Muscle growth, otherwise known as hypertrophy, begins with a stimulus. The muscle is exposed to increasing volumes of tension over a consistent period of time during an activity such as resistance training. Increased volumes of tension placed on the muscles causes microtrauma, or muscle damage. This damage primes the muscle for anabolism, or muscle growth, by allowing for revascularization and new muscle.
Damaged muscle undergoes a process known as phagocytosis. Phagocytosis is a process in which phagocytes -- white blood cells -- congregate to the damaged area and begin to engulf the damaged tissue cells. Once all of the damaged muscle cells are eaten, the area is primed for new growth and progenitor cells. Progenitor cells act as a repair system for the body. They congregate to tissues in which they can be the most help and once there, with the proper conditions and growth factors, begin to mature into the cells of that specific tissue.
Stimulus causes the nervous system to begin establishing connections between muscle fibers to ensure maximal muscle fiber recruitment. The greater amount of muscle fibers firing at one time leads to the greater amount of force exerted and thus the greater amount of muscle mass being stimulated, which leads to more muscle growth.
Stimulus and maximal muscle fiber recruitment elevates androgen production, specifically testosterone. Androgens help the body make more muscle by increasing protein synthesis, which in turn results in the body’s production of more contractile proteins (actin and myosin). Simply put, androgens tell the the body to grow more muscle.
During resistance training, it appears that muscle fiber increase is due mainly to a net increase in muscle protein synthesis. During training, the body’s protein synthesis decreases. After training, especially when a carbohydrate protein meal is ingested, protein synthesis increases. Research has shown that long-term protein synthesis and decreased protein degradation is accompanied by exercise-induced hypertrophy.
Muscle Fiber Growth
Scientists are still not completely certain about the exact mechanism of muscle growth; however, it is widely believed to result from an increase in the size of the muscle fibers by way of increased protein synthesis. Muscle fiber size could be increased through a number of reasons, including more myofibrils, more actin and myosin filaments, more sarcoplasm and/or more connective tissue.