Every muscle cell has some amount of ATP. Adenosine triphosphate (ATP) is the way your body uses biochemicals to store and use energy. There is enough ATP in the cell that the muscle can use immediately, but only enough to last for about three seconds. The muscle must replenish the ATP levels quickly, using a high-energy phosphate compound called creatine phosphate.
An enzyme called creatine kinase removes the phosphate group from creatine phosphate and transfers it to ADP to form ATP. The cell then turns ATP into ADP, and then the phosphagen turns the ADP back into ATP. The muscle continues to work, and the creatine phosphate levels begin to decrease. The ATP levels and creatine phosphate levels working together are called the phosphagen system. This phosphagen system supplies the energy needs of working muscle, but only for 8 to 10 seconds.
Energy is also produced by the Clycogen-Lactic Acid System and aerobic respiration.