Scott K. Powers • Edward T. Howley
Theory and Application to Fitness and Performance
SEVENTH EDITION
Chapter
Skeletal Muscle:
Structure and Function
Presentation prepared by:
Brian B. Parr, Ph.D.
University of South Carolina Aiken
Copyright ©2009 The McGraw-Hill Companies, Inc. Permission required for reproduction or display outside of classroom use.
Chapter 8
Objectives
1. Draw and label the microstructure of skeletal
muscle.
2. Define satellite cells. How do these cells differ
from the nuclei located within skeletal muscle
fibers?
3. List the chain of events that occur during
muscular contraction.
4. Define both dynamic and static exercise. What
types of muscle action occur during each form
of exercise?
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Chapter 8
Objectives
5. What three factors determine the amount of
force produced during muscular contraction?
6. List the three human skeletal muscle fiber
types. Compare and contrast the major
biochemical and mechanical properties of each.
7. How does skeletal muscle fiber type influence
athletic performance?
8. Graph and describe the relationship between
movement velocity and the amount of force
exerted during muscular contraction.
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Chapter 8
Outline
 Structure of Skeletal
 Alterations in Skeletal
Muscle
Muscle Due to
 Neuromuscular Junction
Exercise, Inactivity,
 Muscular Contraction
and Aging
Overview of the Sliding
Exercise-Induced
Filament Model
Changes in Skeletal
Energy for Contraction
Muscles
Regulation of
Muscle Atrophy Due
Excitation-Contraction
to Inactivity
Coupling
Age-Related
 Fiber Types
Changes in Skeletal
Biochemical and
Muscle
Contractile
Characteristics of
Skeletal Muscle
Characteristics of
Individual Fiber Types
Fiber Types and
Performance
 Muscle Actions
 Speed of Muscle
Action and Relaxation
 Force Regulation in
Muscle
 Force-Velocity/ForcePower Relationships
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Chapter 8
Structure of Skeletal Muscle
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
• Muscle actions
– Flexors
• Decrease joint angle
– Extensors
• Increase joint angles
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Chapter 8
Structure of Skeletal Muscle
Connective Tissue Covering Skeletal Muscle
• Epimysium
– Surrounds entire muscle
• Perimysium
– Surrounds bundles of muscle fibers
• Fascicles
• Endomysium
– Surrounds individual muscle fibers
• External lamina
– Just below endomysium
• Sarcolemma
– Muscle cell membrane
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Chapter 8
Structure of Skeletal Muscle
Connective Tissue Surrounding
Skeletal Muscle
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Figure 8.1
Chapter 8
Structure of Skeletal Muscle
Satellite Cells
• Play role in muscle growth and repair
– Increase number of nuclei
• Myonuclear domain
– Cytoplasm surrounding each nucleus
– Each nucleus can support a limited
myonuclear domain
• More nuclei allow for greater protein
synthesis
• Important for adaptations to strength training
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Chapter 8
Structure of Skeletal Muscle
Microstructure of Muscle Fibers
• Myofibrils
– Contain contractile proteins
• Actin (thin filament)
• Myosin (thick filament)
• Sarcomere
– Includes Z line, M line, H zone, A band, I band
• Sarcoplasmic reticulum
– Storage sites for calcium
– Terminal cisternae
• Transverse tubules
– Extend from sarcolemma to sarcoplasmic
reticulum
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Chapter 8
Structure of Skeletal Muscle
Microstructure of Skeletal Muscle
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Figure 8.2
Chapter 8
Structure of Skeletal Muscle
The Sarcoplasmic Reticulum and
Transverse Tubules
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Figure 8.3
Chapter 8
Neuromuscular Junction
Neuromuscular Junction
• Junction between motor neuron and muscle fiber
– Motor unit
• Motor neuron and all fibers it innervates
• 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
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Chapter 8
Neuromuscular Junction
The Neuromuscular Junction
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Figure 8.4
Chapter 8
Neuromuscular Junction
In Summary
 The human body contains over 400 voluntary
skeletal muscles, which constitute 40% to 50% of the
total body weight. Skeletal muscle performs three
major functions: (1) force production for locomotion
and breathing, (2) force production for postural
support, and (3) heat production during cold stress.
 Individual muscle fibers are composed of hundreds
of threadlike protein filaments called myofibrils.
Myofibrils contain two major types of contractile
protein: (1) actin (part of the thin filaments) and (2)
myosin (major component of the thick filaments).
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Chapter 8
Neuromuscular Junction
In Summary
 The region of cytoplasm surrounding an individual
nucleus is termed the myonuclear domain. The
importance of the myonuclear domain is that a
single nucleus is responsible for the gene
expression for its surrounding cytoplasm.
 Motor neurons extend outward from the spinal cord
and innervate individual muscle fibers. The site
where the motor neuron and muscle cell meet is
called the neuromuscular junction. Acetylcholine is
the neurotransmitter that stimulates the muscle fiber
to depolarize, which is the signal to start the
contractile process.
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Chapter 8
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
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Chapter 8
Muscular Contraction
The Sliding
Filament
Theory of
Contraction
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Figure 8.5
Chapter 8
Muscular Contraction
The Relationships Among Troponin,
Tropomyosin, Myosin, and Calcium
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Figure 8.6
Chapter 8
Muscular Contraction
Energy for Muscle Contraction
• ATP is required for muscle contraction
– Myosin ATPase breaks down ATP as fiber
contracts
– ATP  ADP + Pi
• Sources of ATP
– Phosphocreatine (PC)
– Glycolysis
– Oxidative phosphorylation
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Chapter 8
Muscular Contraction
Sources of ATP for Muscle Contraction
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Figure 8.7
Chapter 8
Muscular Contraction
A Closer Look 8.1
Muscle Fatigue
• Decrease in muscle force production
– Reduced ability to perform work
• Contributing factors:
– High-intensity exercise (~60 seconds)
• 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
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Chapter 8
Muscular Contraction
Muscular Fatigue
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Figure 8.8
Chapter 8
Muscular Contraction
Excitation-Contraction Coupling
• Depolarization of motor end plate
(excitation) is coupled to muscular
contraction
– Action potential travels down transverse
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
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Chapter 8
Muscular Contraction
Step-by-Step Summary of ExcitationContraction Coupling
• Excitation
1. Action potential in motor neuron causes
release of acetylcholine into synaptic cleft.
2. Acetylcholine binds to receptors on motor
end plate, leads to depolarization that is
conducted down transverse tubules, which
causes release of Ca+2 from sarcoplasmic
reticulum (SR).
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Chapter 8
Muscular Contraction
Step-by-Step Summary of ExcitationContraction Coupling
•
Contraction
1. At rest, myosin cross-bridges in weak binding state.
2. Ca+2 binds to troponin, causes shift in tropomyosin
to uncover active sites, and cross-bridge forms
strong binding state.
3. Pi released from myosin, cross-bridge movement
occurs.
4. ADP released from myosin.
5. ATP attaches to myosin, breaking the cross-bridge
and forming weak binding state. Then ATP binds to
myosin, broken down to ADP+Pi, which energizes
myosin. Continues as long as Ca+2 and ATP are
present.
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Chapter 8
Muscular Contraction
Muscle
Excitation,
Contraction,
and Relaxation
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Figure 8.9
Chapter 8
Muscular Contraction
Steps Leading to Muscular
Contraction
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Figure 8.10
Chapter 8
Muscular Contraction
In Summary


The process of muscular contraction can be best explained by
the sliding filament model, which proposes that muscle
shortening occurs due to movement of the actin filament over
the myosin filament.
The steps in muscular contraction are:
a. The nerve impulse travels down the transverse tubules and
reaches the sarcoplasmic reticulum, and Ca+2 is released.
b. Ca+2 binds to the protein troponin.
c. Ca+2 binding to troponin causes a position change in
tropomyosin away from the “active sites” on the actin
molecule and permits a strong binding state between actin
and myosin.
d. Muscular contraction occurs by multiple cycles of crossbridge activity. Shortening will continue as long as energy is
available and Ca+2 is free to bind to troponin.
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Chapter 8
Muscular Contraction
In Summary
 When neural activity ceases at the
neuromuscular junction, Ca+2 is
removed from the sarcoplasmic
reticulum by the Ca+2 pump. This
results in tropomyosin moving to
cover the active site on actin, and the
muscle relaxes.
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Chapter 8
Fiber Types
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
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Chapter 8
Fiber Types
How Are Muscle Fibers Typed?
• Muscle biopsy
– Small piece of muscle removed
– May not be representative of entire body
• Staining for type of myosin ATPase
– Type I fibers appear darkest
– IIa fibers lightest
– IIx fibers in between
• Immunohistochemical staining
– Selective antibody binds to unique myosin proteins
– Fiber types differentiated by color difference
• Gel electrophoresis
– Identify myosin isoforms specific to different fiber
types
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Chapter 8
Fiber Types
Immunohistochemical Staining of Skeletal Muscle
Blue = Type I fibers
Green = Type IIa fibers
Black = Type IIx fibers
Red = dystrophin (protein in sarcolemma)
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Figure 8.11
Chapter 8
Fiber Types
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
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Chapter 8
Fiber Types
Characteristics of Muscle Fiber
Types
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Comparison of Maximal Fiber Types
Shortening Velocities Between
Fiber Types
Chapter 8
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Figure 8.12
Chapter 8
Fiber Types
Do Fast Fibers Exert More Force Than Slow
Fibers?
• Maximal force per cross-sectional area
– 10–20% higher in fast fibers (IIa and IIx)
compared to slow (Type I) fibers
• Force production related to number of myosin
cross-bridges in strong binding state
– Fast fibers contain more cross-bridges per
cross-sectional area
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Chapter 8
Fiber Types
In Summary
 Human skeletal muscle fiber types can be
divided into three general classes of fibers
based on their biochemical and contractile
properties properties. Two categories of fast
fibers exist, type IIx and type IIa. One type of
slow slow fiber exists, type I fibers.
 The biochemical and contractile properties
characteristic of all muscle fiber types are
summarized in table 8.1.
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Chapter 8
Fiber Types
In Summary
 Although classifying skeletal muscle fibers
into three general groups is a convenient
system to study the properties of muscle
fibers, it is important to appreciate that
human skeletal muscle fibers exhibit a wide
range of contractile and biochemical
properties. That is, the biochemical and
contractile properties of type IIx, type IIa,
and type I fibers represent a continuum
instead of three neat packages.
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Chapter 8
Fiber Types
Fiber Types and Performance
• Nonathletes
– Have approximately 50% slow and 50% fast fibers
• Power athletes
– Sprinters
– Higher percentage of fast fibers
• Endurance athletes
– Distance runners
– Higher percentage of slow fibers
• Fiber type is not the only variable that determines
success in an athletic event
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Chapter 8
Fiber Types
Distribution of Fiber Type in
Athletes
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Chapter 8
Fiber Types
In Summary
 Successful power athletes (e.g., sprinters)
generally possess a large percentage of fast
muscle fibers and, therefore, a low
percentage of slow, type I fibers.
 In contrast to power athletes, endurance
athletes (e.g., marathoners) typically
possess a high percentage of slow muscle
fibers and a low percentage of fast fibers.
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Chapter 8
Alterations in Skeletal Muscle Due to Exercise, Inactivity, and Aging
Exercise-Induced Changes in
Skeletal Muscles
• Strength training
– Increase in muscle fiber size (hypertrophy)
– Increase in muscle fiber number (hyperplasia)
• Limited evidence in humans
• 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
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Chapter 8
Alterations in Skeletal Muscle Due to Exercise, Inactivity, and Aging
Effects of Endurance Training on
Fiber Type
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Figure 8.13
Chapter 8
Alterations in Skeletal Muscle Due to Exercise, Inactivity, and Aging
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
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Chapter 8
Alterations in Skeletal Muscle Due to Exercise, Inactivity, and Aging
Age-Related Changes in Skeletal
Muscle
• Aging is associated with a loss of muscle mass
– 10% muscle mass lost between age 25–50 years
– Additional 40% lost between age 50–80 years
– 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
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Chapter 8
Alterations in Skeletal Muscle Due to Exercise, Inactivity, and Aging
In Summary


Both endurance and resistance exercise training
have been shown to promote a fast-to-slow shift in
skeletal muscle fiber types. However, this exerciseinduced shift in fiber type is typically small and
does not result in a complete transformation of all
fast fibers (type II) into slow fibers (type I).
Prolonged periods of muscle disuse (bed rest, limb
immobilization, etc.) result in muscle atrophy. This
inactivity-induced atrophy results in a loss of
muscle protein due to a reduction in protein
synthesis and an increase in the rate of muscle
protein breakdown.
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Chapter 8
Alterations in Skeletal Muscle Due to Exercise, Inactivity, and Aging
In Summary
 Aging is associated with a loss of muscle
mass. This age-related loss of muscle mass
is low from age 25 to 50 years but increases
rapidly after 50 years of age.
 Regular exercise training can improve
skeletal muscle strength and endurance in
the elderly but cannot completely eliminate
the age-related loss of muscle mass.
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Chapter 8
Muscle Actions
Types of Muscle Action
• 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
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Chapter 8
Muscle Actions
Muscle Actions
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Chapter 8
Muscle Actions
Isometric and Isotonic Muscle Actions
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Figure 8.14
Chapter 8
Speed of Muscle Action and Relaxation
Speed of Muscle Action 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
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Chapter 8
Speed of Muscle Action and Relaxation
Muscle Twitch
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Figure 8.15
Chapter 8
Force Regulation in Muscle
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
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Chapter 8
Force Regulation in Muscle
Relationship Between Stimulus Strength and
Force of Contraction
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Figure 8.16
Chapter 8
Force Regulation in Muscle
Length-Tension
Relationships in
Skeletal Muscle
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Figure 8.17
Chapter 8
Force Regulation in Muscle
Simple Twitch, Summation, and Tetanus
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Figure 8.18
Chapter 8
Force-Velocity / Power-Velocity Relationships
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
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Chapter 8
Force-Velocity / Power-Velocity Relationships
Muscle Force-Velocity Relationships
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Figure 8.19
Chapter 8
Force-Velocity / Power-Velocity Relationships
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
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Chapter 8
Force-Velocity / Power-Velocity Relationships
Muscle Force-Power Relationships
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.
Figure 8.20
Chapter 8
Force-Velocity / Power-Velocity Relationships
In Summary



The amount of force generated during muscular
contraction is dependent on the following factors:
(1) types and number of motor units recruited, (2)
the initial muscle length, and (3) the nature of the
motor units’ neural stimulation.
The addition of muscle twitches is termed
summation. When the frequency of neural
stimulation to a motor unit is increased, individual
contractions are fused together in a sustained
contraction called tetanus.
The peak force generated by muscle decreases as
the speed of movement increases. However, in
general, the amount of power generated by a
muscle group increases as a function of movement
velocity.
Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.