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 with the help of oxygen to generate energy. In the absence of these carbohydrates, fat metabolism is initiated instead. The latter is a slow process,

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.  developed world, there is a substantial market for products which promise to make weight loss easier, quicker, cheaper, more reliable, or less painful.

doubles tennis, with brief bursts of activity punctuated by more frequent breaks, may not be predominantly aerobic. Some sports are thus inherently aerobic, while other aerobic exercises, such as fartlek training or aerobic dance classes.