The Science Behind Muscle Hypertrophy

A deep dive into the process of building muscle.

If you are interested in health and fitness or bodybuilding, you’ve probably come across the term “hypertrophy”.

But do you even know what hypertrophy means?

Something to do with building muscle, right?

I had no idea what it meant when I started coming across the word in internet articles and on YouTube when I researched about building muscle.

It’s just not a word you ever hear outside of fitness circles.

So, what exactly is hypertrophy, how does it work, and what factors impact our ability to maximize muscle growth?

What is Hypertrophy?

In basic terms, hypertrophy is simply an increase in muscle size.

The most effective way to achieve hypertrophy is through resistance training, often by lifting weights in the gym.

Some of the common reasons people have for wanting to build muscle are:

• change their appearance and body composition
• prevent injury
• improve our ability at a sport or activity
• a goal-setting exercise
• give a sense of accomplishment.

How does hypertrophy work?

Two critical components stimulate muscle growth — resistance training and protein synthesis.

These two components work hand in hand. For example, without working out, consuming a high-protein diet is just added calories. And, without the protein, our muscles can’t recover from resistance training.

Protein consumption decreases muscle protein breakdown to repair damaged muscle cells/fibres.

“Muscle hypertrophy occurs when muscle protein synthesis exceeds muscle protein breakdown and results in positive net protein balance in cumulative periods. This could be achieved with both RT and protein ingestion, which stimulates muscle protein synthesis and leads to decreases in muscle protein breakdown.” — Krzysztofik et al., 2019

A consistent intake of high-protein foods or supplements over a sustained period achieves a net positive balance of protein. I won’t get into the specifics here — if you want to know more, I explore protein in great depth in the below article.

The easy part is the protein.

The hard part to get right is resistance training — there’s far more to it than just attending the gym.

There are many training variables that we need to adjust to optimise our results in the gym, such as intensity, rep ranges, rest periods, load, frequency, tempo, and exercise selection.

However, the scientific community still has a lot to learn about how exactly the hypertrophy process works and how it is optimised.

The quest to increase lean body mass is widely pursued by those who lift weights. Research is lacking, however, as to the best approach for maximizing exercise-induced muscle growth. — Schoenfeld, 2010

“Conclusively identifying major hypertrophy stimuli and their sensors is clearly one of the big remaining questions in exercise physiology. However, experimentally this is difficult to achieve, which explains why there is still a large amount of uncertainty despite many studies.” — Wackerhage et al., 2019

Sarcoplasmic & Myofibrillar hypertrophy

Despite people often referring to hypertrophy in singular terms, two forms of hypertrophy affect the growth of muscle fibres in distinct ways.

Our muscles contain sarcoplasmic fluid, which comprises substances like adenosine triphosphate, glycogen, creatine phosphate, and water. This fluid surrounds the myofibrils, and more of this fluid moves to the muscles to provide us with energy during exercise.

Sarcoplasmic hypertrophy is a physical increase in muscle size through a higher volume of sarcoplasm within the muscle fibre. Our muscles appear bigger; however, this hypertrophy doesn’t contribute to strength gains.

“…The hypertrophic response to resistance training may include fiber growth through sarcoplasmic expansion (e.g., intracellular fluid, sarcoplasmic proteins, and glycogen) prior to the accretion of contractile proteins.” — Haun et al., 2019

Sarcoplasmic fluid is everything in a muscle fibre/cell that is not myofibrils. Myofibrils make up myocytes, which are bundles of muscle fibres that help our muscles to contract.

Myofibrillar hypertrophy is increased myofibrils within a muscle cell/fibre. The fibres within a muscle become denser, contributing to muscle growth. More muscle fibres increase the number of contractile units, contributing to strength gains.

“We contend that myofibrillar hypertrophy has a causal impact on gains in muscular strength, but that it is a contributory causal relationship.” — Taber et al., 2019

Stimulating Hypertrophy

To build muscle, there are a few crucial factors that must be present during a workout: progressive overload, mechanical tension, metabolic stress, and muscle damage.

Progressive overload

Progressive overload is the foundation of improvement in the gym. It involves gradually increasing the difficulty or intensity of an exercise over time.

Whether strength, power, endurance, or hypertrophy is your underlying goal, making incremental improvements is the key driver of progress.

Your exercise programme should focus on progressively overloading to stimulate adaptions. Techniques to progressively overload include increasing weight for the same reps, more reps at the same weight, or adding variations.

You can learn more about progressive overload science in the article below.

The current literature also commonly identifies three significant influences of hypertrophy: mechanical tension, metabolic stress, and muscle damage.

“It has been shown that many factors mediate the hypertrophic process and that mechanical tension, muscle damage, and metabolic stress all can play a role in exercise-induced muscle growth.” — Schoenfeld, 2010

“There may be a “sweet spot” whereby a combination of mechanical, metabolic, and damage-related signals interacts synergistically to promote a maximal hypertrophic response.” — Wackerhage et al., 2019

Mechanical tension

Mechanical tension is the force stretching/contracting our muscles. For example, a heavy deadlift provides a lot of mechanical tension on our lats, lower back, hamstrings, and glutes.

Exercises with low repetitions of a heavy load tend to emphasise mechanical tension. Anything over 80% of your one rep max (>80% 1RM) is heavy. For example, if your maximum squat one rep max is 100 kg, a set might be three repetitions of 85 kg.

These exercises producing high mechanical tension are often used as strength builders. These exercises are heavily fatiguing, meaning volume should be low with long rest periods between sets, which isn’t optimal for maximising hypertrophy.

“Effective hypertrophy-oriented training should comprise a combination of mechanical tension and metabolic stress.” — Krzysztofik et al., 2019

Metabolic stress (fatigue)

Metabolic stress is a physiological process during exercise in response to low energy, leading to metabolite accumulation in muscle cells. It occurs when we exhaust our muscle fibres of their available supply of ATP — an energy component in our muscles, and we can no longer complete a proper rep during a set of given exercises.

Similarly to mechanical tension, much of the current hypertrophy literature indicates that metabolic stress (or fatigue) is an essential stimulus for muscle hypertrophy.

In contrast to mechanical tension, moderate loads (60 — 80% 1RM) and repetitions (6–15) tend to elicit the most metabolic stress.

“Current research suggests that maximum gains in muscle hypertrophy are achieved by training regimens that produce significant metabolic stress while maintaining a moderate degree of muscle tension.” — Schoenfeld, 2010

Muscle damage

The lifting community has long assumed that muscle damage significantly influences muscle growth.

During resistance training, our muscles must generate force to overcome the resistance. Structural damage to our muscle cells/fibres then stimulates a response by our body to repair these damaged fibres. These fibres then grow bigger, stronger, and more abundant.

However, as I discussed, the hypertrophic process isn’t quite this simple.

Furthermore, muscle damage may not actually be an essential or even required stimulus for hypertrophy. Findings from a 2017 study indicated that a resistance training approach that promoted muscle damage stimulated comparable hypertrophy to resistance training methods that didn’t create muscle damage.

The eccentric and concentric portions of resistance training

An exercise has two moving parts, called eccentric and concentric. The concentric portion is also known as the positive, and the eccentric portion is the negative.

During the concentric phase of an exercise, the muscle shortens or contracts under load. During the eccentric, it lengthens we release the tension.

A lot of trainees either don’t understand, or just neglect the importance of the eccentric phase of a lift. This often comes from the mentality that we just want to lift as much weight as we can (the concentric) and form/technique is secondary. This approach reduces our ability to build muscle. Previous studies (here and here) have identified that the eccentric phase of an exercise may stimulate more adaption and improved performance than the concentric.

To effectively focus on the eccentric phase of an exercise, we need to slow our tempo right down and decrease the amount of weight we’d usually lift. Sounds counterproductive, right? Build more muscle with less weight…

“The inclusion of eccentric loads not constrained by concentric strength appears to be superior to traditional resistance training in improving variables associated with strength, power and speed performance.” — Douglas et al., 2017

The eccentric phase of movement provides significant muscle damage and mechanical tension. As we’ve just discussed, both are key for stimulating a hypertrophic response.

Furthermore, how much we emphasise an exercise’s eccentric or concentric phase could provide a different muscular architecture adaptation.

The current science suggests that training with the concentric-only phase increases muscles in width eccentric-only contractions increase fascicle and muscle length.

“Designing a resistance training programme is a complex process that incorporates several acute programme variables and key training principles.

The effectiveness of a resistance training programme to achieve a specific training outcome (i.e. muscular endurance, hypertrophy, maximal strength, or power) depends on manipulation of the acute programme variables.” — Bird et al., 2005

Training to maximise hypertrophy

It is crucial to programme your workouts to optimise hypertrophy if your main goal for exercising is building muscle.

Hypertrophy-focused resistance training uses specific techniques to change the size and shape of your muscles. We must provide a sufficient trigger of our muscle proteins to stimulate a repair response in the body, thus increasing muscle fibre/cell size.

This article will touch on a few of the general components of a workout that will influence your ability to build muscle — volume, load, and intensity.

For more depth on programming the individual components of a workout to maximise hypertrophy (e.g., rep ranges and exercise choice), I will discuss these in a follow-up article.

Volume, load, and intensity

There are a few general components of a workout that have a direct impact on its effectiveness for stimulating adaption in terms of muscle growth.

The three things that I will touch on are load, volume, and intensity.

The load is the weight that we use for an exercise. Low loads are associated with high reps, and high loads are associated with low reps (if something is easy, you can do more repetitions).

“Low-load training can exert similar effects on muscle mass and characteristics as high-load training by increasing the number of repetitions, even when not performed to failure.” — Ikezoe et al., 2020

Exercise volume is typically defined as the total number of repetitions and sets we do.

Much of the current research on training volume suggests that it has a dose–response relationship with hypertrophy, meaning our ability to build muscle is increased with higher training volumes.

“The importance of weekly training volume on muscle hypertrophy is provided by the meta-analysis of Schoenfeld et al. From the 15 included studies, a significant effect was reported in muscle size due to in7creased weekly RT volume. — Ralston et al., 2018

However, it is not just how many sets or reps we do or how much weight we lift. A third factor mediates our results in the gym, regardless of volume and load.

It is intensity. Think about it like effort.

For example, for most people weighing 50 kg, it is quite an effort to deadlift 60kg for ten reps. However, in most cases, it would be much easier for a person weighing 100kg to lift that weight for the same number of repetitions.

Regardless of our pursuit, results in the gym come down to putting in the effort — which is how we measure the intensity of an exercise. How hard we push ourselves relative to our maximal ability.

“Both high-load and low-load training to failure can elicit significant increases in muscle hypertrophy among well-trained young men.” — Schoenfeld et al., 2015

We tend to measure the intensity of an exercise set as a percentage of our one-rep maximum (1RM) using the rate of perceived effort (RPE).

The reality is that most people don’t tend to train hard enough to build muscle effectively. The intensity of our workout really does heavily influence our results.

Other factors contributing to maximising muscle growth are rep ranges, tempo, exercise choice, and time under tension.

I will save these topics for a follow-up article discussing programming your workouts to maximise muscle growth. You can subscribe to receive an email when it is available!

Conclusion

In simple terms, hypertrophy is increased muscle size induced through protein consumption and resistance training.

Hypertrophy-focused resistance training challenges our muscles to stimulate an adaptive response from our bodies to repair damaged muscle fibres, making them bigger, stronger, and more abundant.

It becomes harder to induce further muscle growth as a person becomes more advanced in the gym. Therefore, we must be more strategic with our exercise programming.

This article has explored hypertrophy, explaining how the process works and touching on characteristics of a training session that must be present for a person to stimulate hypertrophy effectively.

I hope you found the information in this article helpful. And keep training hard!

Thank you for reading.

If you enjoyed the content, you might be interested in the follow-up articles about programming your workouts to maximise hypertrophy. You can check them out below.

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