Steven S. Segal
Physical exercise is often the greatest stress that the body encounters in the course of daily life. Skeletal muscle typically accounts for 30% to 50% of the total body mass. Thus, with each bout of muscular activity, the body must make rapid, integrated adjustments at the level of cells and organ systems—and must tune these adjustments over time. The subdiscipline of exercise physiology and sports science focuses on the integrated responses that enable the body to convert chemical energy into mechanical work. To understand these interdependent processes, one must appreciate where regulation occurs, the factors that determine physical performance, and the adaptations that take place with repetitive use.
The cross-bridge cycle that underlies contraction of skeletal muscle requires energy in the form of ATP (see pp. 234–236). Skeletal muscle converts only ~25% of the energy stored in carbon-carbon bonds (see p. 1170) into mechanical work. The rest appears as heat, due to the inefficiencies of the biochemical reactions (see pp. 1173–1174). Thus, the dissipation of this heat is central to cardiovascular function, fluid balance, and the ability to sustain physical effort—an example of an integrated organ-system response. Moreover, because muscle stores of ATP, phosphocreatine, and glycogen are limited, the ability to sustain physical activity requires another set of integrated cellular and organ-system responses to supply O2 and energy sources to active muscles.