Actively contracting muscles obtain Adenosine Triphosphate (ATP) from glucose stored in the blood stream and the breakdown of glycogen stored in the muscles. Initially, pyruvic acid and small amounts of ATP are generated from the breakdown of glucose. The pyruvic acid mixed with oxygen is converted to carbon dioxide, water, and ATP. When muscles contract vigorously for long periods, the circulatory system begins to lose ground in the delivery of oxygen. In these conditions, most of the pyruvic acid produced in the breakdown of glucose is converted to lactic acid (LA). As the lactate is produced in the muscles, it leaks into the blood and is carried around the body. If this condition continues, the functioning of the body will become impaired and the muscles will fatigue very quickly. When oxygen becomes available, the lactic acid is converted to pyruvic acid and then into carbon dioxide, water and ATP.
Since a high level of lactate is detrimental to performance, endurance training is used to train the body to perform with a minimal amount of lactate. This may be accomplished by long steady runs, which develop the aerobic capacity by means of capillarisation (formation of more small blood vessels, thus enhancing oxygen transport to the muscles) and by creating greater efficiency in the heart and lungs. With increased aerobic capacity, more oxygen is available to the working muscles and should delay the onset of lactic acid at a given work intensity.