The effects of different treadmill running programs on the muscle morphology of adult rats

The biochemical adaptations of different muscle fiber types to endurance training of various intensities and durations have previously been investigated. The objective of this study was to determine the effects of two different endurance training programs on muscle fiber morphology. Twenty-four male Sprague-Dawley rats were randomly assigned to three groups: high intensity/low duration endurance trained (HILD), low intensity/high duration endurance trained (LIHD), or untrained (controls). Following the twelve week training period, muscle fibers of the gastrocnemius muscle of the rats were histochemically classified as type I, type IIa, or type IIb following myosin ATPase staining with pre-incubation at pH 4.6. Muscle fiber type distribution and cross-sectional areas were then determined. Neither HILD nor LIHD rats demonstrated significant differences in fiber type distribution compared to controls. When all fiber types were pooled together and analyzed, there were no differences between the three groups with respect to fiber size. However, when the three fiber types were analyzed individually, HILD animals demonstrated a significant reduction in the size of type IIa fibers while LIHD rats experienced a significant diminution in the size of type I and type IIb fibers. Thus, the morphological adaptations of the muscle fibers in the HILD and LIHD groups were fiber type specific.