Amy gained cardiovascular health benefits from running consistently over the past year. Now she runs inconsistently and has lost some of those health benefits. This principle is known as tedium progression specificity reversibility

Reversibility.

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.'