V̇O max (also maximal oxygen consumption, maximal oxygen uptake or maximal aerobic capacity) is the maximum rate of oxygen consumption attainable during physical exertion. The name is derived from three abbreviations: "V̇" for volume (the dot over the V indicates "per unit of time" in Newton's notation), "O" for oxygen, and "max" for maximum and usually normalized per kilogram of body mass. A similar measure is V̇O peak (peak oxygen consumption), which is the measurable value from a session of physical exercise, be it incremental or otherwise. It could match or underestimate the actual V̇O max. Confusion between the values in older and popular fitness literature is common. The capacity of the lung to exchange oxygen and carbon dioxide is constrained by the rate of  blood oxygen transport to active tissue. 
The measurement of V̇O max in the laboratory provides a quantitative value of endurance fitness for comparison of individual training effects and between people in endurance training. Maximal oxygen consumption reflects cardiorespiratory fitness and endurance capacity in exercise performance. Elite athletes, such as competitive distance runners, racing cyclists or Olympic cross-country skiers, can achieve V̇O max values exceeding 90 mL/(kg·min), while some endurance animals, such as Alaskan huskies, have V̇O max values exceeding 200 mL/(kg·min).
In physical training, especially in its academic literature, V̇O max is often used as a reference level to quantify exertion levels, such as 65% V̇O max as a threshold for sustainable exercise, which is generally regarded as more rigorous than heart rate, but is more elaborate to measure.