Some sports nutrition authorities emphasize protein as the most important nutrient for building muscle while other sports nutrition authorities emphasize carbohydrate as being most important for endurance exercises. It is true that during endurance exercises such as cycling or swimming replenishing the carbohydrate stores in your muscle is vital in preventing fatigue, carbohydrate also plays an important role in building muscle. Protein breaks down into amino acids which are transported into the muscle and re-assembled as protein. Carbohydrates raise the levels of the hormone insulin which also helps transport amino acids into muscle to build muscle protein.
However, none of this happens without resistance exercises such as weight lifting, isometric posture exercises such as yoga or using resistance bands. When muscles are stretched in so-called eccentric movements such stretching of the biceps in the down part of a curl movement, the muscle fibers are pulled apart leading to a signal being sent to bring satellite cells into the muscle to merge with the fibers of the muscle. Muscle cells are not circular like most cells but more like long fibers that have many nuclei. The muscle cells grow by the merging of these satellite cells into the muscle cells along with more protein being made. To get the greatest benefit have 20 or more grams of high quality protein within 30 minutes to 1 hour of a bout of resistance exercise.