Cardiorespiratory fitness (VO2 max) is typically measured during a maximal incremental exercise test to exhaustion on a treadmill, involving gas exchange measurement via a mask.

“it's measured during maximal exercise hence the term maximal oxygen consumption so one of the ways that it's typically measured say if you were in an exercise physiology steady you would get on a treadmill you would run at progressively faster speeds until eventually you just couldn't go anymore so you would exercise until exhaustion all the while having um your gas exchange measured so you're going to be wearing a mask to measure your oxygen intake um and that's able to you know measure the rate of oxygen utilization by your body and at the point of maximal exertion however much oxygen your body is utilizing that's your maximal oxygen your rate of oxygen consumption”