Muscle Fiber Type
Slow Twitch / Type I / Slow Oxidative
Fast Twitch / Type II (IIa, IIx, IIc) / Fast
Oxidative,Glycolytic / Fast Glycolytic
Recruitment order I IIa IIx IIc.
On average most muscle composed of 50% type I, 25%
type IIa, remaining 25% type IIx with type IIc making
up 1 – 3% of the muscle.
Exact percentage varies greatly in various muscles
and amongst individuals.
Difference in speed of contraction results primarily
from different forms of myosin ATPase.
Myosin ATPase – enzyme that splits ATP to release
energy to drive contraction.
Type I – slow form
Type II – fast form, allowing cross bridges to cycle
Histochemical method of identifying fibers by their
Type II have a more highly developed SR.
This is thought to contribute to the faster speed of
contraction of type II fibers.
The amount of force generated by the type II and
type I fibers having the same diameter is about the
The calculated power of a type II fiber is 3 – 5 times
greater than that of type I fibers, because of faster
Composed of single α-motor neuron and the muscle
fibers it innervates.
α-motor neuron can determine whether the fiber
type is type I or II.
Type I neuron has:
Smaller cell body
Innervates a cluster ≤ 300 muscle fibers
Type II neuron:
Larger cell body
Innervates a cluster ≥ 300 muscle fibers
Hence type II fibers reach peak peak tension faster
and collectively and generates more force than type
Fiber Type and Exercise
Type I fibers
High level of aerobic endurance.
Very efficient in producing ATP from the oxidation of
carbohydrate and fat.
With prolonged oxidation, ATP production will
influence sustained fiber activity.
Recruited more often during low intensity endurance
events and during most daily activities.
Relatively poor aerobic endurance.
ATP mostly formed through anaerobic pathways
Motor units fatigue easily due to limited endurance.
Primary fiber used during shorter, higher intensity
endurance events (mile run/ 400m swim).
Type IIx not easily activated by the nervous system and
is thought to be used more in high explosive events
(100m dash / 50m sprint swim).
Determination of Fiber
Characteristics of muscle fibers appear to be determined early
in life (first few years).
The inherited genes determines which α-motor neurons
innervate our individual muscle fibers.
After innervation is established, muscle fibers become
Recent studies have suggested that endurance training,
strength training, and muscular inactivity may cause a shift in
the myosin isoforms.
Hence, training may induce a small change in the percentage of
type I and II fibers.
Endurance and resistance training has been shown to
reduce the percentage of type IIx fibers and
increasing the fraction of type IIa fibers.
Aging also alters the distribution of fibers, muscle
tends to lose type II motor units, which increases the
percentage of type I fibers.
α-motor neuron carries an action potential to the motor
unit, which stimulates the muscle fibers and generates
Activating more motor units greater force generation.
When little force is needed only a few units are
As intensity of an activity increases, the # of fiber
Type I Type IIa Type IIx
Principle of orderly recruitment = fixed order of fiber
Size principle = motor units with smaller motor
neurons will be stimulated first.
Fiber Type and Athletic
Athletes with a high percentage of type I fibers
might have an advantage in prolonged endurance
Those with predominance of type II fibers could be
better suited for high intensity, short term, and
Generation of force
Whenever muscle contracts, the force developed must
be graded to meet the needs of the task or activity.
The amount of force developed is dependent on:
The number and types of motor units activated
The frequency of stimulation of each motor unit
Muscle fiber and sarcomere length
Speed of contraction
Motor Unit and Muscle Size
Type II motor units generate more force than type I
Larger muscles produce more force than smaller
Single motor unit can exert varying levels of force
depending on the frequency at which it is stimulated.
Twitch: smallest contractile response of a muscle fiber
or a motor unit to a single electrical stimulus.
Summation: a series of stimuli in rapid sequence, prior
to complete relaxation from the first stimulus
producing an increase in force or tension.
Tetanus: continued stimulation at higher frequencies
resulting in the peak force or tension of the muscle
fiber or motor unit.
Term used to describe the process by which the tension
of a given motor unit can vary from that of a twitch to
that of tetanus by increasing the frequency of
stimulation of that motor unit.
Muscle Fiber and Sarcomere Length
Optimal sarcomere length: length where there is
optimal overlap of the thick and thin filaments , thus
maximizing cross bridge interaction.
When the sarcomere is fully stretched or shortened,
little or no force can be developed.
Speed of Contraction
Ability to develop force also depends on the speed of
During concentric contraction, maximal force
development decreases progressively at higher
On the other hand, fast eccentric contractions allows
for maximal application of force.