows the size principle
• Small neuron cell body and axon activated first
• Larger cell body and axon recruited later
• Example
• S MU FR MU FF MU

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of effort level (Intensity of exercise)
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Gradation of Muscle Strength

• By increasing of MU recruited
• By increasing frequency of stimulation

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Local Control of Muscle Action

• Muscle spindle muscle length monitor
• Consists of 1) afferent nerves, 2) intrafusal
  fibers 3) ?(gamma)-motor neurons
• Golgi Tendon Organ muscle tension monitor

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Structure of muscle spindle
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Action of muscle spindle
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Nerve impulse pattern of afferent nerves
Rest Stretch Contraction Return to
rest
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Speed of stretch on impulse discharge pattern
Clinical implications for individuals with
spastic muscle?
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Golgi Tendon Organ

• GTOltspindle in given muscle
• Composed of network of unmyelinated
• nerve fibers enclosed by fine capsule
• Activated by either muscle stretch or
• muscle contraction. More sensitive to
• muscle contraction.
• Activates inhibitory interneuron in spinal
• cord, which, in turn, inhibits ?-motor
• neuron of contracting muscle (agonists).

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Impulse discharge pattern of GTO during stretch
and contraction
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Plasticity of Muscle

• Metabolic and morphological changes to changes in
  stimulus
• Increased stimulus exercise training
• Decreased stimulus non-weight bearing, bed rest
  and aging

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Endurance training

• Mode jogging, running, cycling, swimming, etc
• Adaptations
• of mitochondria
• size of mitochondria
• Oxidative enzyme activities
• Krebs cycle, beta-oxidation, ETS
• Some glycolytic enzymes
• Capillary density

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Resistance training

• Strength
• Neural factor
• Muscle fiber enlargement (hypertrophy)

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5-6 month resistance training using triceps
brachii
MacDougall et al, EJAP 4325-34
6 wk
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Resistance Training
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Limb suspension(Non-weight bearing)
Berg et al., J. Appl. Physiol. 701882-1885, 1991
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Limb unloading on muscle strength and X-area
X-area
Knee extensor Strength
Dudley et al, in ACSMs Resource Manual, p.201
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Selective muscle atrophy with non-weight bearing
Dudley et al, in ACSMs Resource Manual, p.201
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Bed Rest
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Muscle strength change with bed rest
(soleus and gastroc.).)
Dudley et al, in ACSMs Resource Manual, p.203
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Changes in skeletal muscles with aging

• of muscle fibers
• Muscle area
• Fiber type distribution
• Muscle strength

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McArdle et al, in Exercise Physiology, p639
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Muscle fiber distribution with aging
Fiber Area
Age Type II Type I Type II 26.1 59.5 2944 36
63 35.3 63.2 2854 3509 42.6 51.8 3133 3361
54.5 48.3 2877 2802 61.6 45.0 2264 2120
Larson et al, J. Appl. Physiol 46451-456, 1979
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Muscle strength with aging

• A decline in muscle strength is associated with a
  decrease in muscle mass.
• A decline in lower extremity muscle strength is
  related to poor functional performance
• walking ability, balance, stair-climbing ability,
  falls

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Trainability of skeletal muscles with aging

• Frontera et al, J. Appl. Physiol., 641038-1044,
  1988
• Untrained old men (60-72 yrs)
• 8 reps/set, 3 sets/day, 3days/week at 80 of
• 1 RM for 12 weeks training
• Thigh muscle X-area
• Knee extension and flexion strength.

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Leg strength
Frontera et al
1 RM max
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X-area of quadriceps
Frontera et al
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Physicians Role for Physical Activity
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Summary

• Know followings names functions
• Muscle structure connective tissues, pennation,
• myofibrils
• Muscle fiber characteristics
• Muscle action E-C coupling
• Sliding filament theory changes during
  contraction
• Muscle mechanics force power-length-velocity
• MU elements, function, characteristics.
• Muscle action monitor muscle spindle and GTO
• Changes in muscle in training
• Changes in muscle w/ suspension, bed rest aging

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The End