It has been suggested that, on a practical level, the transformation from one muscle fiber type to another is impeded by a number of natural conditions. In the case of the transformation of slow twitch into fast twitch fibers, any transformation stimulated by training may be countered by the use of the trained muscles for postural reasons. The transformation of FT fiber types to ST is probably impeded by the fact that considerable effort is required in order to reach the threshold necessary to activate the FT muscles often enough to transfer them to a slower type.
Training appears to selectively hypertrophy muscle fiber types.
Most of hypertrophy is due to increases in the diameter of fast glycolytic fibers. It should be noted that an increase in the diameter of the muscles fiber is caused primarily by increased synthesis of actin and myosin filaments, which leads to a greater opportunity for cross-bridge interaction and hence an increase in contractile strength. ST fibers appear to grow less easily. Nevertheless, hyperplasia through a lengthwise split of an enlarged muscle fiber may occur to some small extent with unusual levels of training stress. Endurance training can increase the amount and size of mitochondria, the muscle's ATP-synthesizing capacity, as well as capillarization within muscle tissue.