In order for the muscle fatigue to end the muscle cells must be provided with

oxygen

Explanation:

Everything needs oxygen, even your cells because oxygen is crucial in process of ATP production which basically powers all of our actions. Hence, when you cells have low O2 your muscles will tire out quickly due to lack of energy thus you need to replenish them with Oxygen so that you can put an end to fatigue.

it will increase due to the time span