The Microtrauma Muscle Growth theory holds that the main mechanism of increasing the muscle mass volume is:
- Heavy resistance training causes multiple very small injuries to the involved muscle
- The first response to injury is catabolism (muscle degeneration) accompanied by inflammation
- Muscle degeneration is followed by the activation of muscle repair process
- Skeletal muscle satellite cells play the main role in the swift muscle repair and contribute to the muscle growth
It should be noted that muscle repair is not limited to muscle fibers (myofibers) regeneration but it includes revascularization, reinnervation, and reconstitution of the extracellular matrix.
Healthy adult has between 1x1010 and 2x1010 satellite cells located between the sarcolemma and basement membrane of terminally-differentiated muscle fibres.
Under normal circumstances, these cells have almost no turnover (meaning that no new cells are created). During injury – including the microinjury caused by resistance training – muscle satellite cells start to proliferate, differentiate and fuse leading to new myofiber creation (http://www.ncbi.nlm.nih.gov/pubmed/12757751, http://www.ncbi.nlm.nih.gov/pubmed/14715915 ).
The new satellite cells originate in bone marrow-derived and muscle-derived stem cells in a process very similar to that of embryonic myogenesis (muscle growth in embryos).
Using various markers of muscle satellite cell increase it has been proved that their increase 24 hours following resistance training is very significant (http://jp.physoc.org/content/588/17/3307.abstract ).
The Microtrauma Theory considers the mechanisms described above as more important factors of muscle growth than hormonal stimulation. It has been shown that load-induced muscle hypertrophy still occurs in mice that are deficient in IGF signaling following IGF-I receptor knockout (http://jp.physoc.org/content/586/1/283.abstract?ijkey=9ec03f42872ed4d1614c668830c9a5ebc4f60a97&keytype2=tf_ipsecsha ).
In fact, there are more reasons to believe that hormonal factors which are usually considered to be dependent on or launched by IGF-I (Insulin-like Growth Factor 1) are not as important as previously believed (http://jap.physiology.org/content/108/6/1821.full#ref-33 ). The research in this field gives ever more credibility to the Microtrauma/Satellite cell theory.
While the Microtrauma Theory describes very precisely the processes behind the muscle growth it fails to explain the differences between male and female muscles which are very likely caused by different testosterone levels in both sexes.
To fully explain the increases in human muscle mass it is apparently necessary to consider a combination of several theories.