Metabolic Stress... What is it and why is it important for muscle growth?

There are many factors that influence muscle growth, and one that seems to be more important than most is the metabolic stress caused by training. If we consider some of the other training principles we have talked about in the previous articles in this series, then we know that in order to force an adaptation, in this case muscle growth, we need to place enough stress on the muscle to make it adapt. This obviously relates to the load that we place on the muscle and that by increasing the load, frequency and volume of training, we can create more and more stress leading to consistent progression in muscle size.


We have long established that this mechanical stress causes muscle growth, but what is less clear is how this mechanical stress influences processes within our muscle cells and body that actually stimulates pathways at a much smaller, cellular, ‘metabolic’ level. This metabolic level of stress that is caused by increasing the mechanical stress on the muscle is measured using various outcomes, and in this article we are going to discuss the outcomes that are most associated with metabolic stress and how these can play a key role in converting mechanical loads to cellular, hypertrophic responses with our muscle fibres.


There are many anabolic (muscle building) and catabolic (muscle destroying) pathways that are BOTH essential for muscle growth. It is important to remember that some breakdown of tissue is required to allow remodelling and regrowth to take place. However, ultimately, anabolism wins and leads to more protein synthesis (creation of muscle protein) than protein degradation (the breakdown of muscle tissue). Ultimately we want to focus on promoting anabolism as much as possible, whilst stopping excessive catabolism, and this will be guided not only by resistance training but also our nutritional state.


Arguably, the most important metabolic pathways that have been linked to muscle growth are the mTOR and MAPK pathways, although there are many others that work independently and interact with each other in synergy to create the desired hypertrophic response. In short, these pathways allow transmission of the mechanical signals from training, directly into the creation of new muscle and are influenced by our hormonal status, energy provision and amino acid availability. mTor has a specific pathway that is directly stimulated by mechanical loading, getting us from A to B on our muscle building journey pretty directly, that is potentially linked to a type of special signaller called phosphatidic acid which has given rise to much research in the use of this as a potential muscle building agent and use as a supplement.


Metabolic stress caused by resistance training manifests itself in the accumulation of metabolites; these are the bi-products, typically of the breakdown of nutrients to provide fuel and thus the type of fuel we use to perform resistance training is likely to be important to cause the hypertrophic response. These metabolites include phosphate that is the result of ATP breakdown (the body’s energy currency), hydrogen and lactate, and the metabolite of glucose metabolism when it occurs without oxygen through a process called anaerobic glycolysis. The inability to provide enough oxygen to fully use a glucose molecule as a fuel is caused by exercising at an appropriate intensity and volume where oxygen demand outstrips supply and interestingly, this idea of creating hypoxia (a lack of oxygen in a cell or tissue) in muscles MAY be a key determinant as to why certain repetition ranges cause more muscle growth compared to others.


This notion that creating a hypoxic environment is fundamental to generating muscle growth is employed in blood flow restriction (BFR) training where a medical ‘cuff’ is used to restrict blood and therefore oxygen to the muscle, and also prevents the removal of these potentially anabolic metabolites. Indeed, BFR, even at very low intensities, has shown good results in terms of stimulating hypertrophy, although using this technique should only be done under proper instruction and is limited by the capacity to restrict blood flow only really to the upper and lower limbs; anything on the torso is pretty much impossible to try this on. It is also likely that by training to failure at the appropriate intensity, you can generate the right hypoxic conditions. So whether there is any benefit of BFR compared to conventional training to failure is, in my opinion unlikely, and the potential risks, limitations and inconvenience probably make it pointless unless it is used to illicit a hypertrophic response whilst reducing the loading at the joint after injury.


If we consider training at ‘bodybuilding’ intensities of typically between 60-85% of one repetition max, then the energy system used would be the glycolysis system. This accumulates lactate and signals (potentially through the release of other hormones such as growth hormone) muscle building pathways. If this was the case, this would likely be a large part of the explanation as to why training at these intensities illicit hypertrophy, whereas higher intensity (90+% of 1RM) training, which is reliant more on ‘neural drive’ and energy systems that don’t involve the accumulation of lactate, is less effective at generating hypertrophy. This also applies to the other end of the training continuum, where intensity is low enough that we enter into endurance training, and energy can be provided through aerobic pathways as oxygen supply can meet the demand of the activity.


Muscle hypertrophy is a complex business and is facilitated by many, many metabolic pathways that interact in even more complex ways. Therefore, the science of metabolic stress and its influence on hypertrophy and through which pathways is not only a relatively new one, but also has some important questions still to answer. However, what we do know is that by resistance training using the right intensity to place enough stress on the appropriate energy systems, this will create sufficient metabolic stress to force cellular adaptations to generate hypertrophy, as long as the right nutritional conditions to support muscle growth are met.


Thanks for reading,

Dr P.