Under normal levels of activity, muscle cells have ample oxygen to allow the pyruvate produced during glycolysis to be converted to acetyl-coenzyme A and enter the Krebs cycle within the mitochondria. But when an individual has engaged in intense physical activity without being conditioned for it, a muscle may lose its ability to contract.
When oxygen in muscle tissue has been used up during strenuous exercise, pyruvate enters fermentation pathways instead of the Krebs cycle, and an accumulation of lactic acid is the result. Muscles fatigue as lactic acid accumulates, lowering muscle pH. Activity ceases when the person is too weak, and in too much pain, to use his or her muscles further. Over a period of time, the blood carries away the lactic acid to the liver, where it is converted back to glucose. In fact, the oxygen debt is the amount of oxygen needed to convert the lactic acid back to glucose plus the amount needed to restore lost ATP in muscle tissue.
Interestingly enough, athletes or individuals who engage in regular aerobic exercise do not accumulate lactic acid as rapidly. There appear to be several reasons for this. First, their muscles are conditioned and stronger and have a better blood supply. Regular exercise promotes the development of more capillaries within muscle, so the delivery of oxygen to muscle tissue is better than in nonathletes. Second, the hearts of athletes are stronger and can deliver more blood per pumping action. And last, physical exercise induces the production of more mitochondria in muscle cells, so they can carry on the Krebs cycle at an increased rate, supplying ATP longer.