Only heavy makes heavy? What factors lead to muscle growth through training!

When it comes to building muscle and strength, training is arguably the most important pillar of all. While nutrition and recovery are of course also essential for the optimal conditions for hypertrophy, little will happen without an appropriate training stimulus. However, to get the best possible end result, it’s not enough to simply move dumbbells from A to B or play soccer in hopes of getting legs like Tom Platz. A recent review therefore looked at the stimuli or factors that produce muscle growth [1].

The review, whose title translates as “Stimuli and sensors that initiate skeletal muscle hypertrophy as a result of resistance training.”, distinguishes between different stimuli or factors for muscle growth that sensitize specific sensors or receptors, which then trigger a signalling cascade that ultimately leads to the build-up of muscle mass. In today’s article we will focus on the factors for muscle growth as a result of training and leave the sensors and receptors out of it. This would go into too much detail at this point and would not be relevant to the vast majority of readers. However, the nerds among you can look up the details in the paper itself, as it has free access.

For the practical implementation of the information from this paper into your own training, the illumination of the stimuli discussed in the paper is completely sufficient, which is why we will go through them one by one in the following.

Factor 1: Mechanical tension and load, respectively.

Mechanical tension is probably the most important determining factor for muscle growth [1]. The easiest way to illustrate this is to compare the training of a bodybuilder and a marathon runner. The ultra-endurance athlete performs a much greater volume of training than the bodybuilder and records a similar degree of muscle damage. However, why is the bodybuilder more muscular? The answer lies in the weights used! Strength training induces large amounts of mechanical tension or exposes the muscle to a significantly greater load, which is considered the most important factor in muscle growth [1].

The reason that mechanical tension is so important for muscle growth is the gravitational force that we are exposed to on Earth. In the course of evolution, our organism has adapted to this omnipresent tension or force. That is why we all have such a well-developed musculoskeletal system of muscles, joints and bones. Today, our bodies respond very well to mechanical loads and we possess several sensors that are responsible for detecting loads and forces. These sensors can generate growth signals in response to constant increased loads such as the weights during weight training [1].

We see how important the mechanical load is in bedridden patients. If the muscles are neither actively used for movement nor subjected to a significant load, they atrophy [2]. We can observe something similar in studies where researchers were able to mechanically load muscle fibers without damaging them or inducing an accumulation of metabolites [3]. In this regard, even without these other putative factors for muscle growth, mechanical tension alone was able to induce fiber buildup. In practice, we observe this among others in people who work a lot with their hands, such as carpenters or mechanics. They usually have pronounced forearms, simply due to the constant mechanical tension that acts on these muscles.

How can we incorporate this knowledge into our training? There are two ways to induce greater amounts of the mechanical tension. The first of these is to use heavier weights. Studies show that individuals training in an intensity range of 60 to 90 percent of their maximum strength had significantly greater muscle protein synthesis (MPS) as a result of training than individuals training with lighter weights [4]. The second option is to use light weights until muscle failure is reached. Studies have observed that an intensity of 30 to 40 of maximum force is thus sufficient to increase hypertrophy as much as with heavy weights [5].

Factor 2: Metabolic stress and pump effect.

The term metabolic stress refers to the accumulation of metabolites in muscle cells due to exercise to exhaustion. Mainly these are lactate, phosphates and hydrogen ions, which are by-products of each contraction [1]. They also represent the cause of the pump effect during exercise [6]. First, the muscle is supplied with more blood to flush away the metabolites, causing the muscle cells to swell. Furthermore, these substances ensure that more fluid enters the cells, making them larger. This then causes the smallest blood vessels between the cells to constrict, preventing blood from leaving the muscles.

 

When the muscle fibers are relaxed, blood can easily flow between them. However, once they swell, they constrict the veins that carry blood back to the heart. This causes blood to be pumped into the muscle faster than it leaves, causing it to swell further. The more often the muscle contracts, the more metabolites accumulate in the muscle cells and the greater the swelling [6]. In other words, the pump effect represents a temporary increase in muscle cross-section due to an accumulation of blood and fluid.

Metabolic stress represents one of the most important factors for muscle growth and at the same time induces the swelling of the muscles [6]. We know this as the pump effect.

The first evidence that metabolic stress was a factor in muscle growth also came from bedridden patients. Researchers from Japan found that stopping venous outflow of blood from an immobilized limb reduced muscle breakdown compared to a limb that was not tourniquetted [7]. This observation was then used to combine this principle of so-called “blood flow restriction” (BFR) with strength training, which in some cases allowed subjects to build more muscle than training at low intensity alone, or to reach muscle failure and thus maximal recruitment of motor units more quickly with lower weights [8, 9, 10].

There are several theories about the reasons why metabolic stress is one of the factors in muscle growth. The first, and certainly the most difficult to test, is that the accumulation of metabolites serves as a sensor that initiates growth signals [1]. Another, much more plausible hypothesis, is that activation of motor units increases when the level of metabolites in muscle begins to rise. The reason for this could be that lactate and hydrogen ions lower the pH in muscle, accelerating fatigue [11]. We then perceive this as a burning sensation in the muscles.

This increase in muscle activation may distribute the mechanical tension to more muscle fibers, producing more overall muscle growth compared to a similar exercise with less muscle activation [1]. The third and final theory refers to the muscle pump, discussed earlier, that results from metabolites. The swelling of the muscle cells alone results in some sort of mechanical tension on the muscle fiber from within, which could possibly signal the cell to grow to be able to resist the stress [12].

Which of these theories may end up being the correct one has not yet been conclusively determined. However, since the muscle pump and the accumulation of metabolites in a workout are difficult to separate, it is difficult to explore whether metabolites or the pump alone or only the combination of both should be counted among the factors for muscle growth.

A recent study, which was published after said review and thus could not be included in the results, administered either lactate or a placebo directly into the bloodstream to trained subjects immediately before a training set [13]. While the additional lactate increased muscle lactate levels, it did not increase anabolic signaling cascades or post-exercise MPS. However, muscle pH remained unaffected by the administration of lactate suggesting that training alone already resulted in a maximal decrease in pH.

In addition to BFR training, we can promote metabolic stress as one of the factors for muscle growth by training with lighter weights and higher numbers of repetitions with shorter set breaks [1, 14]. BFR can also be combined with heavy weights because the accumulation of metabolites promotes muscle activation, and thus one may get a greater training stimulus from using heavy weights than from heavy weights alone [11, 15, 16, 17]. However, excessive levels of metabolites may also accelerate muscle fatigue as described, which is why BFR should only be used in a few sets at the end of a heavy training session.

Factor 3: Muscle damage

The last potential factor for muscle growth that has often been discussed in the past is the small injuries that occur in our muscles as a result of training. In fact, this is the factor that was originally thought to be the only reason why a muscle grows as a result of training [1]. Many of you are no doubt familiar with the claim that muscles must be destroyed before they can grow back together stronger. However, over the last ten to 20 years, we have seen increasing evidence that muscle damage does not lead to muscle growth.

 

Muscle damage is the result of repeated muscle contraction under a load or within a movement to which the body is not accustomed [18, 19]. Heavy negative movements over the full range of motion contribute particularly strongly to their development. This damage is so small that it can only be seen under a microscope and affects the structural components of a muscle fiber [20]. When a muscle fiber is damaged, an inflammatory response occurs in the body, increased protein breakdown occurs, and increased levels of enzymes such as creatine kinase can be observed in the blood leaking from the damaged muscle cells [21]. Trained athletes experience a lower degree of muscle damage than beginners because their muscles are already well adapted to eccentric training and they usually do not perform new movements [1].

Muscle damage occurs as a result of strength training and relates to the smallest functional units of the muscle cell, actin and myosin.

The biggest problem with evaluating muscle damage as one of the factors in muscle growth is that you can’t really isolate it in the context of a study [1]. Mechanical tension is considered the most important stimulus for hypertrophy and muscle damage occurs as a byproduct of high mechanical tension. Thus, it is difficult to assess whether muscle growth was caused by mechanical tension or muscle damage.

From exercise science, we know that training within the stretched position of a muscle or over the full range of motion leads to a higher degree of muscle damage [18, 21]. Furthermore, training over the full range of motion often leads to greater muscle growth [22, 23]. Some researchers have interpreted this as a correlation that qualifies muscle damage as a factor in muscle growth. However, in reality, using the full range of motion likely only leads to increased mechanical tension and activation of more muscle fibers due to the length-tension correlation [24].

The active part of the length-tension correlation is determined by the degree of overlap between actin and myosin. This in turn determines how many transverse bridges can form and thus how much force can be applied. This overlap has a maximum (plateau) when the fiber is neither too long nor too short. The passive part of the length-tension correlation is determined by the elongation of the elastic structural elements. The force applied by this increases with increasing elongation at very long extension.

While some muscle groups, such as the quadriceps, the pectoralis major or the latissimus dorsi, can benefit from being loaded over their full range of motion, for other parts of the body, such as the biceps brachiii or the triceps brachii, it is less worthwhile to place the muscle in a stretched position under load. Instead, the increased degree of muscle damage only leads to a prolonged recovery time.

Another scenario in which muscle damage has been linked to muscle growth is eccentric training. While heavy eccentric stress on the one hand leads to more muscle damage, it has also been shown to stimulate muscle growth more than concentric training [25, 26]. The concentric phase describes the positive phase of the movement in which the muscle fibers shorten, whereas the eccentric phase describes the negative phase of a movement in which the weight is released in a controlled manner and the muscle is lengthened.

While the concentric phase describes the contraction of the muscles against a resistance, the eccentric phase is the negative part of the movement in which the weight is being slowed down.

At this point, a simple argument against the theory that muscle damage is one of the factors for muscle growth due to eccentric loading is the fact that more weight can be used in an eccentric movement compared to concentric loading, and thus greater mechanical tension can be achieved [27, 28].

The last theory that states muscle damage is among the factors for muscle growth is based on the fact that muscle damage promotes the recruitment of satellite cells [29]. Satellite cells are mainly held responsible for hypertrophy according to the nucleus-area theory. Muscle cells are among the few cells in the body that have multiple nuclei. The reason for this is their comparatively enormous size. The theory of nucleus areas states that each nucleus can only control a certain area of a cell. As our muscle cells grow, they require more nuclei to maintain their metabolism, including MPS.

The process of adding new cell nuclei requires satellite cells. It is the cell nuclei that are able to control the new protein synthesis [1].

While training that causes muscle damage is associated with increased satellite cell recruitment, studies showed that training without muscle damage also leads to the triggering of this mechanism [30]. The nail in the coffin of muscle damage as one of the determinants of muscle growth is numerous recent studies showing that muscle damage does not correlate with increased MPS [31]. Although short-term increased MPS after exercise does not necessarily mean long-term increased muscle growth, muscle growth cannot occur without long-term increased MPS.

Further studies observed that subjects subjected to muscle-damaging training did not build more muscle than comparison subjects who were not subjected to muscle damage from training [32]. We see the final hook to the theory of muscle damage and muscle building when we consider long-distance runners. This discipline induces a high degree of muscle damage, but no significant muscle growth or even muscle loss, respectively [33, 34]. Certainly, few bodybuilders and strength athletes want to look like a marathon runner and so muscle damage should not be the primary training goal for them.

As we have already noted, it is difficult to look at muscle damage as one of the factors for muscle growth in isolation, since mechanical tension is the most essential cause of muscle damage. However, this is accomplished when we look at muscle injury. In theory, all you would have to do is stab or push hard on the muscle and expect it to grow, provided muscle damage alone increases hypertrophy. Such research shows that muscle damage from injury does not cause hypertrophy, but actually causes some muscle fibers to perish, reducing overall muscle mass. Therefore, it is likely that it is the mechanical tension that leads to muscle growth after muscle damaging exercise and not the repair process that takes place due to the damage [1].

Last but not least, it should be noted that it is not necessary to try to avoid muscle damage as best as possible. They will always occur when we train hard and over a full range of motion. However, we should understand that they are not among the essential factors for muscle growth. Instead, we should focus on increasing mechanical tension through high intensity training or using muscle failure. Methods to increase metabolic stress are good if you feel exhausted from heavy training or training to muscle failure and still want to further increase muscle growth.

Conclusion and summary

According to the current state of science, the mechanical tension on the muscles is considered the most important driving force of muscle building. This can be achieved by heavier weights or the use of lighter weights until muscle failure. The training volume determines the dosage of mechanical tension. Since this type of training can be very exhausting in the long run and the regenerative capacity reaches its limits, the mechanism number two of the factors for muscle growth can be added. It involves the maximization of metabolic stress through the accumulation of metabolites in muscle cells. In addition to a classic pump training, techniques such as Blood Flow Restriction Training may be suitable for this purpose. In this regard, it should be noted that training focused on mechanical tension is also often accompanied by some degree of metabolic stress.

 

Muscle damage, on the other hand, is not a factor in muscle growth. They occur as a byproduct of mechanical tension or the execution of unaccustomed movements. Muscle damage should not be considered negative per se, but it does extend the recovery time of the muscles and central nervous system, and thus the frequency at which we can train. However, as long as we train with a focus on maximum mechanical tension, muscle damage can hardly be avoided.

 

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Primary source: Charlie Ottinger: “What Causes Growth?”, www.themusclephd.com

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