Reversibility in sport means that an athlete that does not train or use his athletic abilities loses them or sees them diminish over time. Conversely, an athlete can regain his athletic abilities over time if he resumes training and uses his abilities.
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.