Answer :
The training principle that states you must continue to exercise to maintain fitness would be reversibility - this states that your muscles can revert into the state of where they were before you started seriously moving and being active.
The training principle states that you must continue to exercise to maintain fitness is reversibility. This principle states that if the mobility training is ceased, the ranges of movement will decline over time or if you stop working out, you will lose the effects of training. Regular use of mobility exercises is crucial for improvement.