ATPAdenosine triphosphate

Adenosine triphosphate stores energy in the bonds between its phosphate groups. Breaking off the terminal phosphate releases energy and leaves ADP; recharging ADP back to ATP is what all of energy metabolism ultimately exists to do.

It matters because muscle cannot use food directly — every contraction is paid for in ATP. The stored quantity is tiny, enough for only a couple of seconds of maximal work, so the real question is never how much ATP you have but how fast you can regenerate it.

In practice three systems do that regeneration at different speeds: creatine phosphate for the first seconds, anaerobic glycolysis for efforts up to a minute or two, and oxidative metabolism for everything longer. Which one dominates is what makes a sprint a different physiological event from a long run.