The Response of Tibialis Anterior Muscle Fibers in Mice to IncreasedMuscle Activity

Cross-sectional areas and oxidative enzyme (succinate dehydrogenase) activities of fibers in the deep, middle, and superficial regions of the mouse tibialis anterior muscle were examined and compared after 30 days of increased muscle activity with voluntary running exercise. Eight 19-week-old male mice were assigned randomly to either a sedentary control (control, n=4) or voluntary running exercise (exercise, n=4) group. There were no differences in the cross-sectional area of fibers between the control and exercise groups, irrespective of the muscle region. The oxidative enzyme activities of fibers in the middle and superficial regions of the muscle were greater in the exercise group than in the control group. These data indicate that increased muscle activity with voluntary running exercise increases the oxidative enzyme activity of muscle fibers that are recruited to perform and adapt to a given condition of activity.