Answer:
the reversibility of training effects
Explanation:The reversibility of training effects means that trainers can loose the effects of thier efforts and n training when they stop, but can achieve the effects back when they resume.
Mark who trained vigorously for a 5K race and improved his time stopped for 2 hrs , when he returned found that his performance had deteriorated by 55 seconds .
This means that all his training effort and effects had been reversed. He will have to start over and be consistent if he wants to continue in his optimum pace and improve further.