Chapter 8
Skeletal Muscle: Structure
and Function
EXERCISE PHYSIOLOGY
Theory and Application to Fitness and Performance,
6th edition
Scott K. Powers & Edward T. Howley
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
Connective Tissue Covering Skeletal
Muscle
• Epimysium
– Surrounds entire muscle
• Perimysium
– Surrounds bundles of muscle fibers
• Fascicles
• Endomysium
– Surrounds individual muscle fibers
Connective Tissue Surrounding
Skeletal Muscle
Figure 8.1
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
Microstructure of Skeletal Muscle
Figure 8.2
The Sarcoplasmic Reticulum and
Transverse Tubules
Figure 8.3
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
The Neuromuscular Junction
Figure 8.4
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
The
Sliding
Filament
Theory
Figure 8.5
The Relationships Among Troponin,
Tropomyosin, Myosin, and Calcium
Figure 8.6
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
Sources of ATP for Muscle
Contraction
Figure 8.7
Excitation-Contraction Coupling
• Depolarization of motor end plate (excitation)
is coupled to muscular contraction
– Action potential travels down T-tubules and
causes release of Ca+2 from SR
– Ca+2 binds to troponin and causes position
change in tropomyosin
• Exposing active sites on actin
– Strong binding state formed between actin and
myosin
– Contraction occurs
Muscle
Excitation,
Contraction,
and
Relaxation
Figure 8.9
Steps Leading to Muscular
Contraction
Figure 8.10
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 (2–4 hours)
• Muscle factors
– Accumulation of free radicals
– Electrolyte imbalance
– Glycogen depletion
• Central Fatigue
– Reduced motor drive to muscle from CNS
Muscle Fatigue
Figure 8.8
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
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
Characteristics of Muscle Fiber Types
Table 8.1
Comparison of Maximal Shortening
Velocities Between Fiber Types
Figure 8.12
Histochemical Staining of Fiber Type
Type I
Type IIa
Type IIx
Figure 8.11
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
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
Effects of Endurance Training on
Fiber Type
Figure 8.13
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
Age-Related Changes in Skeletal
Muscle
• Aging is associated with a loss of muscle
mass
– 10% muscle mass lost between age 25–50 y
– Additional 40% lost between age 50–80 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
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
Muscle Actions
Type of exercise
Muscle
Muscle
Action
Length Change
_________________________________________________
Dynamic
Concentric
Decreases
Eccentric
Increases
Static
Isometric
No Change
Table 8.3
Isometric
and Isotonic
Contractions
Figure 8.14
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 Ca+2 at a faster rate
– Higher ATPase activity
Muscle Twitch
Figure 8.15
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
Relationship Between Stimulus
Strength and Force of Contraction
Figure 8.16
LengthTension
Relationships
in Skeletal
Muscle
Figure 8.17
Simple Twitch, Summation, and
Tetanus
Figure 8.18
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
Muscle Force-Velocity Relationships
Figure 8.19
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
degrees•second-1
– Power decreases beyond this velocity because
force decreases with increasing movement
speed
Muscle Force-Power
Relationships
Figure 8.20
Receptors in Muscle
• Provide sensory feedback to nervous system
– Tension development by muscle
– Account of muscle length
• Muscle spindle
• Golgi tendon organ
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
Muscle Spindles
Figure 8.21
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
Golgi Tendon Organ
Figure 8.22