Myosin ATPase and fiber composition from trained and untrained rat skeletal muscle.

BAGBY, GREGORY J., WALTER L. SEMBROWICH, AND PHILIP D. GOLLNICK. Myosin A TPase and fiber composition from trained and untrained rat skeletal muscle. Am. J. Physiol. 223(6): 1415-1417. 1972.-Myosin ATPase and fiber composition were determined in the gastrocnemius muscle of sedentary, endurance-trained, and sprint-trained rats. The endurance training was running in motordriven wheels 60 min/day (5 days/week) at a speed of 28.4 m/min. The sprint-training program consisted of alternate 30-set exercise and rest periods. At the end of the 1 l-week period, the rats of this group were completing 18 bouts of exercise at a speed of 80.4 m/min. Myosin ATPase at pH 6.8 or 9.4 was not significantly altered by the training programs. Muscle fiber types were identified histochemically as slow or fast twitch on the basis of myosin ATPase activity. No changes in the percentage of the two fiber types existed in either the red or white regions of the gastrocnemius muscle among the three experimental groups. The fiber composition of the entire gastrocnemius muscle was determined from two endurance-trained and two untrained rats. There were 8.1 and 8.37$slow-twitch fibers in the more than 20,000 fibers within the gastrocnemius muscle of the untrained and trained animals, respectively. These results indicate that no change in myosin ATPase or fiber composition occurs in rat skeletal muscle following strenuous training.