What Is Metabolic Stress and Why Is It Important for Muscle Growth?

Muscle growth is influenced by a range of factors; and the one that is perhaps most important is the metabolic stress that training causes. We know by now that in order to promote muscle growth, muscles need to be placed under stress to be forced to adapt. The stress relates to the load that is placed on the muscle, and by increasing the frequency, weight and volume of training, we can in turn, increase the stress on the muscle, leading to consistent progression in muscle growth.

The fact that mechanical stress causes muscle growth is long established, but what is less known is just how the stress influences the processes that take place within the muscles that stimulate metabolic pathways at a cellular level.

There are a large number of both anabolic and catabolic pathways that are imperative for muscle growth. While we largely disregard catabolic processes, some muscle breakdown is necessary to allow for regrowth to take place. Of course, anabolic processes leads to more protein synthesis than muscle degradation and ultimately, we want to focus on promoting anabolic processes and preventing excessive catabolism. This is achieved through both weight and resistance training and nutrition.

The most important metabolic pathways that are linked to muscle growth are mTOR and MAPK pathways. These transmit the mechanical signals from training to the creation of new muscle and are influenced by amino acid availability, energy provision and hormonal status. The mTOR pathway is directly linked by mechanical loading and gets us from A to B on the muscle building journey fairly quickly. There has been much research that looks at the mTOR pathway being linked to phosphatidic acid, which is now thought to be a muscle building agent and effective as a supplement. Although these pathways are key, what’s important to remember is that they are not exclusive, and there are many pathways that work both independently and interact with each other to cause hypertrophic response.

The metabolic stress that resistance and weight training causes manifests itself in the accumulation of metabolites. Metabolites are the bi-products of nutrient breakdown and provide the body with the type of fuel used during resistance training that causes the hypertrophic response.

These metabolites include:

  • Phosphate (from ATP breakdown)
  • Hydrogen
  • Lactate
  • Glucose metabolite (from anaerobic glycolysis)

Anaerobic glycolysis is the metabolism of glucose without oxygen being present. By training at a high intensity, oxygen demand outstrips supply, and the lack of oxygen (hypoxia) in the muscle cells is thought to be a determinant as to why certain repetition ranges leads to more muscle growth.

The understanding behind causing cells to be in a hypoxic state to promote muscle growth is demonstrated in blood flow restriction (BFR) training. Medical cuffs are used to restrict blood (and ultimately oxygen) to the muscles, also preventing the removal of anabolic metabolites; BFR training has reported good results in terms of muscle growth, but it is critical that is it ONLY carried out under proper supervision and is limited to the limbs as it is extremely difficult to implement this on the torso. Also, it is important to remember that unless you are training at the required intensity, you won’t be able to create the hypoxic conditions rendering BFR training ineffective.

Typically training at ‘bodybuilding’ intensities of 60-85% of one rep max means that our bodies use glycolysis for energy. This process accumulates lactate and signals (through the release of hormones) muscle building pathways; this explains why training at these intensities causes hypertrophy while training at higher intensities of 90% of one rep max, uses different pathways that don’t accumulate lactate and fail to provide the same, or improved results. This also applies to the opposite end of the spectrum, where the low intensity means that we enter into an endurance training state and aerobic pathways supply the volume of oxygen that meets the demands of the activity.

The growth of muscle is an extremely complex process and is facilitated by a vast number of metabolic pathways that interact in ways that are even more complicated. Therefore, the science behind metabolic stress and how it influences muscle growth through various metabolic pathways is not a new concept, with an avalanche of questions that still need answers. However, what we do know is that by ensuring we are undertaking resistance training at the right intensity, we can place enough stress on the appropriate energy systems, creating sufficient metabolic stress to force the cellular adaptations to generate hypertrophy – of course, so long as the right nutrition conditions are also met.