Chapter 8 Skeletal Muscle: Structure and Function

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Chapter 8Skeletal Muscle Structure and Function

• EXERCISE PHYSIOLOGY
• Theory and Application to Fitness and
  Performance, 6th edition
• Scott K. Powers Edward T. Howley

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Skeletal Muscle

• Human body contains over 400 skeletal muscles
• 40-50 of total body weight
• Functions of skeletal muscle
• Force production for locomotion and breathing
• Force production for postural support
• Heat production during cold stress

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Connective Tissue Covering Skeletal Muscle

• Epimysium
• Surrounds entire muscle
• Perimysium
• Surrounds bundles of muscle fibers
• Fascicles
• Endomysium
• Surrounds individual muscle fibers

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Connective Tissue Surrounding Skeletal Muscle
Figure 8.1
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Microstructure of Skeletal Muscle

• Sarcolemma
• Muscle cell membrane
• Myofibrils
• Threadlike strands within muscle fibers
• Actin (thin filament)
• Myosin (thick filament)
• Sarcomere
• Includes Z-line, M-line, H-zone, A-band, I-band
• Sarcoplasmic reticulum
• Storage sites for calcium
• Transverse tubules

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Microstructure of Skeletal Muscle
Figure 8.2
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The Sarcoplasmic Reticulum and Transverse Tubules
Figure 8.3
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The Neuromuscular Junction

• Junction between motor neuron and muscle fiber
• Motor end plate
• Pocket formed around motor neuron by sarcolemma
• Neuromuscular cleft
• Short gap between neuron and muscle fiber
• Acetylcholine is released from the motor neuron
• Causes an end-plate potential (EPP)
• Depolarization of muscle fiber

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The Neuromuscular Junction
Figure 8.4
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Muscular Contraction

• The sliding filament model
• Muscle shortening occurs due to the movement of
  the actin filament over the myosin filament
• Formation of cross-bridges between actin and
  myosin filaments
• Power stroke
• Reduction in the distance between Z-lines of the
  sarcomere

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The Sliding Filament Theory
Figure 8.5
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The Relationships Among Troponin, Tropomyosin,
Myosin, and Calcium
Figure 8.6
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Energy for Muscle Contraction

• ATP is required for muscle contraction
• Myosin ATPase breaks down ATP as fiber contracts
• Sources of ATP
• Phosphocreatine (PC)
• Glycolysis
• Oxidative phosphorylation

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Sources of ATP for Muscle Contraction
Figure 8.7
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Excitation-Contraction Coupling

• Depolarization of motor end plate (excitation) is
  coupled to muscular contraction
• Action potential travels down T-tubules and
  causes release of Ca2 from SR
• Ca2 binds to troponin and causes position change
  in tropomyosin
• Exposing active sites on actin
• Strong binding state formed between actin and
  myosin
• Contraction occurs

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Muscle Excitation, Contraction, and Relaxation
Figure 8.9
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Steps Leading to Muscular Contraction
Figure 8.10
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Muscle Fatigue

• Decrease in muscle force production
• Reduced ability to perform work
• Contributing factors
• High-intensity exercise (60 sec)
• Accumulation of lactate, H, ADP, Pi, and free
  radicals
• Long-duration exercise (24 hours)
• Muscle factors
• Accumulation of free radicals
• Electrolyte imbalance
• Glycogen depletion
• Central Fatigue
• Reduced motor drive to muscle from CNS

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Muscle Fatigue
Figure 8.8
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Characteristics of Muscle Fiber Types

• Biochemical properties
• Oxidative capacity
• Number of capillaries, mitochondria, and amount
  of myoglobin
• Type of myosin ATPase
• Speed of ATP degradation
• Contractile properties
• Maximal force production
• Force per unit of cross-sectional area
• Speed of contraction (Vmax)
• Myosin ATPase activity
• Muscle fiber efficiency

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Characteristics of Individual Fiber Types

• Type IIx fibers
• Fast-twitch fibers
• Fast-glycolytic fibers
• Type IIa fibers
• Intermediate fibers
• Fast-oxidative glycolytic fibers
• Type I fibers
• Slow-twitch fibers
• Slow-oxidative fibers

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Characteristics of Muscle Fiber Types
Table 8.1
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Comparison of Maximal Shortening Velocities
Between Fiber Types
Figure 8.12
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Histochemical Staining of Fiber Type
Figure 8.11
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Fiber Types and Performance

• Nonathletes
• Have about 50 slow and 50 fast fibers
• Power athletes
• Sprinters
• Higher percentage of fast fibers
• Endurance athletes
• Distance runners
• Higher percentage of slow fibers

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Exercise-Induced Changes in Skeletal Muscles

• Strength training
• Increase in muscle fiber size (hypertrophy)
• Increase in muscle fiber number (hyperplasia)
• Endurance training
• Increase in oxidative capacity
• Alteration in fiber type with training
• Fast-to-slow shift
• Type IIx ? IIa
• Type IIa ? I with further training
• Seen with endurance and resistance training

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Effects of Endurance Training on Fiber Type
Figure 8.13
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Muscle Atrophy Due to Inactivity

• Loss of muscle mass and strength
• Due to prolonged bed rest, limb immobilization,
  reduced loading during space flight
• Initial atrophy (2 days)
• Due to decreased protein synthesis
• Further atrophy
• Due to reduced protein synthesis
• Atrophy is not permanent
• Can be reversed by resistance training
• During spaceflight, atrophy can be prevented by
  resistance exercise

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Age-Related Changes in Skeletal Muscle

• Aging is associated with a loss of muscle mass
• 10 muscle mass lost between age 2550 y
• Additional 40 lost between age 5080 y
• Also a loss of fast fibers and gain in slow
  fibers
• Also due to reduced physical activity
• Regular exercise training can improve strength
  and endurance
• Cannot completely eliminate the age-related loss
  in muscle mass

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Types of Muscle Contraction

• Isometric
• Muscle exerts force without changing length
• Pulling against immovable object
• Postural muscles
• Isotonic (dynamic)
• Concentric
• Muscle shortens during force production
• Eccentric
• Muscle produces force but length increases

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Muscle Actions
Type of exercise Muscle Muscle Action
Length Change ________________________
_________________________ Dynamic Concentric De
creases Eccentric Increases Static Isome
tric No Change
Table 8.3
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Isometric and Isotonic Contractions
Figure 8.14
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Speed of Muscle Contraction and Relaxation

• Muscle twitch
• Contraction as the result of a single stimulus
• Latent period
• Lasting 5 ms
• Contraction
• Tension is developed
• 40 ms
• Relaxation
• 50 ms
• Speed of shortening is greater in fast fibers
• SR releases Ca2 at a faster rate
• Higher ATPase activity

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Muscle Twitch
Figure 8.15
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Force Regulation in Muscle

• Force generation depends on
• Types and number of motor units recruited
• More motor units greater force
• Fast motor units greater force
• Initial muscle length
• Ideal length for force generation
• Increased cross-bridge formation
• Nature of the neural stimulation of motor units
• Frequency of stimulation
• Simple twitch
• Summation
• Tetanus

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Relationship Between Stimulus Strength and Force
of Contraction
Figure 8.16
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Length-Tension Relationships in Skeletal Muscle
Figure 8.17
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Simple Twitch, Summation, and Tetanus
Figure 8.18
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Force-Velocity Relationship

• At any absolute force the speed of movement is
  greater in muscle with higher percent of
  fast-twitch fibers
• The maximum velocity of shortening is greatest at
  the lowest force
• True for both slow and fast-twitch fibers

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Muscle Force-Velocity Relationships
Figure 8.19
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Force-Power Relationship

• At any given velocity of movement the power
  generated is greater in a muscle with a higher
  percent of fast-twitch fibers
• The peak power increases with velocity up to
  movement speed of 200-300 degreessecond-1
• Power decreases beyond this velocity because
  force decreases with increasing movement speed

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Muscle Force-Power Relationships
Figure 8.20
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Receptors in Muscle

• Provide sensory feedback to nervous system
• Tension development by muscle
• Account of muscle length
• Muscle spindle
• Golgi tendon organ

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Muscle Spindle

• Responds to changes in muscle length
• Consists of
• Intrafusal fibers
• Run parallel to normal muscle fibers (extrafusal
  fibers)
• Gamma motor neuron
• Stimulate intrafusal fibers to contract with
  extrafusal fibers (by alpha motor neuron)
• Stretch reflex
• Stretch on muscle causes reflex contraction
• Knee-jerk reflex

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Muscle Spindles
Figure 8.21
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Golgi Tendon Organ (GTO)

• Monitor tension developed in muscle
• Prevents muscle damage during excessive force
  generation
• Stimulation results in reflex relaxation of
  muscle
• Inhibitory neurons send IPSPs to muscle fibers

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Golgi Tendon Organ
Figure 8.22