Morphology of Immobilized Skeletal Muscle and the Effects of a Pre- and Postimmobilization Training Program*

The hind limbs of mice were immobilized with plaster cast for different periods of time, and the atrophy of the anterior tibial muscle was examined by measuring fiber cross sections. In a second series of experiments, mice were trained on a treadmill before and after immobilization. The most pronounced decrease in fiber diameters was observed during the 1st week; during prolonged immobilization, only a moderate atrophy occurred. Red fibers were found to be more susceptible to immobilization atrophy than white fibers. The ultrastructural observations extended to loss and fragmentation of myofibrils, mitochondria, and the sarcotubular system. Some fibers split and appeared to undergo segmental necrosis, which was followed by invasion of leucocytes into the muscle. Still while immobilized, the muscles exhibited a regenerative capacity; satellite cells differentiated to myoblasts, which fused to myotubes, being the precursors of new muscle fibers. This was already observed during the 1st week of immobilization. The effect of training after immobilization was documented by an increase of fiber diameters. The ultrastructural alterations, however, in these muscles were severe; it was concluded that a postimmobilization training has to be undertaken very carefully. When the muscles were trained before immobilization, the atrophy was almost negligible. A preimmobilization training can probably prevent the muscle from developing severe atrophy.