Functional adaptations to physical activity and inactivity.

Rather than focusing on the performance criteria accompanying adaptation to physical activity, this paper emphasizes the magnitudes of alteration in the function of the circulatory, respiratory, and metabolic systems with adaptation. It is our opinion that the limitation of maximal aerobic power resides in the transport of oxygen to working muscle by the circulation. Increases in maximal aerobic power that accompany physical conditioning are attributed primarily to increased maximal muscle blood flow and muscle capillary density. The increase in the oxidative potential of skeletal muscle after training is presented as the mechanism by which capacity for submaximal work is augmented.