The cellular respiration process that converts food energy into ATP (a form of energy) is largely dependent on oxygen availability. During exercise, the supply and demand of oxygen available to muscle cells is affected by duration and intensity and by the individual's cardiorespiratory fitness level. Three exercise energy systems can be selectively recruited, depending on the amount of oxygen available, as part of the cellular respiration process to generate the ATP for the muscles. They are ATP, the anaerobic system and the aerobic system.
ATP is the usable form of chemical energy for muscular activity. It is stored in most cells, particularly in muscle cells. Other forms of chemical energy, such as those available from food, must be transformed into ATP before they can be utilized by the muscle cells.
Since energy is released when ATP is broken down, energy is required to rebuild or resynthesize it. The building blocks of ATP synthesis are the by-products of its breakdown; adenosine diphosphate (ADP) and inorganic phosphate (Pi). The energy for ATP resynthesis comes from three different series of chemical reactions that take place within the body. Two of the three depend upon the type of food eaten, whereas the other depends upon a chemical compound called phosphocreatine. The energy released from any of these three series of reactions is coupled with the energy needs of the reaction that resynthesizes ATP. The separate reactions are functionally linked together in such a way that the energy released by the one is always used by the other.:8–9
Three methods can synthesize ATP: