Answer:
Reversibility.
Step-by-step explanation:
'Reversibility' principle in the context of sports training asserts that 'an athlete may tend to lose the health benefits of a specific training when they put a halt on their practice and gain these benefits back if they start practicing again.'
As per the given description, Amy's gain of 'cardiovascular health benefits' from constant practice of running and loss of these benefits when she starts practicing inconsistently, it leads her to lose few of those benefits' which clearly justifies the above definition and thus, reflect the principle of 'reversibility.'