Measuring an accurate VO2 max requires an all-out effort (usually on a treadmill or bicycle) performed under a strict protocol in a sports performance lab. These protocols involve specific increases in the speed and intensity of the exercise and collection and measurement of the volume and oxygen concentration of inhaled and exhaled air. This determines how much oxygen the athlete is using.
An athlete’s oxygen consumption rises in a linear relationship with exercise intensity — up to a point. There is a specific point at which oxygen consumption plateaus even if the exercise intensity increases. This plateau marks the V02 Max. It’s a painful point in VO2 max testing where the athlete moves from aerobic metabolism to anaerobic metabolism (See the article: Energy Pathways for Exercise). From here, it’s not long before muscle fatigue forces the athlete to stop exercising. The test usually takes between 10 and 15 minutes and requires an athlete to be completely rested and motivated to endure the pain long enough to find the true VO2 max.
Vo2 max also can be estimated. There are a variety of protocols used to estimate VO2 max, one is called the Bruce Treadmill Test, but none are as accurate as direct testing.