Biochemical Adaptations to Endurance Exercise in Muscle

Two quite distinct adaptive responses can be induced in skeletal muscle by regularly performed, strenuous exercise. The nature of the exercise stimulus determines the type of adaptation. One type of adaptation involves hypertrophy of the muscle cells with an increase in strength; it is exemplified in its most extreme form by the muscles of weight lifters and body builders. The second type of adaptation involves an increase in the capacity of muscle for aerobic metabolism with an increase in endurance and is found in its most highly developed form in the muscles of competi­ tive middleand long-distance runners, long-distance cross-country skiers, bicy­ clists, and swimmers. Although many types of physical activity can bring about varying degrees of both types of adaptation in the same muscle, it does appear that these adaptations can occur quite independently of each other in their most extreme forms. For example, the hypertrophied muscles of weight lifters do not appear to have an increased respiratory capacity (45), whereas the muscles of rodents trained by prolonged daily running, which have a large increase in respiratory capacity, are not hypertrophied (56, 92) and show no increase in strength (12). This review deals with the biochemical adaptations induced in skeletal muscle by the endurance type of exercise and with the physiological consequences of these adaptations.