Definition: A point during exhaustive, all-out exercise at which lactic acid builds up in the blood stream faster than the body can remove it. Lactic acid is a by-product of the anaerobic energy pathway, a process which provides energy to muscles by partially breaking down glucose without the need for oxygen.

Anaerobic metabolism produces energy for short, high-intensity bursts of activity (lasting no more than a few minutes) before the lactic acid build-up reaches a threshold where it can no longer be absorbed and, therefore, accumulates. This point is known as the lactate threshold and is usually reached between 50 to 80% of a athlete's VO2 max.