Histochemical characteristics of muscle fibers from trained and detrained guinea pigs.

FAULKNER, JOHN A., LEO C.MAXWELL, AND DAVID A. LIEBERMAN. Histochemical characteristics of muscle Jibers from trained and detrained guinea pigs. Am. J. Physiol. 222(4) : 836-840. 1972.~Sixweek-old guinea pigs have nearly 12,000 fibers per plantaris cross section. With aging, some 2,700 fibers were lost. Daily endurance running prevented the loss of fibers and fibers were smaller. Trained animals had 62y0 red fibers in the plantaris and 87 y0 in the diaphragm compared to 48 and 71%, respectively, in age-matched controls. To study detraining, 10 young guinea pigs were trained for 8 weeks. Training was terminated and animals were sacrificed after 4, 8, and 16 weeks of detraining. After 4-8 weeks, the percentage of red fibers in the plantaris was 48$?& and 1,100 fibers were lost. Fiber areas increased, but were less than control values. After 16 weeks of detraining, an additional 1,600 fibers were lost and mean fiber areas and percentage of red fibers were comparable to control data. With detraining the percentage of red fibers regressed more slowly in the diaphragm than in the plantaris. Our data indicate that detraining involves a regression of muscle fiber characteristics toward control values.