Skeletal muscles respond to several stimuli changing their phenotype. Muscular fibers adaptation capability is related to the presence of several myosin heavy chains (MHC). These express four types of pure fibers: I, IIA, IID and IIB containing MHCI, IIa, IId and IIb, respectively. Among pure fibers, there are hybrid fibers, which can express two or more types of myosins. In this study, types of fibers constituting male Wistar rats semitendinous and their myosin heavy chains, as well as influence of intermittent training on hypertrophy of these fibers have been checked through MATPase histochemical technique and electrophoretic proteins separation. All types of pure and hybrid muscular fiber have been found, however the fibers of the types IIA, IID and IIB were predominant, featuring muscle as a fast-contracting one. Training has promoted muscular fibers transition with a significant increase of fibers of IC, IIAD and IIDB type. A cross-section increase of fibers of IIDB and IIB type has also been noticed. In summary, semitendinous muscle is essentially constituted by fast-contracting fibers and training could promote transition and hypertrophy of these fast fibers.