The acute hormonal response to weight training... is it essential for muscle growth?

It now seems to be spouted as gospel that lifting heavy weights, especially compound movements involving the bigger muscle groups, is important for muscle growth as this elevates levels of anabolic, muscle building hormones such as testosterone and growth hormone and are therefore essential to muscle growth. Although it can be stated with a degree of certainty that resistance training does indeed cause elevations in muscle building hormones, as well as some muscle ‘breaking’ ones as well, is there actually much evidence to suggest that these acute, short term elevations in hormones contributes significantly to muscle growth, or are there some more factors at play?


I guess the real question would be why would these muscle building hormones be elevated as a response to resistance training if they didn’t play a role in muscle growth? After all, that is one of the main roles they have in the body; it seems an almost ridiculous proposition to suggest they wouldn’t play at least some role… or is it?


It is important to note that when a muscle cell is ‘signalled’ by these hormones, this signal triggers pathways that mean our cells produce proteins, which means more muscle. In theory, if we could increase the levels of these hormones, then they would bind to more receptors more frequently in or on our muscle cells (depending on the hormone and its mechanism of action) and stimulate the creation of new proteins, which would be a potential mechanism by which resistance training promotes new muscle growth.


If you have read my previous article about growth hormone (GH), you will know that it is a very broad term for many different variants of a hormone that has many different functions including fat metabolism and muscle growth. GH is elevated in the acute response to exercise and particularly when blood lactate levels are elevated. Despite elevations in GH caused by exercise, it is interesting that both natural elevations in GH caused by exercise and when GH has been administered medically or under experimental conditions that its effects on muscle protein synthesis, essential for building new muscle, are limited to put it mildly.


It has been suggested that these elevations in GH are actually more to do with the recovery and repair of other tissues, such as the ligaments and tendons, that are also stressed during exercise and particularly under the mechanical loading caused by resistance training with any potential muscle building effects of GH only being carried out through initiating the release of IGF-1.


Of the hormones that are most often cited as key players in this acute response to resistance training, IGF-1 is a hormone that has gotten a lot of attention as it has the potential to prime the body for muscle repair and growth.  IGF-1 (Insulin-Like Growth Factor 1) is named as such as it has structural and functional similarities to insulin, which is a very potent anabolic hormone its own right.


IGF-1 is elevated as a response to mechanical loading and signalling caused by resistance training and has shown a positive relationship with muscle growth. IFG-1 comes in three primary types that are found in muscle tissue, of which mechano-growth factor (MGF) is the type that appears to respond to mechanical stress and loading. However, it is interesting is that it does not appear essential for muscle growth to take place, as in the absence of working IGF-1 receptors muscle growth can still occur.


IGF-1 levels are elevated during intense exercise and this is generated mostly from the muscles, unlike at rest when the liver is the primary source. Aside from the local actions of MGF in the muscle that have been linked to muscle hypertrophy, the systemic effects of IGF-1 (the IGF-1 that enters circulation and is carried to the tissues around the body) appears to have a less significant role in protein synthesis and the development of new muscle tissue. If you recall earlier, we mentioned briefly about hormones signalling the muscle cells to create new muscle proteins, these proteins are created from the genetic code and machinery within our cell nucleus. This is where systemic IGF-1 may have an important role, as it does appear to be important in the creation of new ‘satellite’ nuclei on our muscle cells (called myonuclei) which supports the ability to produce more protein and support muscle growth.


If you haven’t already read our previous ‘hormone series’ article on testosterone and what it does in the body, then please give that a read if you would like a little more background. Testosterone, from a muscle building perspective, is known for its anabolic properties linked to its ability to stimulate protein synthesis and decrease protein break down in muscle tissue directly. As well as this direct role, testosterone mediates the release of GH, IGF-1 and MGF, aiding in promotion of satellite cells that further promote protein synthesis.


Resistance training increases the release of testosterone in the period following training and this, because of the above reasons, has been cited as a potential piece of the puzzle that promotes muscle growth as a response to training. Releases in testosterone are related to muscle size and in those that perform regular resistance training, they get increased elevations in comparison to inexperienced trainers, suggesting that testosterone may play a more important role in muscle growth as our bodies accrete, or grow, more muscle. Despite the apparently ‘obvious’ causal link between training, testosterone and muscle growth it is yet to be proven that the increases in testosterone post-training is directly and profoundly responsible for muscle growth as, like with other hormones, muscle growth can occur at similar rates without the presence of noted increases in testosterone.


It appears then that although individual hormones that are increased as a response to exercise may play both individual and synergistic roles in promoting muscle growth, that there are other mediating factors and mechanisms that are arguably more important, and as yet not fully elucidated, are essential to muscle growth, as elevations in these often cited key hormones are not as yet proven to be essential to promote muscle growth. So the real question is, if it’s not these ‘big name’ hormones that fully govern the process… what is? Well, that my friend is a discussion for another time, but for now, thanks for reading…


Dr P.