Fiber Type Composition in Semitendinous Muscle of Wistar Rats and Effects of Intermittent Training on its Hypertrophy Composición de los Tipos de Fibras del Músculo Semitendinoso de Ratas Wistar y los Efectos del Entrenamiento Intermitente en su Hipertrofia

SUMMARY: Skeletal muscles respond to several stimuli changing their phenotype. Muscular fibers adaptation capability isrelated to the presence of several myosin heavy chains (MHC). These express four types of pure fibers: I, IIA, IID and IIB containingMHCI, IIa, IId and IIb, respectively. Among pure fibers, there are hybrid fibers, which can express two or more types of myosins. In thisstudy, types of fibers constituting male Wistar rats semitendinous and their myosin heavy chains, as well as influence of intermittenttraining on hypertrophy of these fibers have been checked through MATPase histochemical technique and electrophoretic proteinsseparation. 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 isessentially constituted by fast-contracting fibers and training could promote transition and hypertrophy of these fast fibers.