Muscle Fiber Types and Fiber Morphometry in the Soleus Muscle of the Rat

Muscle fiber type composition, distribution and size of fibers were investigated in the soleus muscle of young adult male and female rats, using histochemical and morphometric methods, to determine whether these parameters vary according to the sex of animals. Combined myofibrillar ATPase (mATPase) and metabolic enzyme histochemistry were performed on serial cross-sections from the belly region of muscle. Three fiber types (I, IIA, and IIC) were identified on the basis of mATPase activity after preincubation at pH values of 9.4, 4.5 and 4.3. The type IIC fibers, present in a small number (about 4%), were included within the type IIA fiber class for purposes of quantitative analysis. The fiber type composition of the slow soleus muscle showed type I dominance (78%). There has been observed a variation in the regional distribution of muscle fiber types. A relative number of type IIA fibers increased towards the lateral part of the muscle, and so did type IIC, but the general organization was not as regular as those previously found in the fast tibialis anterior and tibialis posterior muscles. The size of predominating fibers of type I was larger than of IIA type fibers in the soleus muscles from the both groups. No significant difference was detected between male and female rats in respect to the morphometric parameters examined.