-there are several mechanisms of muscle growth, main being reparation of microinjury caused by resistance training and growth factor mediated hypertrophy
-it is not sure if hyperplasia (increase in muscle cell number) takes place
Back in 1960’s, most articles on this topic started with the classical sentence: “although we landed a man on the Moon we still don’t understand what exactly is causing our muscles to grow”.
Well, we really didn’t – the most popular theory at that time was that muscle growth is caused by lactic acid build-up during the exercise.
Now we know this is wrong. But we didn’t improve much in our ability to reach other planets than Moon and we don’t fully understand the muscle grow mechanism yet.
The Microtrauma Theory (full article here)
As it became clear that lactic acid build-up is not responsible for muscle hypertrophy, a new theory occurred: during the resistance exercise – and especially in the passive phase (when lowering the weight) – multiple very small injuries occur in the muscle and the consequent repairing mechanism is responsible for muscle hypertrophy (increase in the size of muscle cells) and possibly muscle hyperplasia (increase in the number of muscle cells).
The Hormonal Theory (full article here)
This theory holds that the most important factors in the muscle growth process are elevated levels of testosterone and growth hormone. They in turn launch a cascade of processes involving a large group of growth factors. Another proven effect of testosterone is removing glucocorticoids from their receptors by occupying their places but not launching the catabolic process normally associated with glucocorticoid presence.
Many effects of testosterone and DHT (dihydrotestosterone, the main testosterone derivate) are mediated by androgen receptors. Androgen receptors concentrations are increased after resistance exercise. Blocking the AR receptors lead to significant decrease of muscle mass growth in laboratory animals. http://www.springerlink.com/content/l651840550t56gm3/
The Intrinsic Theory (full article here)
This new theory is based on studies by D. W. West and colleagues. They argue that since there is no correlation between the post-exercise testosterone and HGH-levels and muscle growth (except for pubertal age group and supra-physiological doses of testosterone) there must be another, more significant biological path leading to hypertrophy of the muscle fibers.
This mechanism is intrinsic to the affected muscle (meaning limited to the muscle and not systemic). It has been described as “acute activation of intrinsically located signaling proteins such as p70(S6K) and the acute elevation of muscle protein synthesis”.
Hypertrophy versus hyperplasia (full article here)
The main unanswered question is whether muscle growth is caused by growth of individual muscle cells (hypertrophy) or by increase in the number of muscle cells (hyperplasia). There are scientific studies supporting both theories.
You probably ask how this is possible. It should be very simple to check it: make autopsy of a small, untrained muscle and of large, trained muscle. If the cells in the large muscle are very large, the hypertrophy theory is correct. If they are of the same size as those in “small” muscle, the hyperplasia theory is right.
Well, believe it or not, this is exactly what several teams of scientists did and the results were different in every study. So while some studies proved the hyperplasia theory, the other studies refuted it.
Sarcoplasmic versus myofibrillar muscle growth theory (full article here)
It should be noted that this theory originated from coaches, not scientists although it (very rarely) found its way to some scientific papers.
The proponents of this school of thought claim that bodybuilders have mostly muscles fibers inflated by sarcoplasmic fluid in the muscle cells as opposed to the muscles fibers of power lifters, who, among others (as the theory goes) have more efficient muscle consisting mostly of proteins.
This belief is not supported by observation of muscular biopsies.
It probably originated from the fact that bodybuilders often cannot develop the same strength in certain specific weight-lifting disciplines as power lifters.
It is, however, well known that CNS (central nervous system) plays a decisive role here. The ability of CNS to activate maximum number of individual fibers during specific movement can be learned and this mechanism is completely independent from growing muscle mass.
Therefore it is almost sure that the extraordinary ability of weight lifters to move the heavy weight is defined by the capability of CNS to activate muscle fibers and by the available muscle mass (read more in the article on STRENGTH).
What we do know
- Muscle growth is caused by increased protein synthesis in the trained muscle.
- Muscle growth is intrinsic, not systemic. Otherwise, the resistance training involving any individual muscle would lead to the growth of all muscles.
- Testosterone plays some role. It is clear that males have bigger muscle mass than females.
- Growth hormone is not directly responsible. GH supplementation leads to growth of tendons but not muscles.
- While increased testosterone levels will lead to some muscle growth regardless of physical activity, most important growth takes place locally in the trained muscle.
- Only resistance training will lead to muscle growth. Other types of physical training may lead to increase in strength or endurance but not to increases in lean muscle mass.
- Insulin and insulin-related growth factors play an important role in muscle growth. Insulin supplementation will, however, not promote muscle growth (and is likely to cause serious health problems).
- Heredity plays a role. Different people have different numbers of various muscle fiber types. This can predispose us to perform better in certain type of sports.
So what does it all mean for us?
For practical reasons, this is what every bodybuilder should know:
- Resistance training is most efficient with 70% - 90% of personal one-repetition maximum
- Three sets of one exercise proved to stimulate the protein synthesis more than one set only
- Protein synthesis is elevated for at least 24 hours after exercise (the window for protein supplementation)
- The passive movement (lowering the weight) is at least as important for muscle growth as the active movement (lifting the weight)
- Insulin and Growth Hormone supplementation do nothing for muscle growth
- Testosterone supplementation will increase muscle growth (but beware of side-effects like impotence; not to speak about the legal issues - testosterone and its derivatives are treated as narcotics in USA and most western countries)
- Improvements in performance (strength) are not necessarily tied to increases in lean muscle mass