The reversibility principle in fitness refers to the concept that workout gains or progress will be lost when an athlete stops training. It is one of the principles of training and is defined as the gradual loss of beneficial training effects when the intensity, duration, or frequency of training is reduced. Reversibility occurs when physical training is stopped, and the body readjusts in accordance with the diminished physiological demand, and the beneficial adaptations may be lost. The extent of losses and deconditioning can depend on several factors, including exercise history, previous length and duration of training, initial strength and fitness levels, age, etc. . The principle of reversibility pertains to the losses in function experienced when training is stopped. The detraining effects can be reversed when training is resumed, but the reversal of the detraining effects will be quicker than the original acquisition of the effects. Understanding the reversibility principle allows clinicians to better develop and implement a plan of care, educate their patients, and enhance their clinical skill set.