Accurately measuring V̇O max involves a physical effort sufficient in duration and intensity to fully tax the aerobic energy system. In general clinical and athletic testing, this usually involves a graded exercise test in which exercise intensity is progressively increased while measuring: 
* ventilation and 
* oxygen and carbon dioxide concentration of the inhaled and exhaled air.
V̇O max is measured during a cardiopulmonary exercise test (CPX test). The test is done on a treadmill or cycle ergometer. In untrained subjects, V̇O max is 10% to 20% lower when using a cycle ergometer compared with a treadmill. However, trained cyclists' results on the cycle ergometer are equal to or even higher than those obtained on the treadmill.
The classic V̇O max, in the sense of Hill and Lupton (1923), is reached when oxygen consumption remains at a steady state ("plateau") despite an increase in workload. The occurrence of a plateau is not guaranteed and may vary by person and sampling interval, leading to modified protocols with varied results.