Metabolic stress is a broad term, and its meaning can vary slightly depending on which area or system you are referring to within the human body. For the purposes of this article, we are focusing on metabolic stress as it pertains to skeletal muscle and hypertrophy and growth.
Metabolic stressors are not widely discussed by those looking to build muscle mass. Usually, people focus on mechanical tension and how it stimulates muscle growth. However, research indicates that metabolic stress can be a potent anabolic agent and plays a significant role in skeletal muscle hypertrophy and growth.
Please keep in mind that mechanical tension plays a crucial role in building human skeletal muscle mass. The research is clear that it is very good at stimulating protein synthesis and hypertrophy. However, we will focus on metabolic stress and its resulting metabolites for this article. Some researchers have even proposed that metabolic stress might be a better stimulator of muscle hypertrophy than mechanical tension.
What is metabolic stress?
Metabolic stress is an accumulation within the working muscle of metabolites such as lactate (lactic acid without H+), hydrogen ions (H+), and inorganic phosphate. These metabolites are primarily byproducts of anaerobic glycolysis, which is the primary way skeletal muscles generate energy during intense resistance training.
These metabolites or byproducts of anaerobic energy production can play a role in promoting and stimulating human skeletal muscle growth.
How to create metabolic stress?
As mentioned above, metabolic stress is generated during exercise that relies heavily on the anaerobic energy system. Hence, activities requiring high intensity for short bursts or duration are the main drivers of metabolic stress within the muscle. The longer the anaerobic energy system is used, the more metabolites build up within the working muscle.
One study demonstrated this nicely. The researchers had subjects perform 1 set of 12 reps to failure, resulting in elevated lactate levels of 91mmol per kilogram. These lactate numbers kept increasing with subsequent sets to failure. After doing 3 sets to failure, the subjects had elevated lactate levels of 118mmol per kilogram.
If you do exercise that does not require an anaerobic energy system or use it for a short period, then the metabolic stress and metabolite buildup will be much less. We can see an excellent example of this with powerlifters, as they do intense exercise but do not take the muscle to failure or do many reps. Heavy resistance training done by powerlifters doing 1 or 2 reps close to 90 percent of their 1 rep max utilized the phosphagen energy system. It thus did not create the same level of metabolic waste byproducts such as lactate.
Another way that resistance exercise causes metabolic stress is by blood flow occlusion. When the skeletal muscles are working and contracting for longer, as seen with repetition ranges of 12 to 20, this causes compression and occlusion of some blood vessels supplying the working muscle. This occlusion leads to hypoxia in the muscle. This lack of oxygen increases the buildup of reactive oxygen species (ROS) and metabolites such as lactate, all of which contributes to the metabolic stress on the muscle.
Bodybuilders have long been using metabolic stress and its metabolites to build muscle mass. They may not have known that it was the metabolic stressors causing the increased muscle growth, but they knew that training to failure with rep ranges ranging from 8 to 12 was the most effective. Most bodybuilding exercise routines focus on muscle failure and the feeling of a “burn” or “pump” caused by the buildup of lactate and other metabolic waste products.
Resistance exercise routines that focus more on exercising to failure as opposed to training with heavy loads have shown in studies to produce significantly more substantial increases in skeletal muscle hypertrophy.
Does metabolic stress build muscle?
Metabolic stress and the corresponding metabolites do build muscle. Studies show that the metabolites such as lactate that are created and contribute to metabolic stress can stimulate muscle hypertrophy.
Resistance exercise routines that focus more on exercising to failure as opposed to training with heavy loads have shown in studies to produce significantly more substantial increases in skeletal muscle hypertrophy. As we know, exercise routines that go to failure generate more metabolic stress and related metabolites.
Some studies show that some of the metabolites that are generated and contribute to the metabolic stress within the skeletal muscle are anabolic. What this means is that these metabolites can stimulate skeletal muscle hypertrophy. The primary metabolite that researchers have focused on is lactate.
In the following section, we will focus in more detail on the specific metabolites that contribute to metabolic stress and can stimulate muscle hypertrophy and muscle growth.
What are the metabolites from metabolic stress that stimulate muscle growth?
There is a growing repository of evidence and studies demonstrating the stimulatory effect of specific metabolites on skeletal muscle hypertrophy. The primary metabolites that researchers have focused on to date are lactate and H+.
Lactate is often interchanged with lactic acid in the athletic and bodybuilding world. You will often hear people refer to the “burn” they feel while working out as being caused by lactic acid, but this is not technically correct. The fact is that the “burn” is caused by lactate and not lactic acid. In fact, lactic acid cannot exist within living cells because when dissolved in water, lactic acid loses its hydrogen ion, which changes it into lactate and means it is no longer an acid.
The growing evidence about lactate and its relationship with muscle hypertrophy is fascinating and compelling. Several in vitro studies demonstrate the anabolic effects of lactate when cells were cultured with lactate. The studies found that when the skeletal muscle cells were cultured with lactate, the satellite cells were activated, and hypertrophy was increased.
In one study, researchers gave test subjects a combination of lactate and caffeine and made them exercise on a treadmill for 30 minutes every other day for 4 weeks. at the end of the study, the researchers noted that the test subjects who received the caffeine and lactate mixture had significantly more muscle growth in their gastrocnemius and tibialis anterior muscles than the placebo group.
The exact mechanism of how lactate increases skeletal muscle hypertrophy and growth is unknown. However, it has been postulated that its mechanism of action might be related to calcium signaling pathways. Another possible mechanism of action could be that lactate has inhibitory effects on histone deacetylase activity, which is an inhibitory regulator of muscle growth.
Hydrogen ions (H+)
Research on hydrogen ions (H+) is limited, and we could not find many studies examining their role in muscle hypertrophy and growth. What is known is that H+ builds up in exercising skeletal muscle and can create an acidic environment that negatively impacts calcium’s ability to bind within the muscle fibers, decreasing the ability of the muscles to contract forcefully. Fast-twitch muscle fibers (type 2) are more susceptible to this than slow-twitch (type 1). Because of this, some researchers speculate that perhaps as the fast-twitch fibers tire because of the H+ buildup, slow-twitch fibers get recruited in more significant numbers, leading to increased hypertrophy within the slow-twitch fibers.
Let’s put it all together
All of this information can be overwhelming and confusing. How do you apply this to increase your muscle-building capacity? The 3 key things to take away from this article and the presented information are listed below.
- Metabolic stress is a critical part of skeletal muscle hypertrophy and growth.
- Lactate is the primary metabolite associated with stimulating muscle hypertrophy.
- Focus your resistance training around creating metabolic stress by doing each set to failure and ensuring you are doing 8 or more repetitions
If you focus on these 3 key points, you should be able to maximize your time in the gym and achieve the best possible gains in lean muscle mass.
All of the information contained within this article should not be taken as medical or health advice. For medical and health advice please speak with your doctor.