Skeletal muscle fiber splitting induced by weight-lifting exercise in cats.

Adult skeletal muscle hypertrophy induced by exercise has been thought to be exclusively related to an increase in cross-section area of individual muscle fibers and not to an increase in the number of muscle fibers. Recent experiments using surgical intervention to cause muscle overload have induced an increase in fiber numbers; however, the muscle also exhibited pathological alterations. The purpose of this study was to determine if an exercise regimen also induced hyperplasia. Cats were trained to lift weights with their right forelimb to receive a food reward. After 19-46 weeks of training, the flexor carpi radialis muscle (FCR) was removed and prepared for histochemical examination. The total number of muscle fibers of the right exercised FCR increased significantly (19.3%) when compared with that of the unexercised left FCR (p less than 0.05). This increase was found to be due to muscle fiber splitting.