A compound, synthesized from the amino acids arginine, glycine, and methionine, that occurs in muscle. In the form of creatine phosphate (or phosphocreatine), it is an important short-term reserve of energy for muscle contraction. This energy is released when creatine phosphate transfers its phosphate to ADP to replenish supplies of ATP, and is itself converted to creatinine, which is excreted in the urine (at a rate of 1.2–1.5 g/day in humans). See also phosphagen.