[Ultrastructural changes in the red muscle fibers of the quadriceps muscle of the rat femur during increased motor activity].

Experiments on rats were made to study the effect of physical exercise running in a treadbahn on ultrastructure of skeletal muscle fibers. The biphasic mechanism of muscle contractile activity was shown. The processes of destruction occurring in red muscle fibers of the intensely working quadriceps femoris were manifested by enlargement of T system and sarcoplasmic reticulum elements, mitochondrial matrix swelling, cryst destruction, vacuolar degeneration of part of the mitochondria, and destruction of individual myofibrils. In addition to destructive changes, the muscle exhibited the recovery processes--physiological regeneration. Those processes included large accumulations of the mitochondria beneath the plasma membrane of muscle fibers, the presence of small mitochondria, division of the mitochondria, transformation of myosatellitocytes to myoblasts, the presence of centrioles in endotheliocytes, and so forth.