ADAPTATION OF RAT SOLEUS MUSCLES TO 4-WK OF INTERMITTENT STRAIN

The effect of repeated strains on rat soleus muscles was investigated by stretching active muscles 3 times/wk for 4 wk with two different me thods of stretching. The adaptation of myofibers and noncontractile ti ssue was followed by histochemical techniques and computer-assisted im age analysis. Muscle hypertrophy was seen in the slow-stretched muscle s, which increased in mass by 13% and increased in myofiber cross-sect ional area by 30%. In the fast-stretched muscle, mass increased by 10% but myofiber cross-sectional area actually decreased. This decrease i n mean fiber area was the result of a population of very small fibers (population A) that coexisted with slightly smaller normal-sized fiber s (population B). Fibers in population A did not have the distribution expected from atrophy compared with atrophic fibers from unloaded mus cles; they were much smaller. In addition, there was a 44% increase in noncontractile tissue in the fast-stretched muscles. Thus, soleus mus cles subjected to repeated strains respond differently to slow and fas t stretching. Slow stretching results in typical muscle hypertrophy, w hereas fast stretching produces somewhat larger muscles but with a mix ture of small and normal-sized myofibers accompanied by a marked proli feration of noncontractile tissue.