Effects of immobilization and subsequent low- and high-intensity exercise on morphology of rat calf muscles.

After a cast immobilization of 3 weeks, the effects of 4-week remobilization by free cage activity or treadmill running on the morphology of the rat soleus and gastrocnemius muscles were studied. The studied morphometric parameters were: percentage volume of intramuscular connective tissue, capillary density, muscle fiber size, number of fibers with a pathological structural alteration, and fiber type distribution. In both muscles, immobilization of 3 weeks produced a significant increase in the connective tissue volume and number of fibers with pathological alterations, with a similar decrease in the capillary number and fiber size. At the same time, the relative amount of type I fibers decreased and type IIA fibers increased. Free remobilization and especially intensified remobilization by treadmill running significantly restored these values towards controls. These findings indicate that in rat soleus and gastrocnemius muscles immobilization-induced accumulation of intramuscular connective tissue, capillary loss, reduction in fiber size, accumulation of fibers with pathological structural alterations, and changes in fiber type distribution are to a great extent reversible phenomena, especially if remobilization is intensified by physical training. In clinical practice, this suggests that in patients with musculoskeletal injuries the postimmobilization rehabilitation should be early and effective.