Creatine is synthesized in the liver from the amino acids arginine and glycine and stored in the major skeletal muscles including the heart. Once inside the muscle, it is phosphorylated to form creatine phosphate which is a high energy substrate that assists in the contraction of the myofibrils (muscle fibers). Creatine phosphate is utilized to maintain higher levels of ATP during exercise. Creatine phosphate maximizes physical performance and reduces exercise fatigue by absorbing hydrogen ions released by muscles as lactic acid. Intense anaerobic exercise, such as weight lifting and sprinting, depletes ATP and greatly increases the demand for creatine.