At rest, between 60% and 90% of creatine stored in the muscle is in the form of creatine phosphate. Creatine phosphate fuels the process of muscle contraction through its vital role in the creation of adenosine triphosphate (ATP), the primary compound which converts food into energy. When muscles contract, ATP is broken down to ADP (adenosine diphosphate) and immediate energy is released. Since very small quantities of ATP are stored within the muscle cell, it must constantly be replenished. Creatine phosphate donates its phosphate group to ADP to rapidly regenerate ATP. When high concentrations of creatine phosphate are present, the body can quickly release the large amounts of ATP needed to sustain muscular contractions.

Creatine phosphate is also involved in the transfer of energy within the muscle cell. ATP, produced in the mitochondria from the aerobic metabolism of fats and carbohydrates, must be transported to the site of muscular contraction. Creatine and creatine phosphate "shuttle" this energy from the mitochondria to the contractile elements where it can be used to fuel energy.