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.
The difference between the amount of oxygen needed by the muscles and the actual amount present is called the oxygen debt. This process requires oxygen, and the amount of oxygen required to recover from strenuous exercise is equal to the oxygen debt.