ATP cannot be stored in the body but must be continuously generated. In anaerobic conditions ATP can be rapidly generated from creatine phosphate which acts as a reserve. Creatine phosphate cannot supply energy directly but can donate its phosphate group to ADP. Creatine phosphate is stored in the muscle and to a lesser extent, the brain, as a reserve supply of phosphate. After it has been used up, it is replenished from ATP when the muscle is relaxed or ATP is in plentiful supply.