During vigorous exercise, the body needs a lot more energy, and therefore has to get more oxygen into the muscle tissue where energy is needed. Therefore it has to breathe deeper and faster. The oxygen is then used to change the chemical energy in glucose to energy which the muscles are then able to use. At some point, a limit is reached – the body cannot breathe deeper or quick enough that aerobic energy cannot cope with the amount of energy needed. This causes the muscles to respire anaerobically. However, with anaerobic respiration, lactic acid is produced (CH3CH(OH)CO2H). This accumulates in the muscles, and causes cramp and muscle tiredness. At the end of exercise, this lactic acid has to be turned into CO2 and O2 immediately after exercise has finished. This is an oxidation reaction and requires oxygen.
CH3CH(OH)CO2H + 3O2 --> 3CO2 + 3H2O
This means your body is working harder and you're breathing in a lot of oxygen but you cannot absorb enough to cope with the level of activity.This extra oxygen needed to remove the harmful effects of anaerobic respiration is called an oxygen debt. The body must continue to breathe deeply once exercise has stopped in order to ‘pay back the debt’. One measure of a person’s fitness is how quickly their breathing returned to normal after exercise. A fitter person will build up less of an oxygen debt while exercising, and therefore will not need as much oxygen to break down the lactic acid in their muscles.