Muscle hypertrophy

How our muscles grow

Gaining muscle mass and increasing muscle volume is among the goals of many professional athletes and ordinary fitness enthusiasts.

But how do our muscles grow and what are the mechanisms by which this happens?

In the following lines you will learn:

  • The main way our muscles grow is called muscle hypertrophy;
  • Muscular hypertrophy is myofibrillar and sarcoplasmic;
  • The main known mechanisms of muscle hypertrophy are mechanical strain, muscle damage and metabolic stress.
  • Mechanical tension is the most important mechanism of muscle hypertrophy.

Structure of muscle tissue

Muscle tissue has an interesting and unique structure. While in most tissues, the cells that make them up have a circular shape and contain a single cell nucleus, muscle cells are different.

Muscle cells are long and cylindrical about the diameter of a human hair and instead of just one, contain many cell nuclei located along the entire length of the cell.

Most of the readers are probably familiar with the term muscle fiber. Well, you might be interested to learn that in practice each muscle fiber is a separate muscle cell that extends from the beginning to the end of each muscle.

Each muscle cell/fiber contains a variety of components, such as mitochondria, glycogen, fat particles, and most importantly for weight trainers, the shortening proteins called myosin and actin, which allow our muscles to contract.

The individual muscle cells/fibres are grouped into individual bundles and the group of all the bundles forms our muscles – as we are used to seeing them when we look in the mirror.

Photo source: Wikipedia

How do muscles grow?

Muscle growth can be described in different terms. In theory, muscles can grow in length (serial) and in width (parallel), but since it is probably clear to everyone that muscle length cannot be changed significantly, at least not through standard weight training, muscle growth for the most part happens in width.

First, muscle growth can happen through:

  • Muscle hypertrophy.
  • Muscle hyperplasia.

In muscle hypertrophy, muscle cells grow in volume/size, while in muscle hyperplasia they grow in number.

Photo source: Wikipedia

At this stage, it is still not entirely clear whether and to what extent muscle hyperplasia is possible (1,2) and the reason is mainly that it is extremely difficult to follow such a thing experimentally. Imagine having to count all the hairs on the human head, but in a much more complicated version. Even experiments on animals can hardly provide an answer. However, most experts agree that even if muscle hyperplasia exists, it contributes minimally to muscle growth.

Accordingly, at this stage, muscle hypertrophy is thought to be the primary way in which our muscles grow.

Muscular hypertrophy in its turn can be:

  • Myofibrillar;
  • Sarcoplasmic.

In myofibrillar muscle hypertrophy, growth occurs in the contractile proteins in muscle cells – the myosin and actin. Myofibrillar hypertrophy is also sometimes called functional hypertrophy, since in practice it alone has a direct and immediate influence on force generation.

In sarcoplasmic muscle hypertrophy, growth occurs due to the other elements contained in the cell – mitochondria, glycogen, and others. The sarcoplasm represents the fluid (cytoplasm) which also contains the myofibrils.

Photo source:

In the fitness world, there is still debate about how certain training protocols lead to (predominantly) sarcoplasmic muscle hypertrophy while others lead to (predominantly) myofibrillar hypertrophy.

Many people explain the difference in strength and size between bodybuilders and powerlifters based on their muscle structure. Bodybuilders are larger but often cannot lift as heavy as powerlifters because their muscle hypertrophy is predominantly sarcoplasmic, while powerlifters’ is predominantly myofibrillar. Or at least, this is what is claimed.

That is why online and offline, you may still come across statements that if you want to become strong, you must train with low repetitions (high intensity), as this targets myofibrillar hypertrophy. Similarly, if you want to be big like a professional bodybuilder, you must train with high repetitions (low intensity). However, this is not exactly true.

Sarcoplasmic hypertrophy can really be targeted without even needing to train. One or two days of high carbohydrate diet to increase the glycogen stores in the muscle cells, a little supplementation with creatine and you have sarcoplasmic hypertrophy.

Certain lower intensity training protocols, allowing more repetitions to be performed, also result in a more significant increase in stored glycogen and water levels. During the first weeks, beginners to weight training also experience greater water retention in the cells due to the more severe muscle damage they are exposed.

In other words – yes, in the short term sarcoplasmic hypertrophy can overtake myofibrillar hypertrophy in degree.

However, in weight trainers, whether and exactly how training protocols affect sarcoplasmic hypertrophy, not only in the short term but especially in the long term, is unclear at this point and is extremely difficult to answer (3,4).

Just like hyperplasia, sarcoplasmic hypertrophy can be ignored at this stage, at least until science advances. It appears that it cannot be avoided and will occur regardless of training protocol, but how this changes over time (months, years) is yet to be understood.

We at PirateTea believe it is far more prudent and practical to train without attempting to target a specific type of muscle hypertrophy, especially given the fact that it has already been repeatedly shown that overall (including myofibrillar) hypertrophy can be roughly equal in magnitude in both low and high intensity training.(5) 

Main mechanisms leading to muscle growth

Our body is a highly adaptive system and muscle growth is one of the types of adaptations it is capable of.

In general, our body wants to maintain a certain balance (homeostasis). In order for it to feel the need to adapt, a stimulus (stress) must be exerted on it to disrupt this balance.

This process of stimulus and adaptation is known in the scientific literature as the General Adaptation Syndrome (GAS)(6), the specifics of which, however, are beyond the range of this article.

For the readers of this article, resistance training appears to be the primary stimulus for muscle growth. When, for example, we train with weights in a gym, this activity appears as a stimulus leading to the activation of various mechanisms leading to muscle hypertrophy.

One of the earliest works on what the mechanisms of muscle growth and muscle hypertrophy are, and in fact one of the milestones in the sports science literature, was Brad Schoenfeld’s The mechanisms of muscle hypertrophy and their application to resistance training (7).

The three main mechanisms of muscle growth are:

  • Mechanical tension (mechanical strain);
  • muscle damage;
  • metabolic stress.

Since writing this work, and as the scientific evidence base has expanded, we have a little more clarity  today about how significant each of these is.

Mechanical stress

Mechanical stress is the tension our muscles are subjected to during their contraction. We generate such tension when we train in the gym, when we run, jump or perform any activity requiring muscle contraction.

When we put muscle fibres under tension, their entire structure is disrupted, which in turn triggers specific chemical activity – a process called mechanotransduction.

Various growth factors (e.g. IGF-1), myokines (e.g. IL-6) and a number of others are secreted, signalling the need for the muscle cells in question to be repaired.

These signals reach an enzyme system called mTOR (mammalian target of rapamycin), which reads the information from these and other signals (e.g. presence of certain amino acids), then sends this information to our genes (translation initiation).

Our genes, in turn, contain the structural blueprints of a variety of proteins, in this case, initiating the performance of processes that build new muscle tissue.

The process that builds muscle tissue is in fact a basic one and it is called muscle protein synthesis (MPS).

Simply stimulating muscle protein synthesis, however, does not guarantee that we will become more muscular, because along with protein synthesis, the process of muscle protein breakdown (MPB) occurs.

The synthesis and breakdown of muscle tissue happens constantly, around the clock and simultaneously. Sometimes one predominates, sometimes the other. Depending on whether synthesis or breakdown predominates in the aggregate over the long term, we gain muscle mass or lose it.

Along with muscle protein synthesis, an equally important process called muscle cell nuclei donation (myonuclear addition) occurs. In this process, so-called satellite cells located around the myofibrils donate new and additional cell nuclei to fuse with the remaining muscle cells.

The donation of new cell nuclei is necessary (although some evidence suggests that it may not be mandatory, at least in the short term (8)) because, as I mentioned above, each muscle cell/fiber contains many cell nuclei that are scattered throughout the length of the fiber. Each nucleus has the ability to govern a strictly fixed region of the muscle fiber (known as the myonuclear domain). As muscle fibers grow, there comes a point where the available muscle nuclei are not sufficient to support and manage the new tissue, so over time and along with muscle growth, there is a need for additional nuclei (9).

Although it is beyond the range of this article, I will briefly mention that a well-supported theory states that each person’s muscle potential is largely dependent on the number of satellite cells they possess and the number of muscle nuclei that can be donated. People with more nuclei can build more muscle tissue, while those with less have a lower limit.(10-12)

The role of mechanical tension in muscle growth is unquestionable and in fact it is considered the most significant mechanism.

Muscle damage

In practice, muscle damage is the breaking and disruption of muscle cell unit, literally.

Recently, muscle damage was also thought to be an important mechanism for muscle growth. Many training programs and recommendations of coaches and professional athletes were (and often still are) aimed specifically at creating more muscle damage, in order for it to lead to greater muscle growth.

The reason for this is that muscle damage itself also stimulates the building of new tissue.

In the last few years, however, a number of data have called into great question the benefit of muscle damage for muscle growth (13-15).

At this point, the opinion of most experts is that muscle damage is part of the muscle growth process, but the stimulated processes as a result of it are purely and simply intended to repair the underlying damage, without, upgrading it in any way. Meaning, more muscle damage does not lead to more muscle growth. They can even have a negative impact, since before the body can start building up and building new tissue, it will first have to repair the existing damage, and the bigger it is, the harder and slower everything will become.

Muscle damage can’t be avoided completely and we don’t need to look for a way to intentionally reduce it, but it’s also not recommended to look for a way to increase it.

Metabolic stress

Metabolic stress refers to the accumulation of secondary substances in cells.

During exercise, relying mostly on the body’s anaerobic energy system, by-products such as lactate, inorganic phosphate, hydrogen ions and others are released because of the glycolysis process.

Metabolic stress is the cause of that burning and pumping during a long run.

Metabolic stress is thought to contribute to muscle hypertrophy by increasing muscle fiber activation, stimulating secretion of various hormones, leading to cellular distention, and more (16), but nowadays the relevance and contribution of this factor to muscle growth is also under big question.

In practice, there are several commonly used ways by which metabolic stress accumulation can be achieved.

One way is to maintain shorter breaks between sets and exercises, as this prevents the body from distracting itself with accumulated secondary substances. However, scientific evidence shows that although hypertrophy is achievable with short breaks (less than 60 seconds), longer breaks (more than 60 seconds, even 120 seconds are recommended) lead to greater hypertrophy. (17,18) The main reason for this is that other things being equal, with longer rests the athlete is able to perform more repetitions each set, resulting in greater training volume, and since training volume is the most significant training parameter for muscle hypertrophy, more volume leads to more muscle growth.

Another way to achieve higher metabolic stress is to train with higher training volume and lower intensity. In other words, using a weight that allows you to perform more reps (6-12 and up). The more reps performed; the more metabolic stress accumulates. Here too, however, the scientific literature shows that with controlled total training volume, muscle hypertrophy is about the same at both high and low intensities (high and low repetitions) (5).

Training to failure also results in significantly higher metabolic stress, but the mass of scientific evidence suggests that training to failure does not result in better performance and better muscle hypertrophy. Often, they can even negatively affect long-term performance. (19,20)

Finally, if there is a method that can provide a clearer answer about the relationship between metabolic stress and muscle hypertrophy, it is the KAATSU training method, also known as blood flow occlusion/restriction training, which by its very nature leads to a very high accumulation of metabolic byproducts, therefore high metabolic stress.

Blood flow restriction training has been repeatedly shown to be effective for muscle hypertrophy. Through them, using very low intensity (20-40% 1RM), the same muscle growth can be achieved as with standard high intensity (70-80% 1RM) training (21-25).

At first glance, this training method is good evidence for the role of metabolic stress in muscle hypertrophy, but the problem is that in the sports literature, the study of metabolic stress is also combined with the main proven mechanism, mechanical loading. This makes interpretation of the results and making any solid conclusions very difficult (26).

It can be argued that if metabolic stress has a role in the muscle growth process, then its influence is indirect, by improving/increasing mechanical loading (by increasing muscle activation), which in turn stimulates hypertrophy (27).

How best to incorporate the increase in metabolic stress into resistance training, and whether it is even worthwhile, is yet to be discovered.

Systemic factors

The systemic factors are mainly hormones, in particular testosterone, growth hormone and IGF-1.

It is known that exogenous (external) intake of super-physiological doses of the hormones can significantly increase muscle hypertrophy, and that experimentally blocking and decreasing the levels of these hormones can compromise gains in muscle mass and strength.

Since a significant but short-lived rise in the levels of these hormones was observed after resistance training, it was not entirely clear for a long time whether these changes had a direct effect on muscle hypertrophy.

It is likely that recommendations of the sort that complex, compound, exercises should be present in training may still be encountered, as some evidence suggests that they stimulate and increase hormone levels slightly more than isolation exercises, and this leads to greater muscle growth.

However, it is important to distinguish between pharmacologically stimulated and natural changes in hormone levels. The increase in testosterone levels after weight training is often on the range of 0.1% to 1% of the dose taken exogenously. The duration of this increase is approximately equally shorter.

At this point, the scientific literature is of the opinion that natural changes in hormone levels after exercise have no effect on hypertrophy. If they do, it is extremely small and should not be taken into consideration by the people who exercise (16,28,29).


The topic of muscle growth is one example of the evolution of sports science literature.

While there is still much to learn, at this stage muscle tension remains the primary mechanism leading to muscle growth and should be the focus of resistance training.

Of course, the activation of a mechanism is not always a guarantee that over time the degree of muscle growth will be significant. This is influenced by several factors, which we will discuss in a separate article.

Learn more:


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