What Happens in the Body When You Build Muscle: The Science of Getting Stronger

As you’re reading fitness content on The Bioneer, chances are that you’re interested in building muscle. But what actually happens inside the body when you lift weights, train calisthenics, or prepare for your sport?

You may think you already know the answer to this. It’s microtears right? A combination of mechanical tension, muscle damage, and metabolic stress?

These statements are not wrong. But they’re also incomplete.

Not only is building muscle and getting stronger MUCH more complicated than that, it’s also much more mysterious. There’s a whole lot that we still don’t know.

In this post, I’m going to take a deep dive into what we do know about muscle growth and how you can use that information to get stronger. In this case, knowledge really IS power.

The Basics of Myofibrilla Hypertrophy

The fundamental notion that microtears build muscle is generally accurate.

Inside the muscle, we have tiny muscle fibers (cells) that contain sarcomeres; essentially tiny telescopic poles formed of contractile proteins.

For now, all you need to know is that these fibers telescopically compress and extend to contract and elongate the muscle.

If we continue to pump out repetition after repetition of curls or squats, we will gradually cause damage to these fibers that needs to be repaired. Imagine an elastic band that slowly gets frayed.

Damage triggers inflammation and in rush cytokines and inflammatory cells – just as they would had you been injured. This is why taking anti-inflammatory pain killers to combat soreness following a workout is a big no-no and can prevent muscle growth! This inflammation is what stimulates the nearby satellite cells to fuse with the muscle fibers and add more myonuclei, triggering further protein synthesis.

Nuclei are like the nerve centers of cells and contain copies of our DNA – the genetic code necessary to produce proteins and send them to the right places for that specific cell. Unlike most cells, muscle cells can contain lots of nuclei, and the more you have, the more protein synthesis can occur: resulting in bigger muscles.

So, this is how muscle damage leads to increase hypertrophy, and it’s sometimes referred to as myofibrillar hypertrophy.

DNA

The contents of that DNA can alter the type and amount of proteins being made. Some people make more and some people make less, which results in a more or less “plastic muscle.” In other words, some people have muscles that respond very quickly to training, while others will struggle. This is somewhat determined by the genetic lottery but the good news is that we can actually alter the expression of DNA through epigenetics.

See also: DNA Analysis Can Help You Optimize Training and Nutrition: My Experience With SelfDecode

The contents of your DNA are identical wherever it is found in your body. So, how does one cell know to produce contractile proteins while another knows to produce digestive enzymes? The answer is gene expression. In short: huge sections of DNA can be “blocked” such that they are no longer “expressed”. Only the relevant areas are expressed in each cell, meaning that only the relevant proteins are produced.

The good news is that gene expression can change in response to our lifestyles AND our training. So, when you train hard and your body receives signals to create more muscle, this is due to gene transcription. Everything from our diet to our thoughts alter gene expression. We may not be able to change our genetic code, but we can massively change the way it impacts on our bodies.

As an example, someone with the ACTN3 gene will encode more alpha-actinin-3, a protein that is expressed more in fast type 2 muscle fiber. Thus, we tend to see this variant more commonly in power-based athletes (study). However, there is some evidence to suggest that supplementing with pequi oil can actually result in altered gene expression, thus increasing transcription for alpha-actinin-3 (study). This is the burgeoning field of nutrigenomics.

Sarcoplasmic Hypertrophy

Also contained within the muscle cells are mitochondria, which provide energy in the form of ATP. These are the “powerhouses of the cell” as we are so often told.

Then there’s sarcoplasmic reticulum: storage units for calcium which is likewise critical for muscle contraction (myosin needs calcium to bind to actin).

Also in here are “t-tubules,” and glycogen and triglycerides that are needed to produce ATP. And all this swims around in a fluid called sarcoplasm.

Increases to all these additional elements can also lead to hypertrophy, think of this like muscle “swelling.” And this type of hypertrophy is sometimes referred to as sarcoplasmic hypertrophy.

The notion that we can train for either myofibrillar or sarcoplasmic hypertrophy has largely fallen out of favor, branded as “bro science.”

Training will always hit both, though from experience, and common sense, it follows that higher rep-ranges and more “bodybuilding-style” training will lean more towards an increase in metabolic products, whereas powerlifting would increase muscle damage.

Higher rep ranges and continuous time under tension flood the muscles with metabolic products.

Hormones

We all know that hormones also play a role in muscle mass. Anabolic hormones such as testosterone, growth hormone, etc. encourage protein synthesis through channels such as the mTOR pathway.

mTOR is Mammalian Target of Rapamycin. This is released in response to the heightened presence of energy, nutrients, and protein and leads to increases in IGF-1 and other hormones that produce testosterone. In other words, the body builds muscle when it is well-fed.

Longer, bodybuilding-style sets can accelerate this effect. That’s because a longer set encourages the build-up of “waste products” such as lactic acid via glycolysis. Now lactic acid isn’t actually a waste product at all and has many important benefits.

Lactate can be used as an additional fuel. It also draws water into the muscle to create muscle swelling (which may act as another stimulus for growth and protein synthesis). And best of all, it stimulates the production of testosterone via the leydig cells (study). Lactate also appears to be necessary for growth hormone production (study).

See also: The Surprising Benefits of Using Partials (Range of Motion)

Differences in diet, hormone production, and even the presence of androgen receptors can all impact how profound the effect will be for a given individual.

This is, as far as we know, why metabolic stress leads to muscle growth.

Though there is another aspect to consider…

Blood Supply

Over time, this demand can even increase blood flow. Through a process called angiogenesis we actually increase the number of blood vessels supplying the muscle. This allows more of those metabolites to enter the muscle where they are needed AND likewise to shuttle away waste products.

Improved blood supply is another great benefit of high volume training then, but the benefits go deeper still.

For example, blood supply is crucial for another type of hypertrophy that gets a lot less attention: connective tissue hypertrophy.

Connective Tissue Hyertrophy

In short: tendons, ligaments, and even the fascia that surrounds the muscle can also increase in strength.

But tendons have a lower blood supply as compared to muscle and thus are slower to respond to training. Whereas muscle growth can occur after just eight days of lifting for a noob, it can take around two months for the same to occur in the tendon (study). On the plus side, atrophy takes longer in connective tissue.

See also: Tendon Training for Injury Prevention and Explosive Power

Fascia, meanwhile, is the kind of “cling film wrap” that surrounds the muscle. This stuff actually contains its own sensory receptors and even muscle cells – meaning it can contribute to force production! Like the muscle, fascia is plastic and can change shape and structure in response to its use – via fibroblast cells that lay down collagen and collagenase in response to stress and pressure signals (just as Tom Myers).

This is “fascial force transmission” may connect disparate muscles and help us to strengthen specific coordinations of muscle exertion. An implication of this, is that we should train with a greater variety of movement to avoid strengthening only specific, linear vectors.

But all this connective tissue doesn’t just surround the muscle.

It also enters it and weaves through it.

A single muscle is wrapped in connective tissue called epimysium. This also protrudes and divides the muscle into its separate bundles of muscle fibers, called fascicles. Each of these is then wrapped in another type of connective tissue, called the perimysium.

In fact, 20% of the muscle is comprised of connective tissue. And we can make this stuff stronger and thicker, contributing to hypertrophy AND strength gains.

Increasing Satellite Cells

So, training with higher volume and time under tension leads to more sarcoplasmic hypertrophy but ALSO more blood supply for the connective tissues. That increased blood supply also allows for improved delivery of blood, oxygen, nutrients, and energy during training AND during recovery. That’s why it’s my strong opinion that we NEED high rep ranges in our training diet.

And there’s more! Increasing blood supply to the muscle may ALSO increase myonuclei and satellite cells. Studies show a correlation between capillarization and muscle satellite cell proliferation (study). Supplementing with creatine also seems to increase satellite cells (study), as does increasing testosterone (study), and training for longer sessions.

Mechanical Tension

The third major mechanism for muscle growth is mechanical tension. This one is simple: loading a ton of weight onto the muscle will trigger muscle growth.

This is possible thanks to mechanoreceptors: sensors in the muscle that can detect mechanical pressure. Mechanoreceptors allow us to gauge our own effort levels, but they can also trigger protein synthesis (study).

We have multiple mechanoreceptors and there are a number of different ways this could occur. One candidate is the “lipid bilayer” which is composed of lipid molecules that could become ruptured during tension, resulting in a cascade of events leading to increased protein synthesis.

This is a deep rabbit hole, and there may even be specific mechanoreceptors that react differently to different types of stress. But what’s clear is that protein synthesis can occur in the muscle purely in response to mechanical tension, independent of other signals (like increased testosterone). Thus, getting under some heavy weight is another important way to grow muscle.

Muscle Fiber Type

As though all this wasn’t already complicated enough, there are also the different types of muscle fiber to consider : Type 1, Type 2a, and Type 2x (although, in truth, it’s more like a spectrum).

Slow type 1 muscle fiber contains more mitochondria for energy-efficient movement. Type 2, or fast twitch fiber, conversely, contains more ATPase which is an enzyme that catalyzes ATP from ADP.

A person can be more or less predisposed to type 1 or type 2 muscle fiber. Different muscle groups are also composed more of specific types. Moreover, a person may find it easier or harder to switch between fiber types.

However, given the right training and stimuli – such as explosive or plyometric training – it is possible to somewhat shift that ratio. This could result in a bigger-looking muscle, but also key to note is that fast twitch fibers produce more lactate versus slow twitch.

See also: Plyometric Training Explained In Depth – The Bioneer

BUT plyometric training of this kind is also less effective at stimulating muscle damage-induced hypertrophy. That’s because there is no or very little eccentric contraction, because you will be using a lighter weight, because there is less opportunity for cross-bridging between the actin and myosin filaments, and because you will use short rep ranges and long recovery times.

It’s also less effective at inducing mechanical tension and metabolic stress.

This is why you can’t build massive legs just by jumping.

The Brain

Then there’s the role of the brain…

You see, you can have the biggest muscles in the world and you won’t necessarily be strong. This is because you also need to be able to send the relevant signals that tell the muscle to contract.

This is why someone physically smaller than you may have a far more impressive powerclean: they’re rehearsed the movement.

See also: The Brain, Movement, and Training

Muscle fibers are bundled into groups called motor units. Each motor unit is “innervated” (triggered) by a different nerve, which is connected to the motor cortex. This brain area serves as a map of the body and activity in each neuron here can trigger the activation of a specific motor unit. The bicep, for example, contains roughly 774 motor units, corresponding to 774 motor neurons in the motor cortex.

No one can activate all the motor units in a given muscle at once, but the strength of the signal we send will determine just how many motor units are recruited: and what size they will be. In accordance with Hahnemann’s Size Principle, we will always recruit the smallest motor units first, which will also be comprised of slow twitch fiber. Only when we need more power do we then add the larger, fast-twitch-containing motor units.

Through repetition, we can strengthen the neural pathways that result in this movement, thereby gaining more efficient use of our muscles and tapping into a greater proportion of our maximum strength. This is how you can get stronger on a given lift without necessarily adding any muscle.

Conclusions

So… what is the lesson, what is the takeaway?

(Don’t mess with Adam when he’s on a breakaway…)

Simple: it’s that there are LOTS of factors to contend with here. Far more than simply mechanical tension and muscle damage. And as usual, this supports the notion of COMBINING training methods, for those interested in “all-round” performance.

For example, if we want to optimize muscle growth, we also need to think about blood supply. That’s why I’ve long been an advocate for pump training AND for running. These activities increase blood supply to the muscle, increasing myonuclei count, thus increasing protein synthesis. This will also ensure that the connective tissue receives adequate blood supply to encourage growth. That may explain why bodybuilders tend to have more connective tissue hypertrophy as compared with powerlifters.

But we then need to combine this with muscle damage and mechanical tension to really kick off that protein synthesis. Combine pump training with more traditional strength training and you might see some really impressive gains.

If we want to increase sarcomeres in parallel, then we need to focus on the concentric portion of the movement. If we want to increase sarcomeres in sequence, we need eccentric training.

Plyometric training alone won’t give you big arms. But use this type of training to increase fast twitch fibers, and you’ll then have more potential for size, and more lactate meaning more anabolic hormone production. Follow that up with bodybuilding or strength training and again, you’ll see compounding results.

More Factors

We also need to think about frequency and how this relates to protein synthesis but also our ability to rehearse the movement.

And there are countless individual differences to take into account. If you aren’t responding to your training, you may find that another method is more effective for you. You might respond well to sarcoplasmic hypertrophy if you have lots of androgen receptors and produce a lot of anabolic hormones, naturally.

Or you might react better to muscle damage. This is why some people find massive gains by switching methods.

Whether that’s because you have a propensity toward slow twitch fibers, or because you have relatively few androgen receptors.

This is such a deep rabbit hole and it’s actually crazy how much we don’t know. But the main takeaway I want to leave you with is that there are LOTS of ways to train muscle and the best results come when you combine all those methods.

Developing strength and size is multifactorial. So your training should be equally multifaceted!

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7 Comments

1. Daniel M says:

   March 29, 2022 at 7:13 pm

   Love love love this type of post Adam! I’ve been thinking about the right sort of training for my goals and landed on doing a little of everything. Slow intentional mobility work most days at home, strength training at the gym with weights every other day, and running around town in my barefoot shoes when I can. You’ve been a huge inspiration in helping me develop my routines and content like this fuels me to keep at it. Keep it up!

   Reply
   • Adam Sinicki says:

     March 31, 2022 at 9:58 am

     Thanks so much Daniel! I really appreciate it: this kind of thing takes a long time and doesn’t get many readers, so it’s really good to know you’ve gotten something from it 🙂 I’ll maybe make this into a video as well, someday!

     Reply
2. James says:

   March 31, 2022 at 11:44 pm

   Really cool article Adam. Fascinating stuff

   Reply
3. marco.picerni says:

   April 3, 2022 at 11:34 am

   It always amazes me how you can make this kind of content super easy to understand! You are a constant source of inspiration, both as an athlete and as a teacher.
   Thanks a lot, Adam! Keep it up!

   Reply
4. Florian says:

   April 8, 2022 at 4:57 pm

   thank you for putting an effort, superb article.

   What’s you thoughts on how much the CNS (central-nervous-system) is playing a role in athletic display? Jay Schroeder has put together an amazing system after his accident. By mind only IsoExtreme (5min pulling a musclegroup) to get out of the hospital bed and later with electronic support (ARP/POV which are advanced ElectroMyoStimulation devices) to push it far beyond anyone thought possible.

   Sadly they are really expensive 30k-40k, but I’m fortunate enough to be using the Neurophone NF3. Amazing device, worth to be checkout

   NF3
   https://www.youtube.com/watch?v=xFXDhpgl6vI

   Schroeder
   https://www.youtube.com/watch?v=idsSZLS8PLQ

   Looking forward to your answer.

   Florian

   Reply
5. Jarrod Macs says:

   March 25, 2023 at 11:58 pm

   Hi Adam! Massive fan of the channel!

   I know this is a post from last year but hopefully I can get a response.

   I’ve been hearing information being put forward on how that muscle damage is actually not correlated to muscle growth at all, and that main contributors to hypertrophy is metabolic stress and mechanical tension. This seems to have become a popular belief with a a lot of people, stating that it’s outdated broscience. They also do provide some pretty convincing studies.

   Have you heard of these comments? What’s your personal thoughts on these claims?

   Thanks ????

   Reply
   • Jarrod Macs says:

     March 26, 2023 at 12:00 am

     Sorry ignore the last couple of question marks haha

     Reply