Final answer:
The principle of reversibility implies that athletes should maintain a moderately high level of fitness during the off-season to prevent loss of previous fitness gains.
Step-by-step explanation:
The principle of training that supports the recommendation that athletes maintain a moderately high level of fitness during the off-season is reversibility. Reversibility refers to the loss of fitness that occurs when an athlete stops training; hence, it's vital for athletes to continue a certain level of exercise to prevent a significant decline in fitness levels.
During the off-season, maintaining a moderately high level of fitness ensures that athletes do not lose the gains they have made and are ready for the upcoming season's training and competitions.