Types of muscle fibres
Slow (type 1) and Fast Twitch (Type 2 a and b)
Muscle Fibres
Skeletal muscle fibres are not all uniform, in fact they can differ in both structure and function.
A single muscle such as the bicep brachi will be composed of two principal types of muscle fibres:
slow twitch (type 1) and fast twitch (type 2).
Slow twitch fibres contract more slowly but are highly resistant to fatigue and are therefore
favoured by endurance athletes, whilst fast twitch fibres can contract more rapidly, generating
greater forces but are moreliable to fatigue.
These fibres are more prevalent in sprinters and power athletes. Fast twitch fibres have
been further classified into type a and type b.
Type 2a fibres, also known as fast oxidative glycolytic fibres (FOG) are more resistant
to fatigue than type 2b fibres which have a greater anaerobic capacity and are termed
fast twitch glycolytic fibres (FTG).
STRUCTURAL AND FUNCTIONAL CHARACTERISTICS OF SLOW AND
FAST TWITCH TYPE FIBRES
Functional Characteristic
Slow twitch (type 1)
Fast oxidative glycolytic Fast twitch glycolytic
(FOG) (type 2a)
(FTG) (type 2b)
Speed of contraction (ms)
Slow (110) Low
Fast (50)
Fast (50)
Low
High
Highest
Resistance to fatigue
Very high
Moderate
Low
Aerobic capacity
Very high
Moderate
Low
Anaerobic capacity
Low
High
High
Fibre Size
Small
Large
Large
Mitochondrial density
High
Moderate
Low
Capillary density
High
Moderate
Low
Myoglobin content
High
Moderate
Low
PC store
Low
High
High
Glycogen store
Low
High
High
Triglyceride store
High
Moderate
Low
Motor neuron size
Small
Large
Large
Force of contraction
Structural Characteristic
KEY TERMS
Slow twitch muscle fibre:
•
a type of muscle fibre that uses oxygen to produce
energy (high oxidative capacity). They are
associated with endurance-based activities
•
Fast twitch muscle fibre:
•
a type of muscle fibre that has a high glycolytic
capacity (anaerobic). They are associated with
speed and power- based activities
Sports for different fibres
Athletes V Non Athletes
Muscle fibre recruitment
The primary function of skeletal muscle is to
contract and facilitate movement of the body.
Muscle contraction involves the interaction of
the muscles with the nervous system. Individual
muscles such
as the anterior deltoid are connected to the
nervous system via a group of motor neurons.
Each muscle fibre within the muscle belly is
supplied by only one motor neuron, however this
neuron can innervate (stimulate) anything from
just a few fibres to several hundred. The motor
neuron plus the fibres it innervates is known as
the motor unit.
The motor unit
The motor unit is the basic functional unit of skeletal muscle. Stimulation of one motor neuron
causes all the muscle fibres in that motor unit to contract simultaneously. Each individual muscle
will be made up of a number of motor units (just like a school is made up of a number of form or
tutor groups). The number of motor units that are recruited at anyone time in the muscle varies
with the amount of strength required for a given movement. The more strength needed the greater
the number of motor units activated.
The number of fibres within a particular motor unit is dependent upon the control of movement
required in that muscle. A small muscle that is required to perform fine motor control suchas those
that enable the eye to focus may only have one fibre per motor neuron; whereas large muscles
responsible for gross movements such as the quadriceps group when kicking a ball may be
innervated by a motor neuron supplying 500 or more fibres.
Motor units are usually made up of the same type of muscle fibre. Consequently we see both
fast and slow twitch motor units in a muscle. Fast twitch motor units are generally recruited during
high intensity activity such as sprinting or throwing the javelin whilst slow twitch motor units are
used during lower intensity exercise such as running a half marathon or cross country skiing.
Innervation
•
The innervation of skeletal muscle is accomplished by a motor neuron transmitting a nerve
impulse or action potential to the muscle fibre. Just how the muscle fibre responds is governed by
the ‘all or none law’.
•
The all or none law
•
•
The all or none principle essentially states that individual muscle fibres within a motor unit
contract either fully or not at all. In other words individual muscle fibres cannot partially contract!
In order to activate these muscle fibres however a minimum amount of stimulation is needed
(termed the threshold).
If the stimulation equals or exceeds the threshold all the fibres within the motor unit will contract
at the same time and to their maximum possible extent. If the stimulus falls short of the threshold,
however, the muscle fibres do not respond and muscular contraction fails to occur.
Spatial Summation
For a muscle to contract the excitatory postsynaptic potential (EPSP) must be of a certain level of
intensity to initiate the sliding filament mechanism of muscle contraction.
Spatial Summation describes the progressive increase in size of the excitatory postsynaptic
potential (EPSP) as a result of the arrival of a number of impulses at the synaptic cleft of
individual muscle fibres
Basically an increase in responsiveness of a nerve resulting from the additive effect of numerous
stimuli. A certain level of intensity is needed before a muscle fibre responds by contracting
Innervation of a muscle fibre
Role of Aceytlcholine
Acetylcholine is a chemical substance that allows
the transmission of an impulse across the
synaptic cleft and enables the muscle to contract.
How does it affect muscle
contraction
On arrival at the neuromuscular junction the transmitter substance acetylcholine is
released which enables the impulse to cross the synaptic cleft and create electrical
potentials (EPSPs) in the muscle fibre (the potential to contract).
However, if the EPSP is not of sufficient intensity the muscle fibre will not contract,
consequently the additive effect of a number of stimuli arriving can be used which
ensures that the excitatory threshold is reached and the muscle fibres contract – all or
nothing!
Variations of muscle strength
Sporting performance requires variations in strength or muscular force from very
weak efforts such as a short putt in golf to all out maximal efforts such as a shot put.
How then does the body cope with these different requirements?
The strength of a muscle can be graded in several ways:
Multiple unit summation
Wave summation
Synchronicity of motor unit stimulation.
Multiple unit summation
The strength of a muscle contraction can be
increased by recruiting more motor units
Maximal contractions will recruit all motor units
within a particular muscle whilst weaker
contractions will recruit fewer units
Fast twitch motor units will be recruited ahead of
slow twitch units for more powerful contractions
Wave Summation
Wave summation considers the frequency with which impulses arrive at the
motor unit.
Typically the motor unit will respond to an impulse (innervation) by giving a
twitch-a very short period of contraction followed by relaxation.
If a second impulse arrives at the motor unit before it had time to completely
relax from the first twitch the motor unit responds with stronger contractions
since the effect of the second stimulus is added to the first.
They summate creating greater tension within the motor unit. When a motor
unit is stimulated many times in quick succession there is little or no time
relaxation.
This produces the highest level of sustained tension referred to as tetanus or
tetanic contraction and will continue until fatigue ensues.
VArying the strength of muscle
contraction with wave summation
Neuromuscular adadptations to
resistance training
Recruitment of more motor units – More motor units may be trained to act synchronously
(together) so that greater forces can be generated therefore resulting in greater strength gains.
Muscle hypertrophy – The size of the muscle belly will increase due to an increase in the
size of individual muscle fibres (hypertrophy) and the possible splitting of fibres
(hyperplasia).
Hypertrophy of fast twitch muscle fibres – As fast twitch fibres are predominantly recruited
during resistance training, these fibres in particular will enlarge.
Hyperplasia of fast twitch muscle fibres – There is some evidence to suggest that muscle
fibres split, particularly with heavy resistance training. This splitting will contribute to the
general hypertrophy of the muscle.
Conversion of type 2b fibres to type 2a fibres – Some studies have shown that
the percentage of type 2b fibres within a trained muscle actually decrease in favour
of type 2a fibres. This could account for the delay in muscular fatigue associated with
prolonged training.
Recap
A motor neuron together with the fibres it innervates is known as the motor unit.
The motor unit is the basic functional unit of skeletal muscle.
When a motor unit is called upon to contract all the muscle fibres under its control will contract
simultaneously.
The all or none law is the principle that explains that individual muscle fibres within a motor unit
will either contract fully or not at all.
In order to cause the muscle fibres within the motor unit to contract fully the level of stimulation
must reach a certain ‘critical’ threshold.
A muscle can vary the strength of its contractile response through multiple unit summation and
wave summation.
There are two principal types of muscle fibre, each having a different structure and function.
Slow twitch fibres (type 1) contract more slowly and are highly resistant to fatigue, so are
favoured more by endurance athletes who use the aerobic system to supply the majority of their
energy during the exercise period.
Fast twitch fibres (Type 2) can contract more rapidly and generate greater forces so occur more
widely in sprinters and power athletes.
RECAP
Type 2a fibres (fast twitch oxidative) have a slightly better
resistance to fatigue than type 2b fibres and so are prominent in
athletes who use the lactic acid pathway to supply energy such as
a 400m hurdler.
Type 2b fibres (Fast twitch glycolytic) have the highest anaerobic
capacity. Although they produce the greatest forces they fatigue
the most rapidly. Type 2b fibres are most widely found in athletes
who predominantly use the ATP-PC system during the exercise
period.
There are a number of neuro-muscular adaptations to resistance
training which include: Muscle hypertrophy, hypertrophy of fast
twitch muscle fibres, hyperplasia of fast twitch muscle fibres,
recruitment of more motor units and possible conversion of type 2b
fibres to type 2a fibres.
Homework
1. Name the three types of muscle fibre and give a sporting example where each of these fibres prevail.
2. Give two structural and two functional characteristics of each.
3. Explain the role of motor units in controlling the strength of muscular contractions in sporting
activity.
4. Name a muscle of the body that will contain motor units composed of several hundred fibres
and a muscle which will contain motor units of relatively few fibres.
5. What types of training would cause hypertrophy (enlargement) of a) slow twitch fibres b) fast
twitch (type 2b) fibres?
6. Outline the stages of the sliding filament process of muscle contraction.
7. Explain how the strength of muscle contraction can be varied in relation to a high jumper and a
distance runner.
8. Give three neuromuscular adaptations to training.