The process by which glucose is completely oxidized in absence of oxygen, to form alcohol or lactic acid and a small amount of energy is known as fermentation.
However, complete oxidation of glucose in muscle cells produces lactic acid. During hard racing or exercise, the oxygen supply may not be enough for the needs of the muscle cells. When this happens, anaerobic respiration takes place, as well as aerobic respiration. The lactic acid that is formed by anaerobic respiration causes muscle fatigue and pain.
