These fibers are to be considered intermediate between the I type fibers and the IIb type fibers; they
are innervated by a phasic motoneuron that has a faster speed than the one of the tonic motoneuron.
Thanks to their features, they can specialize by addressing them towards more aerobic or anaerobic
metabolic properties.
Therefore, the IIa type fiber is able to carry out rapid contractions with a modest strength
development, that are extended during the time due to their relative endurance to fatigue.
IIb type fibers
They are also called FTb fibers (rapid contraction fibers) or FG (glycolytic metabolism rapid
fibers).
This kind of fiber is innervated by a phasic motoneuron that transmits the impulses to the muscle at
a very high speed.
These fibers are white and have a high content in glycogen and glycolytic enzymes to develop a
powerful energetic anaerobic activity.
The contraction is very rapid and develops high strength values; the almost complete lack of
mitochondria makes these fibers not able to support an extended activity and, thus, easy to fatigue.
IIb type fibers are very important in all human activities that have explosive strength expressions
and, naturally, in all power sports: sprints, throws, jumps, etc.
The limits of the actual classifications
The actual classification of muscle fibers is due more to the need of establishing a series of
categories to use for practical aims than to the biological-functional reality of the human muscular
system.
It is certain that the fibers are in a continuous range of different metabolic organization levels that
correspond to the different types of human activity, generally, and sport performance, specifically.
The distribution of the different types of fibers in the muscle
The relation between the two main categories (type I and the type II) can vary in a sensitive way.
There are muscular groups that are typically constituted by fibers of I type, like the soleus, and there
are muscles that have only fibers of II type like the orbicular muscle. However, in the most of the
cases we have a simultaneous presence of various types of fibers.
The studies lead on the distribution of fibers in the muscle have put in evidence the strait
relationship that exists between the motoneuron (tonic or phasic) and the functional features of
fibers from it innervate. They have also shown as a specific motor activity (and sports in particular)
can determine a functional adaptation of the fibers and a modification of their metabolic features. In
the same way, also the training with the electrostimulation enables the training to focus on some
types of fibers rather than on other types, by modifying the parameters of frequency and duration of
the impulse, according to the results achieved.
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