Abstract
Muscle is highly plastic in terms of size (maximum force), speed, maximum power, and endurance. Well-controlled studies in animals have shown that the adult skeletal muscle fiber has a remarkable ability to modify its gene expression so that with long-term substantial changes in the daily activity pattern the contractile phenotype can be modified across the whole spectrum of fiber type found in control muscle. The contractile phenotype in this context includes the isoform content of myosin and therefore the maximum velocity of shortening, the mitochondrial content and therefore the specific force and aerobic capacity (endurance), and the calcium handling proteins and therefore the speed of activation and relaxation. With voluntary exercise in human subjects, similar responses are observed, although the degree of transformation is restricted by the practical limitations of exercise dosing to changes in mitochondrial activity and muscle size rather than the more profound changes in contractile protein isoform that can be induced with artificial activation over a substantial proportion of the day.
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