The anerobic threshold (AT) is an exercise intensity at which lactate (lactic acid) starts to accumulate in the blood stream faster than it can be metabolized . Below the AT any lactate produced by the muscles is removed by the body without it building up.
The only accurate way to measure anerobic threshold is via blood samples. The AT is defined as a lactate concentration of 4 mmols (at rest it is around 1 mmol). There are ways of estimating AT based on heart rate but these can be very inaccurate.
A person's anerobic threshold can be improved by exercising, however the threshold will never exceed the person's VO2max - the point at which the body cannot supply any more oxygen to the muscles. The relation between the two is reliant on the amount of training - an untrained individual's AT is approximately 55% of VO2max, but elite endurance athletes are about 80-90% of VO2max.
See also: aerobic threshold, VO2max
