When we exercise, the muscles contract. This requires energy, which comes from the breakdown of ATP into ADP and a phosphate. ATP is produced by respiration. Initially, whilst there is sufficient oxygen being delivered to the cell by the blood, aerobic respiration takes place. However, when there is not enough oxygen for aerobic respiration to occur, anaerobic respiration occurs instead, which produces less ATP. After exercising has stopped, lactic acid has been produced, and oxygen is needed to break this down in the liver. Therefore, excess oxygen is needed to breakdown the lactic acid produced, and also for aerobic respiration to continue. This excess oxygen is the oxygen debt.