Where does your energy come from in the first seconds of a sprint?

Mostly from a fast backup called phosphocreatine, which rebuilds ATP, the energy currency every cell spends, including contracting muscle. Muscles store only small amounts of ATP and phosphocreatine, so this quick system is exhausted within seconds of vigorous activity, but its value is speed, not quantity. It uses no oxygen, and it is the main system behind very short, powerful efforts like a golf swing, powerlifting or a 100m sprint.

The trick is a simple handoff. When muscle spends ATP, it turns into ADP. Phosphocreatine can donate a phosphate group to that ADP and turn it back into ATP, without oxygen, during the first five to eight seconds of a maximal effort; another summary puts the system's use at up to 10 to 15 seconds at maximum intensity. After that, other systems take a bigger share, like the glycolytic system that dominates intense efforts lasting under two minutes.

The reserve refills itself. During rest or light effort, spare ATP is used to turn creatine back into phosphocreatine, ready for the next burst. About 95 percent of the body's creatine sits in muscle. The molecule was found in 1927 by two teams working separately, Grace and Philip Eggleton at Cambridge, and Cyrus Fiske and Yellapragada Subbarow at Harvard Medical School.