Response of type I fibers to weight lifting in rat plantaris

The purpose of this histochemical study was to determine the effect of weight lifting exercise training on the muscle fibers in 2 muscle regions of the rat plantaris. Adult female Wistar rats were trained to carry weights up to 150 gm on their backs, up a 50 degrees incline, for a period of 6 weeks. A comparison between the exercised and control groups showed no significant muscle hypertrophy or change in the number of fibers. Only the smallest muscle fibers, which are located predominantly in the superficial region of the plantaris, showed a significant increase (19.6%, p less than 0.01) in diameter. The data suggest that the smallest muscle fibers, in the superficial region of the plantaris in this weight lifting training program, were exercised out of proportion to their usual activity and responded to the stress of the activity by enlarging.