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Aerobic exercise refers to exercise that involves or improves oxygen
consumption by the body. Aerobic means "with oxygen", and refers to
the use of oxygen in the body's metabolic or energy-generating
process. Many types of exercise are aerobic, and by definition are
performed at moderate levels of intensity for extended periods of
time. To obtain the best results, an aerobic exercise session involves
a warming up period, followed by at least 20 minutes of moderate to
intense exercise involving large muscle groups, and a cooling down
period at the end.

Aerobic versus anaerobic exercise

Aerobic exercise and fitness can be contrasted with anaerobic
exercise, of which strength training and weight training are the most
salient examples. The two types of exercise differ by the duration and
intensity of muscular contractions involved, as well as by how energy
is generated within the muscle. Initially during aerobic exercise,
glycogen is broken down to produce glucose, which is then broken down
using oxygen to generate energy. In the absence of these
carbohydrates, fat metabolism is initiated instead. The latter is a
slow process, and is accompanied by a decline in performance level.
This gradual switch to fat as fuel is a major cause of what marathon
runners call "hitting the wall". Anaerobic exercise, in contrast,
refers to the initial phase of exercise, or to any short burst of
intense exertion, in which the glycogen or sugar is consumed without
oxygen, and is a far less efficient process. Operating anaerobically,
an untrained 400 meter sprinter may "hit the wall" short of the full
distance.

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