Contractile activity is an important determinant of the size, rate of protein turnover and phenotypic properties of muscle. Animal models that decrease muscle activity invariably accelerate the rate of protein degradation, usually complementing decreases in the rate of protein synthesis. The net effect is muscle atrophy. By contrast, increased activity and/or passive stretch enhance the synthesis of new proteins, whilst protein catabolism may be either decreased or increased. Muscle hypertrophy results. Endurance activities in man and animals usually induce cardiac hypertrophy, and increased fatigue resistance in skeletal muscle. During exercise the whole body and its skeletal musculature exhibit a negative nitrogen balance, and there is general agreement that rates of protein synthesis are decreased. Changes in protein degradation are, however, much less clearly defined. Resistance exercises induce the opposite changes, with the size of the heart remaining unchanged whilst the bulk and strength of skeletal muscle increase. No real consensus currently exists about the nature of the changes in protein turnover with this type of exercise. More carefully designed and executed experiments are required.

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