What REALLY causes our muscles to grow?

Now, this title may seem like a classic bit of clickbait, but if you have read my previous articles looking at mechanisms that play roles in generating muscle growth as a response to resistance training, you may have noticed that although we have discussed concepts such as metabolic stress and the hormonal response to resistance training that despite associations of certain physiological markers with muscle growth and some understanding about the mechanisms by which muscle growth takes place, there are still some ‘hidden’ or not often discussed ways by which the stress caused by pumping iron lead to muscle growth.


There’s plenty of candidates out there, but these are not often discussed in your normal fitness circles for some reason; maybe because it requires stepping away from the familiar hormones and mechanism that are often talked about, and we have to do a little digging around some scientific literature to interpret what is actually going on… and who has time for that?


Well, the answer to that is probably people with more time than sense and an unhealthy obsession with such topics, like me! So, with that said, let me discuss some of these less well-known players and hopefully explain what they are, how they work and how they potentially lead to muscle growth.


As we have discussed previously with regards to hormones, it appears that local hormones produced ‘locally’ in the muscle as a response to training have more of an association with muscle growth than those that are released systemically and circulated throughout the body. It is a reasonable assumption that to cause the growth of a muscle that we are focussing on with training that local ‘signals’ that stimulate muscle-building pathways within our muscle are going to be a huge influence on the hypertrophic, muscle building response.


We know that resistance training causes the release of a large family of cytokines, myokines and numerous different peptides within the muscle. Cytokines are a large family of proteins that are released by cells that have a specific effect within the cell and also act as a method of communication between cells. Myokines are a part of the family of cytokines that are released in muscle cells, typically as a response to muscle contractions, and have been shown to be released in association with metabolic stress, some of which are anabolic in nature and others which help regulates catabolic processes which could influence muscle growth.


A myostatin, a term you’re more likely to be familiar with, is a form of myokine that inhibits muscle growth. Animals such as the Belgian Blue Bull, which have a myostatin deficiency, therefore grow many times more muscle than other breads of bull. There is a small amount of evidence to suggest that resistance training, especially if accompanied with metabolic stress, causes a decrease in myostatin which may create a more favourable environment for muscle growth.


Peptides are also small chains of amino acids, but in this case have certain cell signalling effects. We might be a little more familiar with peptides signallers than we think when we consider that both growth hormone and insulin are ‘peptide’ hormones, and hormones are just the body’s way of transmitting signals. As these local signalling ‘agents’ are known to have hormonal style effects that are associated with stimulating muscle building pathways (I’ll do a full article on these in due course!) and it appears that resistance training amplifies these effects.


I guess the real question is what does this all mean in real terms? Although we know the mechanisms that support muscle growth are complex and likely to have several not yet fully elucidated interactions with each other, one thing does seem clear, and that is that creating metabolic stress seems to act as a starting point for the release of many of these mediators in muscle growth. Therefore, this explains why training in typically bodybuilding repetition ranges and intensities that recruit the right energy systems to create the right metabolic conditions, combined with training in a progressive manner and with high effort levels to produce metabolic stress are going to ensure the best possible conditions for muscle growth.

Thanks for reading,

Dr. P