Muscles increase in size in response to growth factors (testosterone, growth hormone, IGF-1), cytokines (signaling chemicals), nutrients (amino acids, carbohydrates, calories) and resistive exercise. Amino acids, the building blocks of proteins, activate signaling chemicals in muscles that turn on protein synthesis and increase muscle fiber size. Chemicals called ribosomal protein S6 kinase and the target of rapamycin (mTOR) are important cell-signaling chemicals that regulate protein synthesis. They are sensitive to small changes in cell energy levels. They work like biological computer programs to line up amino acids needed to produce new muscle tissue.
After weight training, supplementing carbohydrates and proteins activates protein-signaling chemicals that promote protein synthesis and muscle growth. Muscle tension, blood levels of amino acids (particularly leucine) and insulin are key factors triggering muscle growth. They stimulate signaling pathways inside the muscle cells that promote protein synthesis and modify muscle protein breakdown, remodeling and repair.