What Does Reversibility Mean in Sport?

According to Sports Training Adviser, athletes require rest periods to allow their bodies to recover; however, extended intervals of rest reduce a body's physical fitness. During these periods of extended rest, the physiological effects of fitness training diminish, and the body reverts back to its pretraining condition. The common phrase "if you don't use it, you lose it" derives from and applies to the concept of reversibility in sports.