What is muscle hypertrophy?
Muscle hypertrophy is the increase in the size of skeletal muscle through a growth in the size of its component cells. It is brought about by specific mechanisms within the body which remain somewhat unclear and may differ depending on the overall goals of the resistance training program (Schoenfeld, 2010).
The process of muscular hypertrophy involves several stages (Schoenfeld, 2010). The first stage is the mechanical loading of the muscle fibres. This may involve heavy resistance training, stretching or tearing of the cell membrane, or a combination of these factors. Mechanical overload leads to an inflammatory response in the body that stimulates protein synthesis and angiogenesis within the muscles being exercised (Siff & Verkhoshansky, 1999).
The next stage is the switch from aerobic to anaerobic metabolism. The body begins to produce energy in a way that does not require oxygen and generates the waste product lactic acid. This builds up in the muscles and causes what is commonly known as a “burn” sensation that leads to muscle fatigue (Siff & Verkhoshansky, 1999).
The final stage is the actual growth of the muscle cells from a combination of factors including excess protein within the cell and an increase in the number of nuclei due to the division of pre-existing cells (Schoenfeld, 2010). This last step occurs through a process known as sarcomeres which contain the proteins actin and myosin. These proteins become larger along with the size of the muscle fibre to accommodate for increased stress on the tissue.
Athletes across a variety of sports training for muscular hypertrophy in order to increase their strength and power within a given movement. This has led researchers to study the effects of various training variables on the degree of hypertrophy (Schoenfeld, 2010). It has been demonstrated that both high-load and moderate-load resistance training are able to produce significant increases in muscle size with no differences between groups in some studies. These findings have led experts to believe that mechanical tension may be more important than the load for stimulating muscular hypertrophy (Schoenfeld, 2010).
Muscular hypertrophy is also brought about by improving the mind-muscle connection and learning to recruit different muscle fibres in a given movement. Learning to activate the mind-muscle connection has been shown to lead to increases in strength that transfer outside of the weight room.
There is an apparent link between hypertrophy and the release of testosterone and growth hormone. Neuroendocrine factors have been shown to affect strength gains from training but may no longer be important once a certain level of strength has been achieved. Resistance training does also elicit marked increases in circulating anabolic hormones which may contribute to muscle hypertrophy (Schoenfeld, 2010).
It is possible that muscle hypertrophy from resistance training occurs through a different mechanism than the hypertrophy resulting from aerobic exercise. It has been shown that testosterone levels are linked more closely with increases in strength and power rather than endurance performance.