Be Strong. Be Unstoppable. Be Unbreakable. Leucine, isoleucine, and valine possess a similar structure, and are referred to as branched-chain amino acids (BCAAs). They are essential amino acids that account for about 35% of the essential amino acids in muscle proteins. Branched chain amino acids (BCAAs) refer to a group of proteins with a branched-like structure. It has been well established that BCAAs, especially leucine, promote muscle protein synthesis and help stimulate the repair processes that ultimately lead to larger and stronger muscles. These amino acids are used to help rebuild and repair damaged muscle from workouts. BCAAs have been used to ease muscle soreness and improve muscle function.