L-arginine is a protein amino acid present in the proteins of all life forms. It is classified as a semi-essential or conditionally essential amino acid. This means that under normal circumstances the body can synthesize sufficient L-arginine to meet physiological demands. Arginine is considered to be key to efficient muscle metabolism because of its role in the transport, storage, and elimination of nitrogen. Creatine is derived from arginine, as are guanidophosphate and phosphoarginine, all of which have roles in muscle metabolism. After a vigorous workout, the body has increased testosterone and HGH production for a period lasting approximately two hours. During this timeframe, dietary arginine and ornithine can contribute to a hormonal environment that produces increased protein synthesis and muscle growth.