Training-induced morphological and functional changes in skeletal muscle.

In addition to the histochemical ATPase staining techniques, immunohistochemistry and two-dimensional gel electrophoresis lead to a better understanding of the molecular structure of the contractile proteins actin, myosin, tropomyosin, and troponin in the different fiber types of human skeletal muscle. A great plasticity enables muscle fibers to adapt to different genetic and environmental influences not only on the level of cellular ultrastructure and metabolic function, but also on the molecular level of the contractile proteins. Fiber type transformation is possible with cross-innervation and specific electrical stimulation. Prolonged and intense endurance training probably also transforms type II fibers into type I fibers, whereas with spring or strength training changes in the different fiber types are restricted to ultrastructure (e.g., myofibrillar to mitochondrial volume ration) and metabolic function. The fiber type distribution pattern in top athletes seems to be determined by both hereditary and environmental factors.