Creatine phosphatePhosphocreatine · PCr

Creatine phosphate is a high-energy molecule held in muscle that donates its phosphate to ADP, regenerating ATP almost instantly. It is the fastest of the energy systems and the one that powers the opening seconds of a sprint or a maximal lift.

It matters because the store is small — largely spent within about ten seconds of all-out effort — and takes minutes to fully resynthesise. That single fact explains why rest between heavy sets needs to be measured in minutes rather than seconds if the next set is to be any good.

In practice this is the mechanism creatine supplementation works through: raising muscle stores by roughly 20 to 40 per cent means more available phosphate, more reps at a given load, and over time more accumulated training. It is the best-evidenced sports supplement there is, and this is why.