Muscle Growth | How Do Muscles Grow?

muscle growth

The topic of muscle growth is a popular one, but it is also an incredibly complicated one. There are so many variables that can impact the body’s ability to grow and build muscle. 

In this article, we will cover all aspects of skeletal muscle growth; we will cover everything from the anatomy of the muscle fibers all the way to protein synthesis and mTOR. It might seem overwhelming at first as it is a long article, but each topic and section is important, and if you understand how the interplay of all these topics functions, you will better understand muscle growth as a whole.

skeletal muscle anatomy

In order to understand how muscle hypertrophy happens, you have to understand a bit about how skeletal muscle is structured at the microscopic level. This section will briefly lay out how a muscle fiber is structured at the microscopic level since this is where the growth happens.

Muscle fiber anatomy
Muscle fiber anatomy


Each muscle fiber is wrapped or covered with the sarcolemma, the plasma membrane surface. Each fiber has a long cylindrical shape with numerous oval-shaped nuclei. Skeletal muscle cells are pretty large compared to the average cell in the body. Each one ranges from 10 to 100 micrometers in diameter; this is 10 times larger than normal cells. Their length can stretch up to 30 centimeters or 12 inches.


Hundreds to thousands of myofibrils are contained within the sarcolemma wrapping of just one muscle fiber. These myofibrils are rod-like in shape and run parallel to each other. They are densely packed together, making up 80 percent of the cellular volume. The myofibrils are the contractile units of the muscle fiber. 


The sarcoplasm is the fluid area within the sarcolemma and between the myofibrils. The sarcoplasm is to muscle what the cytoplasm is to normal cells. It contains stored glycogen and oxygen-carrying myoglobin.


Sarcomeres are the muscle’s contractile units, and many are contained within one myofibril. They run one right after the other along the entire length of each myofibril. Each sarcomere is separated by what is called Z discs. A good example would be train cars all lined up end to end and connected to each other. The contractile proteins actin and myosin are contained within each sarcomere. They are what contract on a microscopic level and produce skeletal muscle contractions.

Different ways skeletal muscle grows

There are two main ways that skeletal muscle can grow, either by increasing the size of the muscle fibers, which is known as skeletal muscle hypertrophy. Or, new muscle fibers can be created, called skeletal muscle hyperplasia.

The primary way that growth occurs is by hypertrophy. Hyperplasia is rare and difficult to achieve. Studies show that an untrained average man has the same number of skeletal muscle fibers as a professional bodybuilder.[1] This finding suggests that muscle hyperplasia is rare and extremely difficult to stimulate even in elite bodybuilders.


Skeletal muscle hypertrophy is the increase in muscle mass and also an increase in the cross-section area. Muscle hypertrophy occurs in response to progressively challenging physical stress or load that the muscle has to overcome.[2] This physical stress leads to microtrauma and damage to the muscle fibers, and this damage stimulates the body to repair the damage. During this repair process is when hypertrophy takes place.

There are two types of skeletal muscle hypertrophy. There is still debate within the scientific community on which of these two is responsible for increases in muscle mass or is a combination of both.

sarcomere hypertrophy

As mentioned in the anatomy section, each myocyte or muscle cell contains many myofibrils. Within these myofibrils are the contractile units called sarcomeres. Within the sarcomeres are actin and myosin. The hypertrophy takes place within these myofibrils within the myocyte. It is estimated that each myofibril measures 1 micrometer in diameter, and they make up 80 percent of the volume of a muscle.[3]

Satellite cells within the muscles are triggered to proliferate and divide and go to the site of the trauma and attach to the damaged myocytes (muscle cells). These new satellite cells attach to the myofibrils within the cell and donate their nucleus to the myofibril to help it repair itself.

The satellite cells repair process does not increase the actual number of muscle fibers but instead increases the amount of contractile proteins (myosin and actin) within the myocyte (cell). The satellite cell repair process can last up to 48 hours post-workout or training.[4]

The satellite cells go to the myofibrils within the myocyte damaged during the workout and attach to them and help repair them and make more contractile proteins, which are actin and myosin. These new contractile proteins make the myofibrils larger, leading to an increase in skeletal muscle mass. This growth in the myofibrils is sometimes called myofibrillar hypertrophy, which is the same thing as sarcomere hypertrophy.

Remember, the myofibrils contained within the myocytes account for 80 percent of the total volume of skeletal muscles. So muscle hypertrophy increases the number of myofibrils within the myocyte but does not increase the number of myocytes. So after muscle hypertrophy, you have more contractile units (myofibrils) within the same number of myocytes (muscle cells). 

Muscle fiber hypertrophy
Muscle fiber hypertrophy

sarcoplasmic hypertrophy

Sarcoplasmic hypertrophy is hypertrophy of the sarcoplasm. In basic language, it means a growth in the size of the sarcoplasm, which is the cytoplasm of striated muscle cells. The sarcoplasm is the fluid-filled area surrounding the myofibrils within the muscle fiber.

With sarcoplasmic hypertrophy, it is not about an increase in the number of myofibrils or an increase in their size. It is all about an increase in the volume of the sarcoplasm. This increase in the sarcoplasmic volume causes an increase in the overall size of the muscle fiber.

The first human study to provide strong evidence that sarcoplasmic hypertrophy contributes to overall muscle hypertrophy and growth was done in 1982. The study showed that the test subjects who had never worked out before had a 15 percent increase in the space taken up by the sarcoplasm after 6 months of training. The test subjects who had years of resistance training showed a 2 times greater size of space that the sarcoplasm occupied compared to the other test subjects.[5]

The volume or size of the sarcoplasm can be affected and altered by resistance training. Still, supplements such as creatine and carbohydrate loading can all lead to increases in the volume of the sarcoplasm. Suppose more solutes enter the sarcoplasm, such as creatine or glycogen. These solutes can change the osmolarity and lead to water being drawn into the sarcoplasm, thus increasing its volume and size. 


Skeletal muscle Hyperplasia is the increase in the number of muscle cells or the creation of new muscle cells. In contrast, skeletal muscle hypertrophy refers to the enlargement of the existing muscle cells.

The way that hyperplasia occurs is via satellite cells and their activation and proliferation. When the muscle is damaged after resistance training, satellite cells are activated and stimulated to proliferate, which means they undergo mitosis cellular division. As they divide, their numbers increase. These new cells migrate to the damaged muscle cell and either fuse with the muscle cell, which is what happens with hypertrophy, or they can fuse with each other and create a new muscle fiber is called muscle hyperplasia.[6]

Stimulating hyperplasia in skeletal muscle is not easy, and often people will tell you that it is not possible. However, studies show that hyperplasia is possible in human muscles.[7] Research done in cats shows that resistance training done using a slow tempo or lifting speed was the most effective at stimulating muscle hyperplasia[8]

Skeletal muscle has to be stressed to grow

Now that we know how muscles hypertrophy works and contributes to gains in muscle mass, we need to look at how muscle hypertrophy is stimulated or triggered. The body will not turn on the anabolic process that ultimately leads to increases in muscle mass without a stimulus or stressor.

The body does not waste energy or calories and thus will always try to conserve energy. The body would never expend the energy or the vital amino acids required to increase muscle mass unless it had a reason to, such as a stimulus like resistance training. So what will stimulate a skeletal muscle to grow?

Bicep flexing
Bicep flexing

Physical Stress

Intramuscular Tension

Intramuscular tension is a measure of the force that a contracting muscle must generate during exercise or lifting of a weight. The more force the muscle has to produce to lift the weight, the more intramuscular tension. The more tension or strain generated, the more muscle damage is produced. 

The heavier the weight you lift, the more tension that is generated. This increased skeletal muscle tension leads to more significant skeletal muscle damage, and as we know, muscle hypertrophy occurs when skeletal muscle fibers are damaged and repaired.

Time under tension

A theory in muscle growth stipulates that to stimulate muscle growth, you must achieve a specific time under tension. Basically, the more time a muscle is under tension (strain) during resistance training, the more muscle damage will be caused. Thus, a more significant stimulus is generated for repair and muscle growth.

A great way to increase the time under tension s to use a slow rep tempo, such as 3 to 5 seconds on the concentric contraction, hold for 3 to 5 seconds, and then do the eccentric contraction in 3 to 5 seconds. This example of a slow rep tempo highlights how you can increase the time under tension of the muscle and thus increase the muscular damage and provide a more significant muscle growth stimulus for the body.

Blood vessel occlusion

Another way that muscular tension can stimulate muscle growth is by affecting circulation and blood flow to the working muscles. If intramuscular tension is maintained and the muscle is not allowed to relax while training, the blood supply to the active skeletal muscles can occlude. This reduced blood supply can lead to a build-up of Reactive Oxygen Species (ROS) and lactate, both of which cause the stimulation and proliferation of satellite cells, which we know are essential in the hypertrophy process in skeletal muscle.[9]

Blood Vessels
Blood Vessels

Metabolic Stress

The skeletal muscles contract and produce waste products from anaerobic respiration and energy production by working out and resistance training. One of the top waste products from muscle contraction especially type 2 muscle fibers, is lactate. The build-up of lactate is often referred to as the “burn” or “pump” feeling that you feel when working out intensely. The lactate builds up and creates an acid environment within the muscle. Studies show that lactate can cause the activation of satellite cells within the muscle cells. As we mentioned, satellite cells are critical to the hypertrophy process within skeletal muscle.[10]

Another metabolic byproduct of the anaerobic energy process used by contracting muscles during resistance training is reactive oxygen species or ROS. Studies show that these reactive oxygen species can cause the activation and proliferation of satellite cells within the muscle[11] 

Muscle growth happens when it is repaired

Muscle Repair
Muscle Repair

The first step in the process of building new muscle mass is by stimulating or stressing the muscles, which we covered above. The second step in the muscle-building process is the repair and recovery phase.

The body’s immune system is activated in the recovery and repair phase, anabolic hormones are upregulated, and Mtor and protein synthesis are also activated. This combination of processes and events contributes to creating an anabolic environment in which muscle growth can occur. Building new muscle mass can only happen if all the necessary building blocks, such as the essential amino acids.

You can stimulate and stress the body by working out extremely hard, but if you do not provide the body with the required nutrients it needs, protein synthesis and muscle hypertrophy will stop. In some cases, if you do not consume enough protein and amino acids, you can actually lose muscle due to muscle protein turnover and catabolism. To find out more about how much protein you need, look at our article about how much protein your body can absorb.

Immune Response

after you have caused muscular damage, as mentioned above, your immune system will respond with an inflammatory response.[12] The inflammatory response in the muscle aims to bring in increased blood supply. The increased blood to the damaged area helps bring in key nutrients and building blocks needed to repair and grow the muscle; it also helps to eliminate waste products.

Immune cells called macrophages migrate to the injury site and secrete inflammatory cytokines and growth factors. The cytokines are a way to give direction to the immune system in how it should respond or behave.

The main cytokines that are released after a workout are:

These three proinflammatory cytokines direct the immune system to break down and remove the damaged tissues.[13]

The growth factors that are released after a workout and in response to muscular damage are:

  • Insulin-like growth factor (IGF)
  • Fibroblast growth factor (FGF)
  • Hepatocyte growth factor (HGF)

The above growth factors work synergistically to cause hypertrophy of the damaged muscle.

Anabolic Hormones and Growth factors required to build muscle

Insulin-like growth factor (IGF) 

As the name suggests, IGF controls insulin metabolism and is key in activating protein synthesis. Insulin is an anabolic hormone and is key to creating an anabolic environment and supporting protein synthesis. After resistance training, IGF levels are stimulated and remain elevated to help support protein synthesis.[14]

Hepatocyte Growth Factor (HGF)

HGF is a cytokine that causes satellite cells to migrate to the injured area within the muscle.[15] For more information about Hepatocyte growth factor read of article about myokines.

Fibroblast Growth Factor (FGF)

Fibroblast growth factor has five different forms within the muscle, and each of these helps to cause satellite cell proliferation and division. As mentioned already, satellite cells are critical in growth and muscle hypertrophy. Studies show that the amount of FGF released is proportional to the severity of the muscular damage.[16]


Testosterone is the primary androgen responsible for developing male characteristics such as body hair, deepening voice, and muscularity seen in males. Testosterone is the strongest and most well-known of the anabolic hormones. Testosterone increases muscle mass by stimulating protein synthesis.[17]

Testosterone Molecule
Testosterone Molecule

Growth Hormone (GH)

Growth hormone’s ability to build muscle is somewhat confusing. There is evidence suggesting that its ability to increase muscle size is more related to fluid retention than an increase in hypertrophy or hyperplasia. GH does stimulate IGF release, and IGF does increase protein synthesis, so at the very least, GH does stimulate protein synthesis indirectly.[18]


Insulin is classified as an anabolic hormone, like testosterone and GH. Insulin can increase protein synthesis directly and stimulate mTOR, which also upregulates protein synthesis.[19] Insulin also can help improve the uptake of amino acids into the body’s cells, thus providing the building blocks for growth.

Muscle Protein synthesis

 Muscle protein synthesis is the process of building new proteins inside a muscle cell. It is activated by resistance exercise and can stay active and in an elevated state for up to 36 hours post-workout.[20]

Muscle protein synthesis is always happening within our body and muscles 24 hours per day. But if you are not working out or doing resistance training, your rate of protein synthesis will match or equal that of the catabolism in your body. 

Just like protein synthesis, muscle protein breakdown is always happening within the body. This process of muscle protein turnover happens to free up essential amino acids for the body to use or perhaps clean up old protein and cells to make way for new healthier ones. There is a balance or homeostasis between muscle protein turnover and muscle protein synthesis.

For the body to build muscle or for you to gain new muscle mass, the rate of muscle protein synthesis has to be greater than the rate of the muscle protein turnover. The anabolic stimulus and environment must be stronger or greater than the catabolic stimulus and environment. If the rate of muscle protein synthesis is greater than the rate of muscle protein breakdown, you will build muscle proteins, and the size of your muscles will grow. However, if the rate of muscle protein breakdown is greater, you will be in a catabolic environment, and you will lose muscle proteins, and ultimately, muscle size will decrease.

There are key things that studies show can increase the rate of muscle protein synthesis (MPS).

Resistance exercise

resistance exercise is shown to increase rates of MPS.[21]

Resistance exercise
Resistance exercise

Protein consumption

High protein consumption increases MPS (muscle protein synthesis), especially in the post-workout window when the resistance exercise has already stimulated MPS.[22] The amount of protein required to optimize MPS depends on the amount and intensity of the resistance exercise performed.

Several studies demonstrate this fact. The first study showed that 20 grams of protein were sufficient to maximize MPS in the 4-hour post-workout window, but this study only had participants working out their lower bodies.[23] Another study that had participants do whole-body workouts found that 40 grams of protein produced a 15 percent greater increase in MPS rate than 20 grams of protein.[24]

As you can see, just from comparing these two studies, many variables can come into play that can affect the rate of MPS.



mTOR stands for mechanistic target of rapamycin complex or mammalian target of rapamycin. mTOR plays a pivotal role in activating muscle protein synthesis and thus ultimately leading the body to build muscle. 

Resistance training results in phosphorylation of mTOR. This phosphorylation is what activates mTORC1 or mTOR C1. Once the mechanistic target of rapamycin complex 1 has been turned on by being phosphorylated, it turns on protein synthesis by activating several protein kinases p70S6K1 and 4EBP1. These two proteins promote ribosomal binding to mRNA and thus initiate muscle protein synthesis.[25][26]

mTOR C1 is turned on by resistance exercise. Once activated, mTOR C1 increases the rate of myofibrillar muscle protein synthesis by increasing the efficiency of muscle protein synthesis and increasing the total capacity for protein synthesis by increasing ribosomes.[27][28] 

Studies suggest that mTORC1 can remain activated for up to 18 hours post-workout and that muscle protein synthesis can remain elevated for 36 hours post resistance training.[29]

It is important to remember that mTOR and mTOR C1 are only part of the muscle protein synthesis puzzle, and that mTOR alone does not solely control protein synthesis. Other factors can affect protein synthesis, such as:

  • protein and amino acid consumption
  • Anabolic hormones such as Testosterone, Insulin, GH
  • Amount and type of resistance exercise
  • Growth factors

Delayed onset muscle soreness (DOMS)

DOMS (delayed onset muscle soreness) does not directly play a role in muscle growth but is more a side effect or byproduct. DOMS occurs as a direct result of intense resistance exercise. The resistance exercise causes microscopic trauma or injury to the muscle, which results, as we talked about above, in an inflammatory response that leads to inflammation and swelling.

DOMS does not develop until 8 to 12 hours post-workout and can last up to 72 hours. It is part of muscle growth’s healing, repair, and rebuilding phase. If you are experiencing DOMS, it is a good indication that you worked out or trained hard enough to stimulate growth.

Muscle soreness
Muscle soreness

Does soreness mean muscle growth?

Just because you are experiencing delayed onset muscle soreness does not mean you will gain size and grow. Again it can be a good indicator that your workout was at a high enough intensity to stimulate growth, but what you do after the training will determine if you gain size and grow. You need to provide the body with the building blocks to repair and build after your workout, so eating enough protein and amino acids is vital, and enough rest and sleep are also critical.

Don’t be afraid of DOMS, as it does not mean you hurt yourself or are injured. However, do not go chasing DOMS, do not think that you will grow just because you have DOMS. The opposite is also true, do not think your workout was not good because you do not feel or experience DOMS.

With DOMS, know what it is and understand that it does not play any significant role in building muscle. It is just a side effect of resistance training and is normal if you experience it from time to time.


As you have learned, there are many factors that are at play in the process of building new muscle. It is a very complex process and one that can be impacted by numerous variables, from the amount of protein and amino acids you eat to the amount and intensity of the resistance exercise you do.

Studies are still being done in this area, and new things and theories will come forward in the years to come that will change what the best practices are for stimulating and growing skeletal muscle.

The information contained within this article should not be taken as medical advice. The article is not intended to be health or medical advice. For proper medical and health advice please talk to your doctor.


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