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Muscle Fiber Types and Why It’s Important for Muscle Growth

Muscle Fiber Types and Why It’s Important for Muscle Growth

If you’ve read the “Build by Muscle Group” part of my blog, then you know that I often refer to muscle fiber types at some point of each article. This is because muscle fiber types have an influence on building that muscle group and training specific to that fiber may yield better results.

To clear up confusion and allow my readers to be fully educated on why it’s important, I’ll cover the different muscle fiber types, their functions, and how they relate to muscle growth!


Different Muscle Fiber Types

At the most basic level, muscle fiber types can be classified as Type I and Type II. When fully broken down, there is actually a continuum of the muscle fibers further broken down to better categorize their differences.

There are 7 different types which include (in chronological order): Type IIx, Type IIax, Type IIa, Type IIac, Type IIc, Type Ic, and Type I

Within this article, we will focus on primarily Type IIx, Type IIa, and Type I which are most relevant.


Type II Muscle Fibers

Type II muscle fibers are classified as fast twitch fibers. They contract fast and also relax fast. As the least oxidative, this essentially means that they are easily fatigued. They also have less capillaries surrounding them.

This allows type II fibers to have rapid force development with more anaerobic power. Considering these factors, Type II fibers have a higher recruitment threshold and in theory are recruited as the last pool of muscle fibers.


Type I Muscle Fibers

Type I muscle fibers are basically the exact opposite. They contract slower and relax slower. As the most oxidative, this essentially means that they more fatigue resistant. They also have more capillaries surrounding them.

This allows Type I muscle fibers to sustain work for an extended period of time. For example, postural muscles are primarily Type I dominant because they are constantly firing to keep the body in good posture.

In theory, Type I muscle fibers will be first to be called upon and have a low recruitment threshold.


Muscle Fiber Type Transitions

Remember the continuum mentioned earlier of the 7 different muscle fibers? Well, muscle fibers can actually transition from one type to another throughout this continuum.

The change form Type IIx to Type IIa (technically Type IIx to Type IIax to Type IIa) is commonly seen and usually happens within the first few weeks of weight training and or aerobic endurance training. Then, a smaller percentage of Type IIa change to Type IIc (technically from Type IIa to Type IIac to Type IIc).

In a study1 done on muscle fiber type transitions, Type IIx fibers decreased from approximately 18% to 7% of total muscle fibers in both men and women after eight weeks of training.

The men and women did lower body exercises (leg press, knee extension exercises etc.) two times a week for eight weeks using a 6-12RM load and 2-minute rest periods. Within just two weeks (four workouts), women saw a significant decrease in Type IIx muscle fibers. In men, it took about four weeks (eight workouts).

This is just one study and certain variables may be left out, but science consistently shows this trend!

Interestingly, detraining (cessation of all training) has the opposite effect meaning Type IIa converts back to Type IIx.

When we look at Type I muscle fibers converting to Type II muscle fibers, it seems less likely. There isn’t enough science to prove or disprove this, but it’s doubtful!



Does Muscle Fiber Type Matter for Muscle Growth?

A bodybuilder asking the question "does muscle fiber type matter for muscle growth?"

Yes, considering you can take this new knowledge and work with it, there are three influential reasons muscle fiber types can make a difference in muscle growth.


Reason 1 – Genetic Influence

Type II muscle fibers have a greater growth potential than Type I muscle fibers. Everyone has a slightly different muscle fiber type distribution which will be a factor in your overall genetic potential for muscle growth!


Reason 2 – Training Specific to Muscle Fiber Distribution

All muscles have a different muscle fiber distribution. Typically, muscles of the lower body are more Type I dominant and muscles of the upper body are more Type II dominant with all postural muscles mostly Type I dominant.

Training in a specific rep range may cause better growth for certain muscle fiber types. For example, when training the gastrocnemius muscle (the calf muscle), it is commonly known that higher reps works well for growing the gastrocnemius.

The average muscle fiber distribution of the gastrocnemius muscle is about 60% which is the evidence that supports this proposition. A muscle group with a higher Type II muscle fiber distribution would be thought to respond better to low to mid rep ranges.

Although science doesn’t prove this, it also doesn’t disprove it. “Broscience” shows it to be possibly effective though and something that is recommended to practice and keep in mind.


Reason 3 – The Size Principle

This is good to understand when taking an approach to muscle building. This principle states that smaller motor units will be recruited first before larger motor units. Essentially this means slow-twitch (Type I) muscle fibers will be recruited before the fast-twitch (Type II) muscle fibers. So essentially, using heavy loads will activate the strong Type II muscle fibers. This is important to know because Type II muscle fibers have the greatest growth potential!


Final Thoughts

Is there a whole lot that you can do to specifically target a muscle fiber type for accelerated growth? Not really. The best thing you can get from this article is the understanding of how the muscle fibers work. This may translate into better programming for your workout depending on what you do with this information.

It is important to know that the Type II muscle fibers, which have the most growth potential, are recruited last when there is lots of tension on the muscle; that threshold must be met.

It also may be helpful to keep in mind that training based on the muscle fiber distribution of a muscle group may make a difference and shouldn’t be totally discounted. Training in all rep ranges is still always recommended for maximum muscle growth as well!

Lift smart and lift hard my friends!!



Haff, Gregory and Triplett, Travis. Essentials of Strength Training and Conditioning. National Strength and Conditioning Association, 2016. Print.

  1. Staron, RS,  Karapondo, DL, Kraemer, WJ, Fry, AC, Gordon, SE, Falkel, JE, Hagerman, FC, and Hikida, RS. Skeletal muscle adaptations during the early phases of heavy-resistance training in men and women. J Appl Physiol 76:1247-1255, 1994.

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