Variations in fibre size in the gastrocnemius muscle of rats undergoing training.

Sixty-six Wistar rats of both sexes were divided into six groups. One group was used as control, and the others underwent different training schedules. Muscle samples taken from the red and mixed portions of m. Gastrocnemius (caput lateralis) were stained with m-ATPase in order to identify fibre types I, IIA and IIB. These fibres were measured in order to determine their minimum diameter. The most notable results obtained were: a) the three types of fibre were generally larger in males than in females; b) the mean minimum diameter of fibres from the mixed portion was smaller than those of the red portion, except type IIB fibres, and c) the training schedules involved generally led to slight hypertrophy of all types of fibres, although this finding was more significant in the mixed portion of m. Gastrocnemius.