Effects of prolonged voluntary wheel-running on muscle structure and function in rat skeletal muscle.

We examined the effects of prolonged voluntary wheel-running on skeletal muscle functional and/or structural characteristics in rats. Male Sprague-Dawley rats (5 weeks old) were divided into five groups: (1) 15W-SC, sedentary controls housed in normal plastic cages until age 15 weeks; (2) 15W-VE, housed in a voluntary-exercise (running-wheel) device equipped with housing space until age 15 weeks; (3) 35W-SC, housed in normal plastic cages until age 35 weeks; (4) 35W-VE, housed in the voluntary-exercise device until age 35 weeks, and (5) 35W-MVE, housed in normal plastic cages until age 15 weeks, then in the voluntary-exercise device from age 16 weeks to 35 weeks ("middle age"). At the end of each rat's experimental period, the plantaris muscle was dissected from each hindlimb for analysis of the muscle's functional and/or structural characteristics. Total running distance was similar in 15W-VE and 35W-VE, both being significantly greater than in 35-MVE. The percentage of type IIb myosin heavy chain isoform was significantly lower in each VE group than in the corresponding SC group. This shift from type IIb was significantly greater for 35W-VE than for the other VE groups, which were similar to each other. The cross-sectional area of type IIx fibers was significantly greater in 35W-VE than in 35W-SC, but this was not true for 15W-VE versus 15W-SC or for 35W-MVE versus 35W-SC. No significant difference in citrate synthase activity was detected between any VE group and the corresponding SC group. These results suggest that a prolongation of voluntary wheel-running leads to some advantageous enhancements of functional and/or structural characteristics in rat plantaris.