How Do Muscles Grow? Toned Versus Bulky, and High Reps vs. Low Reps

There are myths circulating about how to look “toned”, which is the most common body type most people strive for. They see the functional, sleek build of a good swimmer or fighter and wish they were there. They try to avoid the “bulky” look of professional powerlifters and bodybuilders, and thus, get scared of lifting heavy.

On the other side of the coin, you have guys who want to get bigger, stronger, and leaner. They hear that high reps do absolutely nothing for strength or size, and that heavy lifting is the way to go, period. They look down upon everyone using bodyweight or smaller weights because, they feel, they will never see results with such wimpy exercises.

I want to examine both of these ideologies with some practical science. Today, you’ll learn the way muscle cell recruitment works, and how understanding the function debunks both myths.

How Do Muscles Grow?

Here’s the question a trainee asked me:

How do the muscles grow under a high intensity workout? I thought that high intensity/high rep meant you get stronger and that higher weight was how you got bigger.”

In any strength movement, you lift a weight. It doesn’t matter if the weight is iron or yourself: a weight is weight.

Moving any weight requires our muscles to twitch. The heavier something is, the faster our muscles must twitch.

Evolution has given us a cool trait. We actually have 3 types of muscle fibers. Some are better at twitching slowly, but continuously. These have more “endurance” for lower weight. Other fibers twitch really fast. They have more “strength” for higher weight, but get tired quicker.

There’s 3 fibers to know about: Type I, Type IIA, and Type IIB.

When you lift heavy, the “lower threshold fibers” are recruited first. These are the slower-twitch (and thus, “weaker”) Type I fibers. If the weight is not lifted, then the next set of fibers are recruited (Type IIA). If it still hasn’t been lifted, then the next set are recruited (Type IIB).

Type IIA and IIB fibers act at a higher threshold than the Type I – meaning they fire AFTER Type I cannot handle a weight alone. Now, also recognize that Type I fibers, while lifting weight, can fatigue further as Type II fibers are twitching. Which means Type II fibers are recruited as compensation of Type I failure.

We assumed that the weight had been lifted at this point. But wait! What happens if all your fibers are recruited, but the weight has not been lifted? If you thought, “Well, you fail the lift because it’s too heavy!”, then you’re wrong!

You don’t simply “fail” at this point. Instead, muscle realizes it must increase the amount of force produced. It begins to twitch even faster to develop more force. THIS happens until “maximum rate coding” is hit, where you’ve either lifted the weight, or you’ve entirely failed.

Ever wonder why you shake when under those final, difficult reps? That would be your muscles twitching with all their might to adapt.

So What’s Best: High Reps/Low Weight Or Low Reps/High Weight?

Now you know how muscle works. It progressively uses faster twitching fibers until it successfully moves something. If it does not move something, it’ll start firing even faster to do so. If even that fails, it fails to lift a weight.

So how does this relate to getting stronger or better looking?

Above is physiological evidence that training to concentric failure – or at least very close to it – is good for maximally activating muscle. And by training near concentric failure – forcing near-max recruitment of all muscle, until the point of exhaustion – you force a muscular adaptation and the signal to repair and grow new muscle under imposed demands.

So back to high reps and lower weight, versus lower reps and high weight. The truth is that you can recruit all your fibers by either making the load heavy enough to prompt recruitment (low reps, high weight), or you can fatigue your lower end fibers to force Type II fibers to work (high reps, low weight). As long as you hit concentric failure, your body will want to grow more muscle (signaled by the fatigue) and increase its strength (due to both new muscle growth and better access to “maximum rate coding” per unit of muscle, aka strength by better/more neuronal firing).

Whichever works best for you will be personal preference. Some find the intensity of heavy weights scary. Others find that they don’t possess the grit to push through shaky near-failure sets of high-rep bodyweight exercises. The choice is up to you.

Also consider that, physiologically speaking, high reps with lower weight will more easily exhaust Type I fibers, making it more likely to result in muscular endurance, but the psychological intensity of near-failure reps may prevent newer trainees from adequately exhausting Type II fibers. Low reps with higher weight will more quickly exhaust Type II fibers, making it more likely to result in strength gains, to the possible detriment of your Type I fibers (they still fire, but are not as easily exhausted as they would be in high rep protocols as they are underutilized by unit of time).

Any ideal training program will include aspects of both heavy lifting and high rep lifting to cover all bases and optimize growth. But there’s no harm in choosing one over the other for the bulk of a program.

Now that you know how muscle actually grows, look at your program. Are you hitting concentric muscle failure, or getting really close to it? If not (and your goal is to get both stronger and more muscular/lean), you might not be putting in enough intensity to grow. Every workout, you should approach concentric failure to efficiently exhaust your muscle fibers. This ensures the response from your body to adapt and grow stronger.

So now that you know the truth, how will your program change (if at all)? Let me know in the comments below. And if you want to be the cool kid on the block, please spread the message to destroy these strange workout myths.


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By Aaron Roam

Aaron Roam is a fitness advocate from Berkeley, California.

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