In humans, the term reserve capacity can be defined as the excess capacity, or reserves, above that needed for normal functioning (load), as in carrying out activities of daily living. Many systems of the body have reserve capacities (e.g., functional reserves of the lungs and heart, fuel depot reserves for metabolism, or reserve of a single enzyme). Such reserves are necessary for the body's systems to make appropriate adjustments to normal fluctuations and unanticipated increases in load, to respond to emergencies, and to act as safety nets. Simply put, reserve capacity allows for proper adjustments and adaptations to alterations in the surrounding environments.

The multiple reserve capacities composing the body's systems, collectively and coordinately, culminate in producing the functional reserves of the whole person (e.g., physiological, ...