VO2 max, or maximum oxygen consumption, is an index of physical fitness and a measure of the maximum amount of oxygen that the body can utilize during exercise. Generating the energy needed in the muscles during exercise requires oxygen. Transporting oxygen from the outside air to the muscles involves the integrated function of several parts of the body, including the lungs, heart, blood, blood vessels, and the muscles which are the engines that produce the energy to do the physical work. Regular physical activity increases the body's ability to transport and utilize oxygen. This results in improved exercise tolerance and VO2 max, reflecting better fitness and aerobic capacity.
ANDREW L. RIES
American College of Sports Medicine (2000). ACSM's Guidelines for Exercise Testing and Prescription, 6th edition. Philadelphia, PA: Lippincott Williams & Wilkins.