Increase of muscle fiber number induced by spontaneous running exercise

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The effect of spontaneous running exercise, which induced muscular enlargement, on fiber number was studied using female weaning rats. Animals were randomly assigned to either an exercising (E) group or a sedentary control (C) group. Exercising animals were placed in steel cages free access to running wheel for spontaneous exercise. After 7 weeks of exercise, muscle weights i.e., heart, gastrocnemius, quadriceps and sartorius increased significantly in E group. Muscle fiber number was investigated in sartorius of which most fibers run the length of each fascicle from its origin to insertion. Sartorius, long muscle, was divided into several sections for the investigation. Fiber number was counted through profile projector by two persons isolating each single fiber under a dissecting microscope after nitric acid tratment.In another different approach, fiber number was counted on photomicrographs of the transverse sections of the muscle. The two approaches confirmed significant increase o … More f fiber number in enlarged muscle following spontaneous running exercise.Fiber number varied in different regions of the muscle, showing the peak of number in the middle of sartorius and decreasing number in regions closest to the origin and insertion. In the middle of the muscle, the total fiber number of exercised muscle increased significantly as compared with control muscle, although muscle fiber area did not change. On the other hand, in the regions of origin and insertion of the muscle, fiber area of E group increased significantly but the total fiber number did not change. There appeared many small fibers in the middle of exercised muscle, which was found to be tapering fiber under a differential interference microscope. DNA and RNA contents in E group were much more than those in C group, espeially in the middle of the muscle. Thus, we concluded that muscle hypertrophy was attributed to increased fiber number in the middle of the muscle and increased fiber area in regions of the origin and insertion, being supported by the biochemicala data. Further, an interference microscopy demonstrated that increase of fiber number in the middle was partly due to appearance of tapering fiber. Less