We know that muscle will grow when properly trained and when proper nutrients are supplied. But what biological system is responsible for this growth stimulus? Is it one or is it all of them? Is growth caused by the forces applied to the muscles through weight training , or on a more cerebral level, is resistance exercise merely a means by which we expose, through recruitment, as many muscle fibers as possible to some other stimulus responsible for growth?
All tissues undergo a formation period (synthesis) and a breakdown period (degradation). This typically occurs in a state of homeostasis where very little variation occurs. Old tissues are replaced by new ones which look and perform essentially the same as their predecessor. However, muscular hypertrophy requires an imbalance in this equation as the period following the degradation results in the laying down and retention of a greater amount of tissue than previously.
D. A. Jones, et al., theorizes that there may be three possible mechanisms responsible for muscular growth: hormonal stimuli, metabolic stimuli, and mechanical factors. We know there are a number of powerful hormonal reactions to exercise, but it is unlikely that hormonal stimuli are primarily responsible for muscle growth. Because of their generalized effects and the fact that muscular growth is typically limited to only those muscles which are trained, it would appear that hormones only work with some other localized muscular change with a greater influence on muscular growth.
However, there are situations in which localized hormonal changes in fibroblasts (which become connective tissue) produce growth factors which may have an effect on the muscles in their vicinity. It’s not uncommon to “go for the burn” or to “enter the pain zone” during hard, heavy training. This is typically associated with high concentrations of metabolites as a result of reactions which take place during energy production for muscular contraction (note: This burning sensation is typically associated with lactic acid accumulation, however, there are people who cannot produce lactic acid due to a metabolic disorder, and yet experience the “burn” anyway… food for thought).
However, these metabolites are those typically associated with failure of the energy systems (muscular fatigue) rather than force production. They may induce an increase in mitochondria and/or capillary growth, but this contributes very little to muscular size. For example, marathon runners have extensive mitochondrial and capillary proliferation, yet they have relatively weak and small lower extremity muscles.
High force mechanical stress, whether by contraction or repeated stretching, obviously plays a key role in the muscle growth equation. Its role varies in several theories. It may cause the fracturing of myofilaments which split and regenerate to full size part of which is supported by electron micrograph photos. It may also cause microtrauma within muscle cells allowing nearby satellite cells to divide and contribute their DNA material to the muscle cell nucleus allowing it to synthesize more ribosomes (the organelle responsible for protein synthesis) and produce more protein for regeneration and hypertrophy.
Based on Jones, et al., this appears to have the strongest case.
Jones, D.A., Rutherford, O.M., Parker, D.F. Physiological Changes in Skeletal Muscle as a Result of Strength Training. Quarterly Journal of Experimental Physiology. 74: 233-256. 1989.