Download Chapter 3: "Muscle Structure and Function"

In This Chapter:
Types of Human Muscle 62
Skeletal Muscle 62
Properties 62
Muscle Teamwork 64
Structure 64
Muscle Fibre Types 69
Nerve–Muscle Interaction 70
Motor Unit 70
Intra-muscle Coordination 72
Inter-muscle Coordination 72
Sport-specific Training 72
Muscle’s Adaptation to Strength Training 74
Summary 75
CHAPTER 3
CHAPTER
Let’s explore muscle structure and function....
3
Muscle Structure
and Function
After completing this chapter you should be able to:
n
describe the macro and micro structures of skeletal muscle;
n
describe muscle contraction and explain the sliding filament theory;
n
demonstrate an understanding of nerve–muscle interaction;
n
differentiate among types of muscle fibres;
n
describe group action of muscles;
n
discuss muscle’s adaptation to strength training.
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62
Structure
determines function. This is a
statement that defines the essence of human
anatomy and physiology. Muscle tissue – the
contraction specialist – provides a prime example
of how the structure of a tissue is well-adapted to
perform a specific function. With approximately
660 muscles in the adult human body, comprising
nearly half of our body weight, the importance of
muscular activity is obvious. The various structures
and types of muscle tissue support numerous life
functions, such as ventilation, physical activity
and exercise, digestion, and of course, pumping
life-sustaining blood throughout the body via
specialized cardiac muscle. The focus in this
chapter will be on skeletal muscle, which permits
voluntary movement, and is unique among other
types of muscle in other important ways.
We often look at muscle as a single entity,
but in so doing, fail to recognize the molecular
complexity and hierarchical structure of this
tissue. This specialized structure enables muscle to
shorten and develop tension, allowing a myriad of
human movements to occur. From movements as
simple as waving good-bye or picking up a book,
to more complex actions such as those required
in athletics, the muscular system is vital to our
daily functioning. But how does muscle activity
integrate with the nervous system to produce
movement? And what are the fundamental
contractile properties of muscle?
Types of Human Muscle
On the basis of their structures, contractile
properties, and control mechanisms there are three
types of muscle in the human body: (1) skeletal
muscle; (2) smooth muscle; and (3) cardiac muscle.
Most skeletal muscle is attached to bone
and its contraction is responsible for supporting
and moving the skeleton. The contraction of
skeletal muscle is initiated by impulses in the
motor neurons to the muscle and is usually under
voluntary control.
Smooth muscle is under the control of the
Foundations of Exercise Science
autonomic nervous system and is called involuntary.
Smooth muscle forms the walls of blood vessels
and body organs, such as the respiratory tract,
the iris of the eye, and the gastrointestinal tract.
The contractions of smooth muscle are slow and
uniform and are very fatigue resistant. Smooth
muscle functions to alter the activity of various
body parts to meet the needs of the body at the
time.
Cardiac muscle, the muscle of the heart,
has characteristics of both skeletal and smooth
muscle. Cardiac muscle functions to provide the
contractile activity of the heart and has its own
intrinsic beat. Like skeletal muscle, the contractile
activity of cardiac muscle can be graduated;
however, cardiac muscle is very fatigue resistant.
Like smooth muscle, the activation of cardiac
muscle is involuntary.
Although fitness training can benefit all three
types of muscle systems, this chapter will deal
primarily with the skeletal muscle.
Skeletal Muscle
Properties
Skeletal muscle refers to a number of muscle fibres
bound together by connective tissue and is usually
linked to bone by bundles of collagen fibres, known
as tendons. Tendons are located at each end of the
muscle (Figure 3.1 A). During muscle contraction,
skeletal muscle shortens, and as a result of the
tendinous attachments to bone, functions to move
the various parts of the skeleton with respect to
one another (joints) to allow changes in position
of one skeletal segment in relation to another.
Positioning several muscles on each “side” of a joint
allows movement in several planes, and through
graded activation the speed and smoothness of the
movement can be graduated.
Skeletal muscles are capable of rapid contraction and relaxation. Intensive activity causes them
to show early signs of fatigue. The assessment
of the movement and the sequential pattern of
muscle activation acting through joints to move
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63
Origin–Insertion
In order for muscles to contract they must be
attached to bones to create movement. This is
accomplished by tendons, strong fibrous tissues
at the ends of each muscle. The end of the muscle
attached to the bone that does not move is called
the origin, while the point of attachment of the
muscle on the bone that moves is the insertion
(Figure 3.1 A). The origin tends to be the more
proximal attachment (closer to the body), while
the insertion is the more distal attachment (further
from the body).
A
Tendon (origin)
Tendon (origin)
Biceps
Triceps
Tendon (insertion)
Tendon (insertion)
B
Biceps
contracts
Triceps
contracts
Extension
Flexion
Figure 3.1 Bending or straightening the elbow requires the coordinated interplay of the biceps and triceps
muscles.
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Foundations of Exercise Science
body segments is termed biomechanics of human
movement (see Chapter 7).
Striated or Voluntary Muscle
Muscle attached to the skeleton to make it move
is known as skeletal muscle. It is also known as
voluntary or striated muscle. Skeletal muscle
is considered striated because of the alternating
light and dark bands (created by the organization
of the muscle fibres, or cells) that appear when
viewed under a light microscope. Its description
as voluntary comes from the fact that we can
contract skeletal muscle when we want to,
voluntarily, i.e., flex the biceps.
Muscle Teamwork
Muscles work in perfect synchrony. When one
muscle contracts (draws together) to move a bone,
the other relaxes, allowing the bone to move. The
muscle or group of muscles producing a desired
effect is known as the agonist, the prime mover. A
muscle or group of muscles opposing the action is
called an antagonist.
An agonist–antagonist relationship occurs
between the biceps and triceps of the upper arm.
When the biceps (agonist) contracts to bend the
elbow, the triceps (antagonist) relaxes and allows
the bend. When the triceps (agonist) contracts to
straighten the arm, the biceps (antagonist) in turn
relaxes (Figure 3.1 B).
The cooperation of biceps and triceps is typical
of what takes place throughout the body. When
entire groups of muscles get involved the interaction
between agonist and antagonist muscles becomes
more complex.
The muscles surrounding the joint being
moved and supporting it in the action are called
synergists (complementing the action of a prime
mover). Other muscle groups called fixators
will steady joints closer to the body axis so that
the desired action can occur. For example, if you
want to climb a rope hand over hand, the muscles
holding your shoulder girdle tightly to your rib cage
are fixators, enabling you to use the muscles acting
over the shoulder, elbow, wrist, and finger joints to
perform their job and pull you up the rope.
Structure
Skeletal muscle is comprised of numerous
cylinder-shaped cells called muscle fibres, and
each fibre is made up of a number of myofilaments
(Figure 3.2). The diameter of each fibre varies
between 0.05 and 0.10 mm, with the length
being dependent mainly on the distance between
skeletal attachments (in the case of the biceps, the
length of a fibre is approximately 15 cm). Each cell
(fibre) is surrounded by a connective tissue sheath
called the sarcolemma, and a variable number of
fibres are enclosed together by a thicker connective
tissue sheath to form a bundle of fibres (Figure 3.2
B). Each fibre contains not only the contractile
machinery needed to develop force (Figure 3.3),
but also the cell organelles necessary for cellular
respiration (see Chapter 5, Energy for Muscular
Activity). Also located outside each fibre is a
supply of capillaries from which the cell obtains
nutrients and eliminates waste.
A large number of individual thread-like fibres
known as myofibrils run lengthwise and parallel to
one another within a muscle fibre. The myofibrils
contain contractile units that are responsible for
muscle contraction (Figure 3.2 D).
Muscle’s Tug of War
In some muscles, the individual fibres extend
the entire length of the muscle, but in most the
fibres are shorter. The shorter fibres, anchored
to the connective-tissue network surrounding
the muscle fibres, are placed at an angle to the
longitudinal axis of the muscle. When muscle
pulls on the bone during the transmission of
force, it is like a number of people pulling on
a rope, each person
corresponding to a
single fibre and the
rope corresponding to
the connective tissue
and tendons.
Studying Human Movement and Health
65
Muscle: The Contractile
Machinery
here, but you should still
be able to appreciate all
the intricate anatomical
structures involved with
every move we make.
Within each myofibril, a number of contractile
units, called sarcomeres (Figure 3.3 A), are
organized in series, i.e., attached end to end. Each
sarcomere is comprised of two types of protein
myofilaments: myosin, the so-called thick filament,
and actin, termed the thin filament. Looking at
the filaments in a cross-section, i.e., looking at the
myofilaments end-on, we see that each myosin
filament is surrounded by actin filaments (Figure
3.3 A). Examining the sarcomere longitudinally,
i.e., length-wise, we see the distinctive banding
pattern (striations) characteristic of skeletal,
or striated, muscle (Figure 3.4). Projecting out
from each of the myosin filaments at an angle
of approximately 45 degrees are tiny contractile
elements called myosin bridges; from this view,
these elements look similar to the projections of
oars from a rowing shell (Figure 3.3).
The Sliding Filament Theory
D
C
B
Muscle Belly
Muscle Fibre Bundle
Muscle Fibre
Myofibril
During the
contraction of a muscle, it is the sliding of the thin
actin filaments over the thick myosin
filaments that causes shortening of
the muscle to create movement.
This phenomenon is called the
sliding filament theory. It is
far more complex than
described
A
Figure 3.2 Components of skeletal muscle. A. Muscle belly (50 mm in diameter). B. Muscle fibre bundle
(0.5 mm). C. Muscle fibre (0.05-0.1 mm). D. Myofibril (0.001-0.002 mm).
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Foundations of Exercise Science
Myofibril
Sarcomere
Sarcomere
Actin filaments
Myosin filament
Myosin bridge
A
B
C
D
Figure 3.3 Longitudinal section of a myofibril and simplified representation of muscular contraction: A. At
rest. B. Contraction. C. Powerful stretching. D. Powerful contraction.
Studying Human Movement and Health
Rowing Simulation
When a signal comes
from the motor nerve activating the fibre, the
heads of the myosin filaments temporarily attach
themselves to the actin filaments (Figure 3.3 D),
a process termed cross bridge formation. In a
manner similar to the stroking of the oars, and
the subsequent movement of a rowing shell, the
movement of the cross bridges causes a movement
of the actin filaments in relation to the myosin
filaments, leading to shortening of the sarcomere.
A single “stroke” shortens the sarcomere by
approximately 1 percent of its length, and the
nervous system is capable of activating cross bridge
formation at a rate of 7-50 per second. Since the
sarcomeres are attached to one another in series,
the shortening of each sarcomere is additive. The
total amount of fibre shortening amounts to some
25-40 percent of myofibril length.
To produce an efficient rowing stroke, the
oars must be optimally placed, i.e., reaching far
enough, but not too far; similarly, for optimal
cross bridge formation, the sarcomeres should be
an optimal distance apart. For muscle contraction,
this optimal distance is 0.0019-0.0022 mm. When
the sarcomeres are separated by this distance, an
optimal number of cross bridges can be formed
per unit time. If the sarcomeres are farther apart,
or closer together, than this optimal distance, then
fewer cross bridges can be formed, resulting in less
force development. If the sarcomeres are stretched
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further apart, as occurs when the muscle is in a
lengthened (i.e., extended or stretched) position
(Figure 3.3 C), fewer cross bridges can form as
the myosin projections have difficulty in reaching
the actin filaments; this results in a decreased
ability to produce force. When the sarcomeres
are too close together, as would occur when the
muscle is shortened (flexed), the cross bridges
in fact interfere with one another as they try to
form, resulting in a fewer number of effective
cross bridges being formed, and again a decreased
ability to develop force (Figure 3.3 D).
The distance between sarcomeres depends
on the state of muscle stretch, which in turn is
a product of the position of the joint. What this
means to the development of muscle force is that
maximal force is developed when an optimal
number of cross bridges are formed, which occurs
at an optimal joint angle. Thus, as muscle force
depends on muscle length, maximal muscle
force occurs at optimal muscle length. As a joint
moves through its range of motion, the muscle(s)
connecting the two segments of the joint will
move from a stretched position to a compressed
position, and therefore, at some point in the
movement, will pass through a position, termed
the optimal joint angle, at which the muscle is at
optimal length for maximal force development
(Figure 3.5). This means that there would be an
optimal joint angle for maximal force development
Figure 3.4 High magnification view of a single sarcomere within a single myofibril. The characteristic
“striped” appearance of skeletal muscle is a result of the arrangement of the actin and myosin strands.
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Foundations of Exercise Science
Maximal Force (kp)
80
60
40
20
0
30
60
90
120
150
Angle (degrees)
Figure 3.5 Maximal muscle force changes continuously throughout elbow flexion according to the joint angle.
Table 3.1 Relative involvement of muscle fibre types in sport events.
Event
Slow Twitch – Type I
Fast Twitch – Type II
100-m sprint
Low
High
800-m run
High
High
Marathon
High
Low
Olympic weightlifting
Low
High
Barbell squat
Low
High
Soccer
High
High
Field hockey
High
High
Football wide receiver
Low
High
Football lineman
High
High
Basketball
Low
High
Distance cycling
High
Low
Studying Human Movement and Health
69
for each movement of a joint. Knowledge of
the joint angle at which maximal force can be
developed is important in the development of
optimal biomechanics of the movement.
Muscle Fibre Types
There are also different types of skeletal muscle
fibres. Some fibres can reach maximum tension
more quickly than others. Based on this distinction,
muscle fibres can be divided into the categories
of fast twitch (FT or Type II) (also called white
fibres based on their microscopic appearance) and
slow twitch (ST or Type I) (also called red fibres
based on their microscopic appearance). FT fibres
are more anaerobic, larger, fatigue faster, and have
a faster contraction speed than ST fibres. This
makes these fibres ideal for actions that are short
and require quick bursts of power and energy,
such as sprinting or jumping. On the other hand,
events that require endurance, such as long-distance
running, swimming, or cycling, depend on the
smaller, slower contracting, fatigue-resistant ST
fibres that rely on oxygen (Table 3.1). There are also
fibre types that fall in between these extremes with
characteristics of both fibre types. Thus, Type II is
further divided into Type IIa and Type IIb fibres.
The distinction between the two is mainly in their
contractile strength and their capacity for aerobicoxidative energy supply. The Type IIa fibres have
greater capacity for aerobic metabolism and more
capillaries surrounding them than Type IIb and
therefore show greater resistance to fatigue.
The muscle fibre composition of an individual
is dictated through heredity. The fibre composition
of an individual cannot be altered by training, i.e.,
the transformation of a fibre from one type to
another as a result of the training stimulus does
not occur.
Most skeletal muscles, however, contain both FT
Fast Twitch (Type II)
Fibres (large diameter)
Capillary Blood
Vessels
Slow Twitch (Type I)
Fibres (small diameter)
Figure 3.6 Muscle biopsy.
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Foundations of Exercise Science
and ST fibres, with the amount of each varying from
one muscle to another, as well as among different
individuals. Therefore, individual performance
differences occur as a result of varying percentages
of the muscle fibre types, making some individuals
suited to some activities more than others (that
is not to say that training will not improve what
fibres you do have).
information to the brain where it is processed
and acted upon. This sensory information is also
stored away so that future responses to similar
input can be acted upon more quickly. The motor
section (Figure 3.7 D) is involved directly in
conducting the signals from the CNS to activate
muscle contraction (see Chapter 17, Information
Processing in Motor Learning).
Muscle Biopsy
Motor Unit
Muscle fibre type is determined from a muscle
biopsy. In this procedure, a small incision (about
5-7 mm) is made in the skin and fascia of the
muscle following injection of a local anaesthetic
into the muscle. A tiny piece of tissue is cut and
removed from the muscle and then analyzed
under a microscope (Figure 3.6). In addition to
determining muscle fibre type, it is possible to
study the metabolic characteristics of the muscle,
and to assess changes in metabolic capability
following various types of training programs.
Biopsy = bio (life) + opsis (sight)
Nerve–Muscle Interaction
As with bone, muscle is a living tissue, and as such,
is richly supplied with blood vessels and nerves.
Skeletal muscle activation is initiated through
neural activation (Figure 3.7), and therefore, is
under conscious control. The nervous system is
organized at two levels – the central (CNS) and
peripheral (PNS) nervous systems, with the
central system being composed of the brain and
spinal cord (Figure 3.7 A), and the peripheral
system being made up of numerous nerves of
various sizes (Figure 3.7 C).
The nervous system can also be divided in
terms of function, namely motor and sensory
activity. The sensory section (Figure 3.7 E)
collects information from the various sensors
located throughout the body and transmits the
Motor nerves extend from the spinal cord to the
muscle fibres. Each fibre is activated through
impulses delivered via its motor end plate. A
group of fibres activated via the same nerve is
termed a motor unit, the basic functional entity of
muscular activity. A muscle can be composed of a
different number of motor units and each motor
unit can in turn consist of a different number of
muscle fibres.
All muscle fibres of one particular motor
unit, however, are always of the same fibre type
(FT or ST fibres). Muscles that need to perform
delicate and precise movements (the eye and
finger muscles) generally consist of a large number
of motor units (1,500-3,000), each containing
only a few muscle fibres (8-50). Relatively
unrefined movement, however, is carried out by
muscles composed of fewer motor units with
many fibres (approximately 600-2,000), each
of which innervates up to 1,500 muscle fibres.
In the tibialis anterior muscle, approximately
650 muscle fibres are innervated by each motor
unit; in the gastrocnemius muscle, the number is
approximately 1,600, and in the extensors of the
back, it is about 2,000.
The specific number of fibres in a motor unit
of any given muscle can vary. The biceps may be
composed of motor units that innervate 1,000,
1,200, 1,400, or 1,600 fibres. Furthermore, each
muscle fibre can be innervated by only one motor
unit. This cannot be altered through exercise.
All-or-none Principle
Muscle movement is controlled by the motor nerve
Studying Human Movement and Health
71
impulses transmitted from the CNS and spinal
cord out to the motor unit, which when activated
causes the muscle fibres to contract. Whether or
not a motor unit activates upon the arrival of an
impulse depends upon the so-called all-or-none
principle. This principle, discussed in more detail
in Chapter 17, requires an impulse of a certain
magnitude (or strength) to cause the innervated
fibres to contract. The principle is analogous to
firing a gun. Once a sufficient amount of pressure
is placed on the trigger, the gun fires; pulling on
the trigger harder will not cause the bullet to go
faster or further.
Activation Threshold Every motor unit has a
specific threshold that must be reached for such
activation to occur. For the biceps muscle, for
example, all of the 1,500 fibres that may comprise
B
a single motor unit will contract maximally
providing the nerve impulse has reached a certain
magnitude. However, if the nerve impulse does
not reach the required magnitude, then none of
the fibres will contract.
A weak nerve impulse activates only those
motor units that have a low threshold of activation.
A stronger nerve impulse will additionally activate
motor units with higher thresholds. As the
resistance increases, more motor units must be
activated by stronger, more intensive impulses.
An athlete needs increasingly more will power
to exceed the excitatory thresholds of the motor
units. This process is extremely fatiguing as a
result of lactic acid accumulation in the muscle
tissue and blood, the depletion of high-energy
compounds, and the fatigue of the nervous system
processes.
C
D
E
F
A
Figure 3.7 Sensory neurons transfer messages to the central nervous system, where they are analyzed
and responded to by motor neurons. Activation of a motor unit and its innervation systems: A. Spinal cord.
B. Cytosomes. C. Spinal nerve. D. Motor nerve. E. Sensory nerve. F. Muscle with muscle fibres.
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Intra-muscle Coordination
The capacity to activate motor units simultaneously
is known as intra-muscle coordination. Although
it is impossible to use all the motor units of a
muscle at the same time, many highly trained
power athletes, such as weightlifters, wrestlers, and
shot-putters, are able to activate up to 85 percent
of their available muscle fibres simultaneously, thus
generating great strength. Untrained individuals,
on the other hand, can normally activate only up
to 60 percent of their fibres.
Research has shown that under hypnosis a
trained athlete can elevate the maximal force
application for a given muscle by approximately
10 percent. The difference between assisted and
voluntarily generated maximal force is regarded as
the muscle force deficit of the muscle contraction.
For untrained individuals, this deficit is much
larger (approximately 20-35 percent).
Trained athletes have not only a larger muscle
mass than untrained individuals, but can also
exploit a larger number of muscle fibres to produce
force. However, for this reason, such athletes are
more restricted than untrained individuals in
further developing strength by improving intramuscle coordination. For this same reason, trained
individuals can further increase strength only by
increasing muscle diameter.
Inter-muscle Coordination
Any physical movement requires considerable
effort by the muscles or muscle groups to master a
given movement. This requires an optimal level of
inter-muscle coordination.
The interplay between muscles that generate
movement through contraction, the agonists
or prime movers, and muscles responsible for
opposing movement, the antagonists, is of
particular importance to the quality of intermuscle coordination. The cooperation between
agonist and antagonist muscles during the bench
Foundations of Exercise Science
press, for example, provides a useful illustration.
From a supine position, an athlete explosively
stretches his or her arms against a high resistance.
During the movement, a considerable number
of motor units in the triceps and in cooperating
muscles are synchronously activated, while the
motor units of the antagonist muscles relax.
The greater the participation of muscles and
muscle groups, the higher the importance of
inter-muscle coordination for strength capacity.
To benefit from strength training, technically
demanding sport-specific movements are often
broken down into partial movements, so that the
individual muscle groups responsible for these
movements can be trained in relative isolation.
The exercises used closely resemble the movement
structure of the sport-specific movement, such
that the training allows for the key muscle groups
to be loaded relatively heavily.
Sport-specific Training
Consider the following exercises, which are
beneficial to shot-putters: the bench press (Figure
3.8 A), lateral trunk curl (Figure 3.8 B), knee
bend or squat (Figure 3.8 C), and heel or calf
raise (Figure 3.8 D). An athlete whose muscles
have been trained and developed in isolation
using such exercises must subsequently engage in
training that coordinates these muscles within the
complete, sport-specific movement. Difficulties
may occur if the athlete fails to develop all the
relevant muscles in a balanced manner. For
instance, a shot-putter who uses exercises that
increase strength in only the arm and leg extensors,
but not the trunk muscles, may experience major
disturbances of inter-muscle coordination. As a
result, performance may not improve or reach the
level desired by the athlete.
High-level inter-muscle coordination greatly
improves strength performance and also enhances
the flow, rhythm, and precision of movement.
Unlike an ordinary individual, a highly trained
athlete is able to translate strength potential more
effectively into strength performance through
enhanced inter-muscle coordination.
Studying Human Movement and Health
A
B
C
D
Figure 3.8 A shot-putter’s training includes exercises that work several prime movers in isolation. A.
Bench press. B. Lateral trunk curl. C. Knee bend or squat. D. Heel or calf raise.
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Foundations of Exercise Science
Trainable vs. Non-trainable Factors
The performance capacity of muscle is determined by several trainable and non-trainable factors.
Trainable factors:
Non-trainable factors:
•
•
•
•
•
•
•
• number of fibres
• fibre structure (ST or FT fibres)
fibre diameter
intra-muscle coordination
nerve impulse frequency
inter-muscle coordination
elasticity of muscle and its tendons
energy stores of muscle and liver
capillary density of muscle
Muscle’s Adaptation to
Strength Training
In strength training, an individual’s performance
improvements occur through a process of
biological adaptation, which is reflected in
the body’s increased strength. Similar types of
adaptation may occur in any form of training.
Indeed, they are the building blocks for improved
performance in any athletic activity.
In strength training, the adaptation process
proceeds at different time rates for different
functional systems and physiological processes.
The adaptation depends on a variety of factors,
in particular on intensity levels used in training
and on an athlete’s unique biological make-up.
Specific substances of the metabolism, such as
enzymes, adapt within hours; the energy supply in
the liver and muscle increases at a more moderate
pace, within 10-14 days, by which time the first
adaptations in the cardiovascular circulation also
occur (see Chapter 6). The muscle mass increases
slowly, within four to six weeks, its growth caused
by an increase in the structural proteins in the
skeletal muscle fibres.
Recruitment of Muscle Fibres
Proportion of Muscle Fibre
%
100
Fibre
Reserve
80
FT fibres,
Glycolytic
60
FT fibres,
Oxidative
40
ST fibres
20
20
40
60
Resistance
80
100
%
Recruitment of muscle fibres during resistance
work depends on the level of muscle tension. As
the tension rises, more and more of the various
fibre types are recruited into the movement as
shown by the curves. Muscle tension below 25
percent of one’s maximal resistance recruits
mostly ST fibres. At higher resistance, FT fibres
also become active. Furthermore, which fibre
is involved depends upon the muscle force
that needs to be mobilized, and also the rate
of acceleration of the mass to be moved. High
accelerations of small loads and low accelerations
of high loads require the intensive involvement of
the FT fibres. Also, it is primarily the FT fibres that
generate the explosive-type movements requiring
a lot of strength.
Studying Human Movement and Health
Summary
Muscles attached to skeletal bones work together
and with tendons to enable body movement.
Thin fibres called myofibrils constitute muscle,
and end-to-end units called sarcomeres within
each myofibril enable muscles to contract,
causing movement in response to motor nerve
stimulation.
Motor nerves extend from the spinal cord to
muscles throughout the body, and each motor
unit is specific to either fast twitch or slow twitch
muscle types. FT fibres, which are anaerobic
in nature and fatigue faster than ST fibres, are
best suited for activities requiring short bursts of
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power and energy. Endurance events such as longdistance running, swimming, or cycling make
use of the fatigue-resistant ST fibres that rely on
oxygen. Motor units require threshold levels of
nerve impulses before they can react – and some
motor units have higher resistance thresholds than
companion units in the same muscle.
Movement requires precise coordination of
muscles and the muscle fibres themselves. Intramuscle coordination is the ability to activate
motor units simultaneously, while inter-muscle
coordination refers to the synchronization of
different muscles and muscle groups. Cooperation
of the agonists and antagonists is necessary for
smooth, controlled motion.
Key Words
Actin
Agonist
All-or-none principle
Antagonist
Biological adaptation
Cardiac muscle
Cross bridge formation
Fast twitch (FT) fibre
Fixator
Insertion
Inter-muscle coordination
Intra-muscle coordination
Involuntary muscle
Motor end plate
Motor unit
Muscle biopsy
Muscle fibre
Muscle force deficit
Myofibril
Myofilament
Myosin
Origin
Prime mover
Sarcolemma
Sarcomere
Skeletal muscle
Sliding filament theory
Slow twitch (ST) fibre
Smooth muscle
Striated muscle
Synergist
Tendon
Voluntary muscle
Discussion Questions
1. What are the three types of muscle found in the
human body?
4. What are the three types of muscle fibres? Give
two characteristics of each type of fibre.
2. Describe the structure of a muscle from the
largest structural unit to the smallest.
5. Explain nerve–muscle interaction.
3. Explain how the sarcomere contracts, resulting
in muscle shortening.
6. Discuss the differences between inter- and
intra-muscle coordination.