The Reversibility Principle dictates that athletes lose the beneficial effects of training when they stop working out. Conversely, it also means that detraining effects can be reversed when athletes resume training. In short, If you don't use it, you lose it. While rest periods are necessary for recovery, extended rest intervals reduce physical fitness. The physiological effects of fitness training diminish over time, causing the body to revert back to its pretraining condition.
The length of the detraining period and the training status of the athlete dictate how much performance is lost.