To perform optimally, muscles need energy. The body provides the energy needed in situations requiring immediate, high-intensity actions, as in exercise, in the form of ATP or adenosine triphosphate. Since the body has only a limited supply of ATP, usually lasting only a few seconds of intense exercise, ATP is continuously produced to supply energy in order for the muscles to function. The burst of energy is produced by the breakdown of ATP when one phosphate group is released and packs considerable metabolic energy. The body uses creatine phosphate to quickly replenish ATP. The more energy the muscles store, the better they can perform in events, which require intense, immediate energy, such as weightlifting, sprinting, jumping, football, hockey and soccer. Since creatine is stored in the muscle as creatine phosphate, intake of supplemental creatine can increase the production of energy that enables muscles to perform at higher intensity. While the body produces its own supply of creatine, it is not sufficient to supply the muscle with the added energy necessary for intense performance. The benefits of creatine supplementation for endurance athletes have been actively researched. This research has established that creatine can, in fact, extend endurance at a relatively high dose of 20 grams per day. Creatine increases the muscle mass and muscle girth if taken along with a sustained exercise regimen. Initially it may also increase weight due to gain in the muscle mass. This increase may slow down some people, especially swimmers. The "slowing down" may be due to the highly aerobic nature of this exercise and should be reversible after sustained exercise.