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The exact mechanism by which exercise augments strength remains unclear, but its basic principles are understood. Two processes appear to be involved: hypertrophy, or the enlargement of cells, and neural adaptations that enhance nerve-muscle interaction.
Muscle cells subjected to regular bouts of exercise, followed by periods of rest that include a sufficient intake of dietary protein, undergo hypertrophy. (This should not be confused with short-term swelling resulting from water uptake into cells.) Improved muscle protein synthesis and incorporation of these proteins into cells cause the muscle-building effect. When a muscle cell is activated by its nerve cell, the interaction of the proteins responsible for muscle contraction - actin and myosin - generates force via changes in protein structure called power strokes. The total force generated depends on the sum of all the power strokes occurring simultaneously within all the cells of a muscle. Because more potential power strokes accompany an increased presence of actin and myosin, the muscle can exhibit greater strength of contraction. In addition, hypertrophy is aided by certain hormones and has a strong genetic component.