Muscle growth and exercise

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Abstract
This paper first reviews muscle growth and then considers the influence of exercise in growth. Knowledge about how muscle cells grow and some factors that may influence the growth pattern are discussed first since these effects must be considered before the influence of exercise becomes clear. Growth of muscle can occur in three ways: (1) by an increase in muscle cell numbers, (2) by an increase in muscle fiber diameter, and (3) by an increase in fiber length. All three of these mechanisms are involved in muscle growth. However, growth in cell numbers is limited to the prenatal and immediately postnatal period, with the animals and man being born with or soon reaching their full complement of muscle cells. Thus, growth occurs by either hypertrophy of the existing muscle fibers by adding additional myofibrils to increase the muscle mass or by adding new sarcomeres to the ends of the existing muscle fibers to increase their length. Both of these mechanisms occur during the growth process. Growth in the girth of the muscle fibers appears to take place by splitting of the myofibrils. This may be stimulated by development of stress creating an unequal pressure with splitting at the Z‐band and development of additional SR and T‐tubule systems. This adds to the diameter or girth of myofibers without any hyperplasia. The growth in length occurs at either end of the fibers and results in addition of new sarcomeres. In both cases, new myofibrillar protein must be synthesized and deposited in the muscle cells. It is suggested that adaptation by adding or removing sarcomeres is physiologically determined by the degree of force a muscle can generate that is in turn dependent on the degree of overlap of the thick and thin filaments. Thus, the amount of tension would control the number of in‐series sarcomeres in a single muscle fiber. Nutrition is also known to play an important role in muscle and was discussed from the standpoint of the effects of nutritional adequacy and restriction. Although a nutritionally balanced and calorically adequate diet is required to achieve optimum muscle growth, it may be less efficient in terms of protein deposition than a moderately restricted diet. Muscle and bone deposition, however, can be limited on severely restricting the dietary intake. Although fat deposition is the first tissue to suffer on a severely restricted diet, muscle and bone follow next with the nervous system, brain and eyes being the last systems to be affected. Sex effects on muscle growth were also considered. Generally, the intact male is the most efficient in muscle growth. The female, although less efficient than the castrated male in some species (the pig), generally produces muscle more efficiently than the castrated male. Some of the sex effects on meat quality such as darker colored lean and tougher meat from the intact male are considered and discussed. The effects of exercise on muscle are considered in the remainder of this chapter. The factors involved in muscle hypertrophy are discussed, including the mechanism of accumulation of muscle mass, which is readily apparent in weight lifting, wrestling, and similar exercises. Exercise does not appear to greatly alter the proportion of the different muscle fiber types, which is also true for inactivity or immobilization. De‐nervation and cross innervation experiments have demonstrated that it is possible to change muscle fiber types. The changes appear to be the result of the type innervation, which it is suggested may develop due to the demands placed upon the muscle for a particular type of activity. This demand appears to in some way send a signal that causes the muscle cell to synthesize the proper kinds of myofibrillar proteins. The effects of exercise on metabolic activity and some factors influencing it were also covered. Changes in the ATPases, glycogen stores, and oxidative capacity as a result of exercise are also discussed, as were changes in the glycolytic and oxidative enzymes. The effects of exercise on the connective tissues, the influence of pyruvate and lactate concentrations and H+ ‐ions are reviewed as were such interesting physiological adaptations as fatigue, muscular injuries, or soreness, diving, and high altitude acclimatization. Mechanisms and fundamental processes involved in each of these situations are discussed in relation to exercise and its effects on muscle.