The anaerobic threshold: definition, physiological significance and identification.

During exercise, the oxygen consumption above which aerobic energy production is supplemented by anaerobic mechanisms, causing a sustained increase in lactate and metabolic acidosis, is termed the anaerobic threshold (AT). The oxygen consumption at the AT depends on factors that affect oxygen delivery to the tissues. It is increased when oxygen flow is enhanced and decreased when oxygen flow is diminished. Its value is quite low in patients with heart disease. The AT is an important functional demarcation since the physiological responses to exercise are different above the AT compared to below the AT. Above the AT, in addition to the development of metabolic acidosis, exercise endurance is reduced, VO2 kinetics are slowed so that a steady state is delayed, and VE increases disproportionately to the metabolic requirement and a progressive tachypnea develops. The AT can be measured directly from the lactate concentration with precise threshold detection from a log-log transformation of lactate and VO2. This threshold also defines the VO2 above which the lactate/pyruvate ratio increases. As bicarbonate changes reciprocally with lactate, its measurement can also be used to estimate the lactate threshold. But most convenient are gas exchange measurements made during exercise testing which can be used to noninvasively detect the lactate or anaerobic threshold. These methods are based on the physical-chemical event of buffering lactic acid with bicarbonate, and the increased CO2 output which occurs in association with the acute development of a metabolic acidosis.