These processes require oxygen. This is why, when the period of activity is over, a person's breathing rate and heart rate do not return to normal straightaway. The amount of oxygen required to remove the lactic acid, and replace the body's reserves of oxygen, is called the oxygen debt.
The lactic acid builds up and causes an oxygen debt. The lactic acid building up also causes fatigue. Removing the lactic acid after exercise is the cause of the oxygen debt – the oxygen debt is why you breathe deeply after exercise for some time. You are 'repaying' the oxygen debt.
If anaerobic activity takes place over a long period of time, the muscles soon become exhausted. This is due to a condition called oxygen debt. Strenuous exercise uses up all ATP stores and causes a build-up of lactic acid. This is a toxic (poisonous) substance which causes the muscles to stop working.