Creatine is the natural energy source for muscle contraction during high intensity training. There is only enough ATP in the muscle to sustain activity for a fraction of a second. Creatine replaces the ATP which has been used up during muscle contraction, to allow prolonged intense activity. Energy from food is carried within the muscles by creatine itself forming an effective biological energy-transfer system. It is not a drug and is used and endorsed by many top Olympic and other athletes.