Effect of high-intensity running in rectus femoris muscle fiber in rats.

The effect of a 12-week high-intensity intermittent exercise program on fiber type composition and the oxidative capacity of rectus femoris skeletal muscle from 20 male Wistar rats (Trained, n = 10; Sedentary, n = 10) was histochemically determined. The training exercise program was developed in a motorized treadmill. It consisted of four running bouts of 2 min duration at 48 m/min, alternated with recovery intervals of 4 min. Training increased relative cross-sectional area of oxidative fibers (I, IIA, IIX) and decreased the same parameter in type IIB non-oxidative fibers (P < 0.001). Our results suggest that this type of strength exercise program is enough to induce changes in muscle fiber composition. This opens a possibility to use this kind of exercise in preventing and treating muscle atrophy.