If the amount of oxygen required by your muscles during a workout exceeds your aerobic capacity, your muscle cells will .

anaerobic respiration

Explanation:

In the absence of oxygen that is responsible for pulling electrons down the transport chain for ATP generation, oxidative phosphorylation ceases. The cell switches to anaerobic respiration or fermentation

So during strenuous exercise the muscles lacks oxygen. Muscles cells switches to anaerobic condition that convert pyruvate to lactate by lactate dehydrogenase. Lactate tends to accumulate in the muscles which can cause muscles cramps. To prevent lactate accumulation the muscles reconvert lactate to glucose using the Cori cycle.