anaerobic threshold

The level of exercise at which the rate of oxygen uptake into muscle becomes limiting and there is an increasing proportion of anaerobic metabolism to yield lactate. See also aerobic.

If you start running at a slow pace and gradually increase your speed, there comes a point when the energy supplied by aerobic (oxygen-dependent) respiration is insufficient to sustain your activity. Put simply, you start to get out of breath. This point is known as the anaerobic threshold and reflects your capacity to work at a steady state. It is an important indicator of aerobic fitness. A number of methods have been used to determine this threshold (see blood lactate and conversational index), but none is completely satisfactory. Nevertheless, whichever method is used the anaerobic threshold is higher in those who perform regular, vigorous aerobic exercises than in sedentary people.

The level of activity at which the aerobic energy system cannot meet all of the body's demands for ATP. As the intensity of activity increases above the threshold, energy supply becomes increasingly dependent on anaerobic metabolism. A number of methods have been used to determine anaerobic threshold, including ventilatory breakpoint (see minute ventilation method) and lactate threshold. However, no method is completely satisfactory. The lactate threshold, for example reflects anaerobic threshold under most conditions, but the relationship is not perfect.