VO2 max (aerobic capacity)

VO2 max, also called maximal oxygen consumption, is the measure of aerobic capacity, meaning your body's ability to do work using oxygen. It answers a simple question: during hard exercise, at the very peak of the effort, what is the fastest rate at which your body can take in oxygen and keep it up?

That rate depends on teamwork. On one side is cardiac output, the amount of blood your heart pumps. On the other is the difference between arterial blood, which leaves the heart for the rest of the body, and venous blood, which comes back. Arterial blood carries more oxygen, and what venous blood is missing is the oxygen that was dropped off along the way. Oxygen consumption, or VO2, is what you get when you multiply cardiac output by that difference. Think of a delivery truck: how much it delivers depends on how many trips it makes and how much it unloads at each stop.

With training, VO2 max usually goes up because more oxygen gets delivered: the heart increases its cardiac output, and capillary density grows, which means there are more capillaries, the thinnest blood vessels. In the truck example, that's more trips and more streets to deliver to. Age works the other way: in healthy adults over 30, VO2 max drops by about 10% per decade. But that decline doesn't close the door, because with consistent endurance training, the kind that calls for sustained effort, it's still possible to improve it at older ages.