Do Some Muscles Grow Faster With Lighter Weights and Higher Reps?

Some say that you should train a muscle based on its predominant fiber type.

Muscles that are slow-twitch dominant, for example, should be hit with light weights and high reps. Muscles that contain mainly fast-twitch fibers, on the other hand, respond best to heavy weights and low reps.

The first time I came across the idea was in a book called:

“The Poliquin Principles: Successful Methods for Strength and Mass Development”

Here’s a snippet:

“A muscle with a high percentage of slow-twitch fibers responds best to higher reps. The soleus (one of your calf muscles) contains predominantly slow-twitch fibers; therefore, repetitions in the 15-25 range may be needed to give sufficient time under tension for these fibers to hypertrophy. In contrast, the gastrocnemius (the other calf muscle) contains predominantly fast-twitch fibers and responds best to lower reps.”

At the time, this was completely new to me. It seemed to make sense, so I immediately changed what I was doing in the gym.

I started doing seated calf raises, which primarily work the soleus, with higher reps and lighter weights. Standing calf raises, which bring the gastrocnemius into play, were done with heavy weights and lower reps.

The same thing is supposed to apply to all the other muscles in your body.

It’s been said, for example, that your triceps should be trained with heavy weights because they contain mainly fast-twitch fibers. Your delts, on the other hand, are supposed to be trained with high reps because they’re slow-twitch dominant.

However, I’ve never seen any research to put the idea to the test. It’s always been one of those things that sounds good in practice, but there’s no evidence to show whether or not it actually works.

That all changed last month, with the publication of a new study in the journal Physiological Reports.

The researchers took a group of 26 subjects, and got them to train both gastrocnemius and soleus, using either a heavy weight and low (6-10) reps, or a light weight and high (20-30) reps.

If the “train right for your fiber type” theory was true, you’d expect soleus (a predominantly slow‐twitch muscle) to grow faster when it was trained with higher reps.

On the flip side, gastrocnemius (a muscle with a mixed composition of both major fiber types) should respond better to heavier weights and lower reps.

But that isn’t what happened.

After eight weeks, muscle growth was similar in both soleus and gastrocnemius, regardless of the amount of weight used. There was no benefit to training a muscle based on its fiber type.

To quote the researchers directly:

“Collectively, the emerging evidence indicates that hypertrophy can be achieved across a wide spectrum of loading ranges, and it appears that this paradigm holds true for the major muscles of the body regardless of their fiber type composition.”

Bottom line?

There’s very little evidence to show that you need to worry about fiber types when you’re planning your training. As long as you work hard and push yourself, heavy weights, medium weights and light weights can all be used successfully to build muscle, irrespective of the fiber composition of that muscle.