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Muscles need energy to contract. While exercising, the muscles need additional energy as:

• the breathing rate and volume of each breath increases to bring more oxygen into the body and remove the carbon dioxide produced
• the heart rate increases, to supply the muscles with extra oxygen and remove the carbon dioxide produced

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:

• lactic acid levels build up
• glycogen reserves in the muscles become low as more glucose is used for respiration, and additional glucose is transported from the liver.

This build-up of lactic acid produces an oxygen debt.

As body stores of glycogen become low, the person suffers from muscle fatigue.