Muscle contraction requires significant amounts of energy. Like most cells, muscle cells use ATP (adenosine triphosphate) as the energy source. In the presence of calcium ions, myosin acts as an enzyme, splitting ATP into ADP (adenosine diphosphate) and inorganic phosphate and releasing energy to do work. Muscle cells store only enough ATP for about ten seconds worth of activity. Once this is used up, the cells must produce more ATP from other energy sources, including creatine phosphate, glycogen, glucose, and fatty acids.