Section A: Applied Anatomy and
Physiology
7. Muscle fibre types
Syllabus
• Structure and function of slow oxidative, fast
oxidative glycolytic and fast glycolytic muscle
fibre types
Types of Muscular Tissue
• Skeletal
–
–
–
–
Striated
Voluntary
Consciously controlled
Most are subconsciously
controlled (stabilising)
• Cardiac
– Only the heart
– Also striated
– Involuntary
• Smooth
– Located in wall of hollow
internal structures
– NONstriated
– Involuntary
The following slide shows a table of
muscle fibre types. These muscle fibre
types are found in skeletal muscle
tissue.
Slow-oxidative
Fast-oxidative
Fast-glycolytic
mitochondria
many
many
few
capillaries
myoglobin
content
many
many
few
high
high
low
myosin ATPase
activity
low
high
high
contraction
velocity
slow
fast
fast
rate of fatigue
slow
intermediate
fast
muscle fiber
diameter
small
intermediate
large
innervating
neuron size
small
intermediate
large
motor unit size
small
intermediate
large
Type 1 (slow oxidative)
• They are red in colour due to the presence of large volumes
of myoglobin and so oxygen and high numbers of
mitochondria
• Due to this fact they are very resistant to fatigue and are
capable of producing repeated low-level contractions by
producing large amounts of ATP through
an aerobic metabolic cycle.
• For this reason the muscles containing mainly type I fibres
are often postural muscles such as those in the neck and
spine due to their endurance capabilities
• Also, athletes such as marathon runners have a high
number of this type of fibre, partly through genetics, partly
through training
Type 2a (fast oxidative)
• Are a hybrid of type I and II fibres
• These fibres contain a large number of mitochondria
and myoglobin, hence their red colour
• They manufacture and split ATP at a fast rate by
utilising both aerobic and anaerobic metabolism and
so produce fast, strong muscle contractions, although
they are more prone to fatigue than type I fibres
• Resistance training can turn type IIb fibres into type IIa
due to an increase in the ability to utilise the oxidative
cycle.
Type 2b (fast glycolytic)
• They are white in colour due to a low level of
myoglobin and also contain few mitochondria
• They produce ATP at a slow rate by anaerobic
metabolism and break it down very quicky
• This results in short, fast bursts of power and rapid
fatigue
• As mentioned above, this type of fibre can be turned
into type IIa fibres by resistance training
• This is a positive change due to the increased fatigue
resistance of type IIa fibres
• These fibres are found in large quantities in the
muscles of the arms
• Most skeletal muscles of the body are a mixture of all three types of
skeletal muscle fibres, but their proportion varies depending on the
usual action of the muscle. For example, postural muscles of the
neck, back, and leg have a higher proportion of type I fibres.
Muscles of the shoulders and arms are not constantly active but are
used intermittently, usually for short periods, to produce large
amounts of tension such as in lifting and throwing. These muscles
have a higher proportion of type I and type II B fibres.
• Even though most skeletal muscle is a mixture of all three types of
skeletal, all the skeletal muscle fibres of any one motor unit are all
the same. In addition, the different skeletal muscle fibres in a
muscle may be used in various ways, depending on need. For
example, if only a weak contraction is needed to perform a task,
only type I fibres are activated by their motor units. If a stronger
contraction is needed, the motor units of type II A fibres are
activated. If a maximal contraction is required, motor units of type II
B fibres are activated as well. Activation of various motor units is
determined in the brain and spinal cord. Although the number of
the different skeletal muscle fibres does not change, the
characteristics of those present can be altered.
• The fast muscle (what the researchers call
type IIa) moves 5 times faster than the slow
muscle, and the super-fast (called type IIb)
moves 10 times faster than the slow muscle
fibre.
• The average person has approximately 60%
fast muscle fibre and 40% slow-twitch fibre
(type I). There can be swings in fibre
composition, but essentially, we all have three
types of muscle fibre that need to be trained.
•
•
•
Various types of exercises can bring about changes in the fibres in a skeletal
muscle.
Endurance type exercises, such as running or swimming, cause a gradual
transformation of type II B fibres into type II A fibres. The transformed muscle
fibres show a slight increase in diameter, mitochondria, blood capillaries, and
strength. Endurance exercises result in cardiovascular and respiratory changes that
cause skeletal muscles to receive better supplies of oxygen and carbohydrates but
do not contribute to muscle mass.
On the other hand, exercises that require great strength for short periods, such
as weight lifting, produce an increase in the size and strength of type II B fibres.
The increase in size is due to increased synthesis of thin and thick myofilaments.
The overall result is that the person develops large muscles.
•
You can develop your fast-twitch muscle fibre by
conducting plyometric or complex training (combination of plyometrics and
weights) to build the fast muscle (IIa) and performing sprinting types of training to
build the super-fast (IIb) to the point where you can release exercise-induced
growth hormone.
•
The body itself produces the best form of growth hormone. If you want to
accelerate muscle building then use large muscle group targeted weight training in
combination with anaerobic sprinting-types of exercise to increase your body's
natural muscle building steroids.