Muscles need energy to contract. While exercising, the muscles need additional energy as:
If insufficient oxygen is available to the muscles, for instance the exercise is vigorous and/or prolonged, the heart and lungs are unable to supply sufficient oxygen. Muscles begin to respire anaerobically. Lactic acid is produced from glucose, instead of carbon dioxide and water. Muscles continue to contract, but less efficiently.
During long periods of vigorous activity:
This build-up of lactic acid produces an oxygen debt.
As body stores of glycogen become low, the person suffers from muscle fatigue.