Creatine phosphatePhosphocreatine · PCr
A rapid phosphate store that regenerates ATP during the first seconds of hard effort.
Creatine phosphate, also called phosphocreatine, is a high energy molecule stored in muscle that donates its phosphate group to regenerate ATP almost instantly. It is the fastest energy system you have and the one that covers the first seconds of any maximal effort.
Its capacity is small and its speed is unmatched. Stores support something like five to fifteen seconds of all out work, which is why a maximal sprint slows before ten seconds and why a heavy set of three is limited by something quite different from what limits a five kilometre run.
Recovery of the stores is fast but not instant, taking a few minutes and depending on aerobic capacity, since regenerating phosphocreatine is an oxygen requiring process. That is the physiological reason rest periods between heavy sets matter, and why fitter people recover between efforts more quickly.
This system is the reason creatine supplementation works. Taking creatine raises muscle phosphocreatine stores, which extends the immediate system slightly, allowing a little more high quality work per session. Over months that extra work becomes more strength and muscle, which is a training effect delivered through a supplement rather than a direct one.
Loading is optional. A steady daily intake reaches the same saturation within a few weeks that a loading phase reaches in about one, with less chance of stomach upset.
It is one of the few supplements with a large, consistent evidence base and a good safety record, which is why it appears so often on this site as the exception rather than the rule.
In practice, our creatine calculator sets the dose, and the training implication is to rest long enough between heavy sets for this system to refill.