The term aerobic exercise describes any type of exercise typically performed at moderate levels of intensity for extended periods of time that increases the heart rate. In such exercise, oxygen is used to "burn" fats and glucose in order to produce energy in the form of ATP, the basic energy carrier for all cells. Initially, glycogen is broken down to produce glucose, but in its absence, fat metabolism is initiated instead. Aerobic exercise is an exercise performed at a moderately high level of intensity over a long period of time. For example, running a long distance at a moderate pace.