Adenosine triphosphate (ATP)

Adenosine triphosphate (ATP) is the molecule that carries usable energy in the body.^{1Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 42.} It consists of adenine, ribose and three phosphate groups, with energy stored in the bonds of the two outer phosphates.^{1Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 42.} An enzyme uses water to split off one phosphate, releasing energy for biological work, including crossbridge cycling in muscle.^{2Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 60.} Muscle activity and growth depend on an adequate ATP supply.^{1Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 42.}

The body stores only about 80 to 100 g of ATP, too little to serve as an exercise energy reserve.^{3Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 61.} The phosphagen system, glycolysis and the oxidative system replenish it.^{2Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 60.} Even during very intense exercise, muscle ATP doesn’t fall by more than about 60%.^{4Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 51.}

In practice

• Harder efforts can’t last as long: they depend more on the fastest ATP-producing systems, which have the least capacity.^{5Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 52.}
• A full lifting repetition uses more metabolic energy and depletes phosphagens more than an isometric hold.^{4Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 51.}
• ATP breakdown also releases a hydrogen ion.^{6International Sports Sciences Association, Strength and Conditioning, p. 115.} ISSA identifies it as the muscle’s main potential source of hydrogen-ion buildup, which can impair power in the short term.^{6International Sports Sciences Association, Strength and Conditioning, p. 115.}