The Definitive (and Practical) Guide to Muscle Hypertrophy


If you want to know what muscle hypertrophy is, how it works, and how to stimulate it best, then you want to read this article.


You’ve probably heard a lot of things about muscle hypertrophy (the technical term for muscle growth).

  • You’ve probably heard that there are different types of muscle fibers.

And you’ve heard that these fibers are made for different types of activities and respond differently to training.

  • You’ve probably heard that there are two distinctive types of muscle growth.

One is “sarcoplasmic” growth and the other is “myofibrillar” growth.

Many people claim that you can program your training to emphasize one more than the other (and that you should, according to your goals).

  • You’ve probably heard that how you train your muscles determines how they grow.

If you want bigger muscles, you’re told, you want to maximize sarcoplasmic hypertrophy, and you do that by training primarily in higher rep ranges.

And if you want stronger muscles, you want to maximize myofibrillar hypertrophy, which means emphasizing lower rep ranges.

And to make matters even worse,  you’ve probably also heard that much of this is pseudoscientific nonsense.

You’re only a Google search away from a number of evidence-based arguments that you can’t selectively train for one type of muscle growth and even that the growth classifications themselves have no basis in good science.

That is, some people claim that muscles can only grow in one fundamental way and that all rep ranges and weight loads are equally effective at stimulating it.

So if you’ve had trouble connecting all the dots and incorporating your newfound knowledge into your training…I understand.

Well, in this article, I want to give you a simple, science-based crash course in muscle hypertrophy and show what we know, what we don’t, and what you can do to build muscle and strength faster than ever before.

By the end of this article, you won’t have the “final word” on the physiology of muscle growth, but you’ll know enough about how and why muscles grow to build the type of physique you really want.

So let’s get to it.

The Two Types of Muscle Fibers

muscle hypertrophy reps

Let’s start this review with what we know for certain.

Muscle tissue is a complex structure, with bundles of long strands of muscle cells (known as fibers) sheathed in a thick band of connective tissue known as a perimysium.

Here’s how it looks:

muscle hypertrophy physiology

There are two types of muscle fibers:

  • Type I

Type I muscle fibers, also known as “slow-twitch” muscle fibers, are dense with capillaries and rich in mitochondria and myoglobin, which makes them very resistant to fatigue.

They also, however, have the least potential for growth and power output.

  • Type II

Type II fibers, also known as “fast-twitch” muscle fibers, grow faster than type I fibers and contract faster as well, which gives them a higher strength and power potential.

The “downside,” however, is they fatigue much faster, making them less suited to endurance activities.

Now, various muscles in your body have varying amounts of Type I and Type II fibers, and there’s an ongoing debate about what determines this distribution.

The question is why do some people’s bodies have more Type I muscle fibers than Type II, and vice versa?

Well, genetics are the most likely culprit because studies suggest it’s unlikely that we can convert one type of muscle fiber into another through training.

That is, there’s no significant difference in the proportions of type I and II muscle fibers between untrained and resistance trained individuals.

That said, there is evidence that weightlifting can change the physiology of type II muscle fibers, causing it to shift from what’s know as type x to type a fiber.

There’s also evidence that muscle fiber types are affected differently by different types of resistance training:

High-rep, low-load training appears to preferentially stimulate type I muscle fiber and low-rep, high-load type II.

This would help explain why research shows that emphasizing heavy weights (80%+ of 1RM) in your training is superior to lighter weights for muscle building purposes.

For example, one well-designed study published earlier this year separated 33 physically active, resistance-trained men into two groups:

  1. A high-volume, moderate-intensity group that did 4 workouts per week consisting of 4 sets per exercise in the 10 to 12 rep range (70% of 1RM).
  2. A moderate-volume, high-intensity group that did 4 workouts per week consisting of 4 sets per exercise in the 3 to 5 rep range (90% of 1RM).

Both groups did the same exercises (which included the bench press, back squat, deadlift, and seated shoulder press), and both were instructed to maintain their normal eating habits (which was monitored with food diaries).

And the result?

After 8 weeks of training, scientists found that the high-intensity group gained significantly more muscle and strength than the high-volume group.

It’s no surprise that the high-intensity group gained more strength, but many people wouldn’t have expected them to gain more muscle as well.

Researchers cite two main reasons for why the heavier training beat out the lighter:

1. Higher amounts of mechanical stress imposed on the muscles.

The high-volume training, on the other hand, caused higher amounts of metabolic stress.

2. Greater activation of muscle fibers.

And this, in turn, results in a greater adaptation across a larger percentage of the muscle tissue.

Use this workout and flexible dieting program to lose up to 10 pounds of fat and build muscle in just 30 days…without starving yourself or living in the gym.

How to Best Stimulate Muscle Hypertrophy

muscle hypertrophy training

There are three primary ways to stimulate muscle growth:

  • Progressive tension overload

 Progressive tension overload refers to increasing tension levels in the muscle fibers over time.

The most effective way to do this is to progressively increase the amount of weight you’re lifting over time.

  • Muscle damage

Muscle damage refers to just that—actual damage caused to the muscle fibers by high levels of tension.

This damage necessitates repair, and if the body is provided with proper nutrition and rest, it will adapt the muscle fibers to better deal with future stimuli.

  • Cellular fatigue

Cellular fatigue refers to pushing muscle fibers to their metabolic limits through the repetition of actions to muscular failure.

You can think of these factors as separate growth “pathways,” and they can be emphasized differently in your training.

For example, low-volume, high-weight training emphasizes progressive overload and muscle damage, and high-rep, “pump” training emphasizes cellular fatigue.

Now, out of the three pathways, progressive tension overload is the most important, and is the one you have to emphasize most if you want to build large amounts of muscle as quickly as possible.

This takes us back to a point I made in the previous section of this article:

Your primary goal as a natural weightlifter is to get stronger, and especially on key compound exercises like the squat, deadlift, and bench and overhead press.

Here’s a simple rule of thumb:

The more weight you can push, pull, and squat, the more muscular you’re going to be.

I learned this lesson several years ago.

For my first seven years of weightlifting, I more or less trained exclusively in the 10 to 12 rep range (70 to 75% of one-rep max) and did a lot of traditional (isolation) “bodybuilding” exercises.

I also spent about 10 hours per week in the gym.

Here’s what it got me:

muscle hypertrophy training

As you can imagine, I wasn’t thrilled.

I didn’t look horrible but that’s not exactly what you’d expect for seven years of consistent weightlifting.

Soon after that picture was taken, I dramatically changed my training regimen.

I started focusing on the 4 to 6 rep range (80 to 85% of 1RM) and centered my training around progressing on the big, compound movements.

Here’s a shot of me about 2.5 years later:

muscle hypertrophy training program

I learned how to diet as well, but as you can see, every aspect of my physique markedly improved.

My strength went through the roof, too. In those 2.5 years, I added close to 100 pounds to my bench and overhead presses and doubled my squat and deadlift.

And better still, I did it in just 4 to 6 hours in the gym each week.

Now, I was baffled by my results because I had thought, like most people, that heavy weightlifting is good for building strength but not muscle.

Well, it turns out that’s one of the worst weightlifting myths out there.

And that’s why my Bigger Leaner Stronger (men) and Thinner Leaner Stronger (women) programs focus on heavy, compound weightlifting.

The Two Types of Muscle Hypertrophy

muscle hypertrophy causes

A discussion of muscle hypertrophy isn’t complete without addressing the two ways for muscles to grow:

  • Myofibrillar hypertrophy

Myo means “muscle” and a fibril is a threadlike cellular structure.

Myofibrillar hypertrophy, then, refers to an actual increase in size and number of the individual muscle fibers, which are made up of proteins that can contract.

  • Sarcoplasmic hypertrophy

Sarco means “flesh” and plasmic refers to plasma, which is a gel-like material in a cell containing various important particles for life.

Sarcoplasmic hypertrophy would be an increase in the volume of the fluid, non-contractile components of the muscle (glycogen, collagen, water, minerals, etc.).

Here’s a simple visual of how it works:

muscle hypertrophy causes

Now, I mentioned earlier in this article that this is a “controversial” aspect of muscle building.

While it’s obvious that the sarcoplasmic elements of muscle cells (necessarily) expand as myofibrillar growth occurs, and it’s equally obvious that you can temporarily increase sarcoplasmic volume by loading creatine or carbs or damaging muscle tissue, the unanswered question is twofold:

  • Can the sarcoplasm expand at a rate significantly faster than the myofibrils expand?
  • And can this expansion result in long-lasting increases in muscle size?

That is, can sarcoplasmic hypertrophy contribute significantly to the overall long-term increase in muscle size, or is it more a “byproduct” of myofibrillar hypertrophy?

Well, this is where things get muddy and complex.

Some people like to point to the differences in muscle size and strength between bodybuilders and strength athletes as evidence of sarcoplasmic hypertrophy.

“How can a 170-pound powerlifter out-squat a 250-pound bodybuilder?” they ask.

Sarcoplasmic hypertrophy would seem to answer this question. The bodybuilder, in his pursuit of building the biggest muscles possible, has (apparently) accumulated more non-contractile proteins than the powerlifter.

The problem with this is it relies on an unproven theory (sarcoplasmic hypertrophy) over a more plausible one:

Strength athletes squat, deadlift, and press far more frequently than bodybuilders, and the more you do something, the better you get at it.

This is why you can find many examples of bodybuilders that switch to powerlifting and rapidly increase their strength.

That said, just because sarcoplasmic hypertrophy isn’t the likely reason for “big but (relatively) weak” bodybuilders doesn’t mean it isn’t real.

The good news, though, you don’t need a conclusive answer on sarcoplasmic hypertrophy to effectively program workouts and build muscle.

So long as you train with heavy loads and a moderate volume, and you will have no trouble building muscle and strength.


The Bottom Line on Muscle Hypertrophy

muscle growth

Muscle hypertrophy is an extremely complex process that involves scores of physiological functions and adaptations.

You can spend hundreds of hours studying it and barely scratch the surface.

Fortunately, however, actually building muscle isn’t nearly as complicated. You don’t need to be a scientist to build a fantastic physique.

You just need to have a good basic understanding of muscle hypertrophy and of how to properly create workout and diet plans, and then you just need to put in the work and stay patient.


What’s your take on muscle hypertrophy? Have anything else to share? Let me know in the comments below!


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