Aerobic capacity refers to the maximum amount of oxygen that the body can utilize in an exercise session, usually measured as a brief period of high intensity exercise. It is possible to improve aerobic capacity over time, and it is also possible to see a decline in aerobic capacity in response to a variety of factors, including declines in physical activity, chronic illness, and aging. For athletes, aerobic capacity, sometimes known as VO2 max, is an important aspect of their physical health.

When people exercise, the body needs to be able to utilize oxygen. The lungs and cardiovascular system work in tandem to deliver oxygen to the body. The more people exercise, the more efficient the process becomes, and the more oxygen the body can utilize in a given period. As people are able to use more oxygen, they can work at a higher level of intensity, and their endurance also tends to be extended because they can work harder, longer.

Cardiovascular exercise is used to improve aerobic capacity by strengthening the heart muscle and developing the rest of the cardiorespiratory system. As many people who have started exercise plans have noted, at the start, they tire quickly and breathe hard because their bodies can't get enough oxygen. As their fitness program continues, their aerobic capacity improves, allowing them to exercise more vigorously and for longer periods of time.

Athletes and other people who are interested can take an aerobic capacity test, in which the function of their cardiorespiratory system is assessed. During the test, someone engages in aerobic exercise while inhaled and exhaled gases are monitored. When the exhaled gases reach a steady state which does not change, the athlete has reached aerobic capacity; no more oxygen can be utilized by the body, no matter how much harder the athlete works.

A number of things can impact aerobic capacity. Aging is a major factor, with people over age 40 usually experiencing a decline even when they keep fit. Chronic illness and disease can also cause declines, which may be permanent or temporary, depending on the nature of the illness. Failure to keep up with a physical fitness plan will also contribute to a reduction in VO2 max, because the body needs to be worked regularly to retain a high efficiency level when it comes to utilizing oxygen. Even small amounts of physical activity every day can help people get and stay fit.