Considering the diversity of activities carried on in the day-to-day, human body developed muscles capable of performing specific functions according to the need. It is justified, therefore, the very existence of fast-twitch muscle cells (explosion) and slow (resistense), which, despite different features work together, alternating the predominance of only using each according to activity performed.

The muscle fibers type I are reddish in color because they have lots of myoglobin and mitochondria (cellular organelles responsible for energy production). Their cytoplasm is rich in enzymes that use oxygen as the main energy substrate, characterizing them therefore as slow twitch fibers. When the human body needs more muscle strength for a prolonged activity, is used predominantly this type of muscle fiber (POWERS; HOWLEY, 2005). The muscle fibers of type IIa are a subset of fast twitch fibers and contains numerous mitochondria, although unless the fibers type I.

This type of fiber is also resistant to exhaustion and appears to be suitable for resistance activity and strong short-lived contractions (GENESER, 2000). Have a paler color for presenting a smaller number of blood capillaries and, therefore, less oxygen supply. Because of this, using glucose as the main source of energy.

As for the fiber type IIb constrict quickly and match the white fibers. Are generally thicker fibers and form large motor units. Contains only few mitochondria, for the myofibrillar mass, and run out very quickly. Their main energy source a compound called ATP (adenosine triphosphate), ideal for activities that require intense muscular work with a high force generation, but in a short time (explosion) (GENESER, 2000). The majority of the muscles consisted of a mixture of these three types of fibers, but the percentage distribution of the three types is distinguished from a muscle to another and from one individual to another.

According Gitirana (2007), on page 228 of his work: In normal muscle contractions involved first the type I fibers by low contraction forces, for example, corresponding to the activity of the muscles in standing position. Faced with increasing contraction forces are incorporated into the type II fibers, but the type IIb fibers are then included before hard acceleration or quick short contractions.

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