How Do You Actually Grow Muscle?

April 3, 2023

Consistent strength training produces muscle growth. But... What's actually happening, on a cellular level? The process of hypertrophy isn't as complicated as it sounds!

Skeletal Muscle Fibers (Myofibrils)

Repairing damaged muscle fibers

When you exercise, you create microscopic muscular tears in your muscle fibers (AKA myofibrils). So, your body gets to work fusing muscle fibers together and forming new strands of myofibrils.

When the number of myofibrils in your body/muscle(s) increases, you're undergoing hyperplasia. When the size of your muscle cells increase, you're undergoing hypertrophy. Both phenomena result in muscle growth, or GAINS.

To grow your muscles, the rate of muscle protein synthesis must exceed that of muscle protein breakdown.

When you're a child, your muscle cells do divide (or, they undergo mitosis). When you're an adult, however, no myofibril mitosis occurs. Instead, satellite cells must differentiate into myofibrils.

Satellite cells supercharge the process

You've heard of stem cells - satellite cells are basically just muscle stem cells. Located on the outer surface of your muscle fibers, these cells are activated in response to exercise, mechanical tension, or hormonal signals.

The primary functions of satellite cells are to a) protect/repair muscle and b) form new muscle fibers.

Satellite Cells and Muscle Tissue Repair

When muscle fibers are damaged and require repair, activated satellite cells differentiate into new muscle cells.

The new cells donate their nuclei to and fuse with existing, damaged muscle fiber cells to repair them. Without satellite cells, our muscle tissue would degrade.


Satellite Cells and Muscle Growth

In order to grow your muscles, satellite cells divide and differentiate into new muscle cells. These cells then fuse with existing cells to increase the size and strength of your muscles.

Genetics and % Activation of Satellite Cells

Your genetics play a role in the % activation of your satellite cells. People who tend to put muscle on more easily may have a greater % activation of satellite cells, or they may have greater abundance of satellite cells.

If you happen to be somebody with more satellite cells to begin with, your body will more rapidly respond to training and you'll see quicker muscle growth.

You may be teased for having bodybuilder genes. There is no 'L' to take here, though.

Not a "born bodybuilder"? No problem - You can also increase the number of satellite cells you have through resistance training!

mTOR activation and signaling

Similar to satellite cells, a protein kinase dubbed mTOR (mechanistic target of rapamycin) is a key regulator of muscle protein synthesis.

In addition to helping you make GAINS, mTOR is important for regulating cell growth, proliferation, and survival in various tissues.

When mTOR is activated, it signals to the muscle cells to increase protein synthesis by increasing the production of ribosomes, which are the cellular structures responsible for synthesizing proteins.

...mTOR, and the powerhouses of the cell!

mTOR is also associated with increased numbers of mitochondria in muscle cells - you know, the "powerhouses of the cell".

Mitochondria are like teeny tiny cell generators. The more mitochondria you've got, the better endurance you'll have during exercise!

mTOR. Source:

What factors activate mTOR?

Factors that activate mTOR include exercise, amino acids, and insulin. Amino acids, you say? Yes. Protein consumption is a major activation factor, so check to see that you're actually getting enough.

Supplementing leucine, or eating a diet rich in leucine, supports mTOR signaling and leads to increased muscle protein synthesis. Leucine is an essential amino acid most abundantly found in animal proteins - for example, beef, chicken, eggs, milk, and fish.

Mechanical tension and metabolic stress created during your workout makes exercise a particularly powerful mTOR activator!

Testosterone and muscle growth

Higher levels of testosterone in the body are associated with greater strength and increased muscle mass.

You might consider testosterone the main driver for muscle growth. It is actually not, though some people may emphatically claim otherwise. Testosterone is certainly involved in muscle growth, but it is not ALL you need.

Testosterone promotes muscle growth by increasing protein synthesis and by activating satellite cells. It also reduces muscle breakdown, such that your body can preserve and maintain your muscle mass.

Testosterone molecule.

Strength training helps release MORE testosterone while simultaneously increasing how sensitive your muscle cell receptors are to testosterone. This is also called androgen receptor upregulation, if you're interested.

Testosterone also increases the number of certain neurotransmitters present at the site of muscle damage, which stimulates the release of growth hormone. And growth hormone... leads to GROWTH!

Supplementing Testosterone??

Testosterone supplementation can lead to significant strength and muscle gains in men, particularly in men who are deficient in testosterone or in men whose testosterone levels have decreased with the process of aging.

Testosterone replacement therapy (TRT) does come with side effects including some that may negatively impact your fertility. Please make sure you're well informed if you're choosing supplementation!

Nutrient Availability

In order to grow muscle, you've got to give your body the right materials! Muscle growth requires adequate nutrient intake, particularly protein and carbohydrates.

Prioritize eating high-quality protein.

Muscle protein synthesis literally cannot happen without adequate amino acid supply, so make sure you're eating lots of high quality protein.

You'll also need to eat enough carbohydrates, because your muscles primarily use glycogen for muscle contraction and recovery.

Glycogen = Fuel

Muscles use carbohydrates as a source of energy. When your body needs energy, it can quickly break down glycogen, a complex carbohydrate.


The process of breaking down glycogen into glucose is called glycolysis. The glucose is then used by your muscles to produce ATP (adenosine triphosphate, the energy currency of the body!) and power you through your reps.

Glycogen is a fairly large polymer - a polymer is a molecule made up of repeating units, also called monomers. In glycogen, the monomer is glucose.

(The monomer unit for glycogen.)

When you contract and use your muscles, you are obviously also using energy. Whatever glucose is in your bloodstream is used first to feed your muscles, and beyond that your body starts dipping into its glycogen stores for additional fuel.

The majority of your glycogen is stored in your muscles and in your liver.

Delicious, delicious fruit. Mmm.

Get enough carbs in, friends!

If you're totally out of glycogen and your stores are depleted, you'll experience a decrease in performance. Your parents call it low blood sugar; runners may refer to this as "hitting the wall".

To prevent this, try eating carbs before and/or during exercise to maintain your glycogen levels and provide your muscles with a steady source of energy.

Low blood sugar can also make you cranky. Maybe eating appropriate amounts of carbs will make you a happier person, too? No guarantees, but it's worth trying. :)


You do get a pump when lifting, but the actual growth happens while you rest, post-workout. Protein synthesis is highest during periods of rest!

Rest and recovery is necessary for replenishing energy stores, properly repairing those damaged muscle fibers, and removing waste products from your muscles. With sufficient rest, your body adapts to the stress of exercise and your athletic ability greatly improves.

If you do not rest, you're not allowing your body to fully recover and ready up for your next strenuous workouts. Overtraining sets you up for injury and can really set back your muscle building progress.

(You, prioritizing rest)

And, that's it for the crash course! If you liked reading, please share this article. :)

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