Maximal oxygen consumption (often abbreviated to VO₂ max) is the maximum volume of oxygen consumed per minute. The fitter a person is, the more oxygen he or she can draw from the blood. Oxygen is extracted from air in the lungs, transported in the blood, and then utilized by respiring tissues to release energy from food. The energy that is released is used to synthesize ATP, a high energy compound which is the only direct source of energy for the body's activities. Maximal oxygen consumption is therefore a measure of a person's ability to use aerobic respiration as a source of energy. As such, it also reflects a persons aerobic work capacity, endurance capacity, and maximal aerobic power.

Maximal oxygen consumption is usually expressed in units of millilitres of oxygen consumed per kilogram body mass per minute. It is determined during a large muscle group activity (e.g. cycling or running). The intensity of the activity is progressively increased until exhaustion. Sometimes maximal oxygen consumption is expressed as litres of oxygen consumed per minute, to indicate total work capacity. The average value for a 20-year-old female is between 32-38 ml/kg/min; for a 20-year-old male it is 36-44 ml/kg/min.

Maximal aerobic capacity can improve with training. The amount of improvement is highly individualized and inversely related to the initial level of fitness. A sedentary person may experience as much as a 25 per cent increase in VO₂ max after only 8 weeks training; someone used to aerobic exercise may experience 5 per cent improvement or less in the same time. There is an upper limit of oxygen consumption beyond which training has no effect. This limit seems to be genetically determined and may be reached after 18-24 months of intensive endurance training.

Food and Fitness: A Dictionary of Diet and Exercise. Copyright © 1997, 2003 by Oxford University Press. All rights reserved.