Phosphocreatine disodium tetrahydrate, primarily found in the skeletal muscles of vertebrates and one of organic compounds known as alpha amino acids and derivatives, is a substrate for the determination of creatine kinase and used to regenerate ATP during skeletal muscle contraction[1].
[1]. Feldman EB, et al. Creatine: a dietary supplement and ergogenic aid. Nutr Rev. 1999 Feb;57(2):45-50.