doing pullups with a chain to build muscle

Fast Twitch vs. Slow Twitch Muscle Fibers: A Complete Guide

One of the first things that most people come across during their fitness research journey is the two different types of muscle fibers.

Probably not a stretch to assume you've heard the terms, "slow twitch" and "fast twitch."

But do you know exactly what these muscle fibers are, what they do, and how important they are?

Let's take a deep dive into these muscle fibers. We'll cover exactly what you can do to strengthen both types of fibers and how this will translate to better results on armed forces entrance tests.


What is Muscle Tissue?

Let's start from the outside and go deeper into exploring your muscles.

Just underneath the skin, you'll find your muscle tissue. But there's more than just the muscle tissue that you associate with hitting the gym and feeling sore the next day.

You have three types of muscle tissue:


Smooth Muscle

These involuntary muscles are located within your internal organs and eyes. As an involuntary muscle, it cannot be consciously controlled by you. For example, smooth muscle is responsible for digestion, a process that occurs regardless of what you do.


Cardiac Muscle

Like smooth muscle, cardiac muscle is also involuntary. By the name, you shouldn’t be surprised to learn that your heart is where you’ll find cardiac muscle. Specifically found within the walls of your heart, this muscle ensures your ticker keeps ticking.


Skeletal Muscle

Now we get to the muscle type that you’re most familiar with. Skeletal muscle is latched on to your bones thanks to tendons, which is a type of connective tissue. Unlike the other two types of muscle tissue, skeletal muscle can be directly controlled by you. Whenever you move, you are controlling your skeletal muscle tissue.

For the purpose of this article, we’ll only be talking about skeletal muscle and the muscle fibers found within.


What are Muscle Fibers?

Muscle fibers are the muscle cells that make up your muscle tissue. There are two main types of muscle fibers: slow-twitch (Type I) and fast-twitch (Type II).


Type I: Slow-Twitch Muscle Fibers

Type I muscle fibers are slow to contract but can sustain long periods of contraction.

They're also known as red muscle fibers because they have a characteristic reddish color. This is due to the elevated numbers of mitochondria and myoglobin in the muscle fibers.

Slow-twitch muscle fibers are triggered during activities that require endurance, such as long-distance running.


Type II: Fast-Twitch Muscle Fibers

Type II muscle fibers are the opposite of Type I muscle fibers. They contract quickly but they tire out quickly too.

There are two subtypes of fast-twitch muscle fibers:

  • Type 2A: These fibers are perfect for short burst activities that demand immediate but not extended periods of strength and power. But as mentioned above, they burn out pretty quickly.
  • Type 2B (Type 2X): Type 2B, also called Type 2X fibers, can be used for the same type of activites as 2A but these fibers provide a higher degree of strength and power. The drawback is that Type 2B fiber are incredibly inefficient, burning out faster than Type 2A. Interestingly enough, Type 2B fibers will convert to Type 2A fibers when you work out consistently. But the moment you lead a sedentary lifestyle, they will revert back to Type 2B again.


How Many Type I vs Type II Muscle Fibers Do You Have?

Your muscles are a mix of both type I and type II muscle fibers. The muscle fibers you have is determined by your genetics.

You may be born with a higher percentage of one type of muscle fiber than another, which can affect your athletic ability.

Is that to say that if you’re born with more Type I fibers, you’ll never be able to do Type II fiber activities?

Not at all!

Regardless of which muscle fiber type you have more of, you can still train your body to achieve impressive results in any activity or workout.


Matching the Muscle Fiber to the Activity

Depending on which type of muscle fiber ratio you're born with, you might be better suited for certain athletic endeavors over others.

Again, that doesn’t mean you should give up on an activity because you don’t have as many muscle fibers in that category.

If you're born with more fast-twitch muscle fibers, you may be better suited for short bursts of speed and power.


  • Sprints
  • Bodybuilding
  • Powerlifting
  • Olympic Lifting

If you're born with more slow-twitch muscle fibers, you may be better suited for endurance activities that require sustained muscle contractions.



How Do I Know Which Muscle Fibers I Have More Of?

The big question: How do you know which muscle fibers you have more of in your body?

There are a few ways to determine which muscle fibers you have more of.


Muscle Biopsy

One way is to look at your muscle fiber type distribution. This can be done through a muscle biopsy, which involves taking a small sample of muscle tissue to be analyzed in a laboratory.

Admittedly, it’s not very pleasant and you’ll have a scar, but it’s the most accurate way to do it. This is the gold standard of muscle fiber testing.


Electrical Stimulation

Another way to estimate your muscle fiber type is to look at your muscle fiber type profile. This can be done with a muscle fiber type test, which uses electrical stimulation to determine the makeup of your muscle fibers.


Muscle Appearance

The easiest way to estimate muscle fiber type is to look at your muscle size and shape.

This method is not as accurate as the other two methods, but it can give you a general idea of which muscle fibers you have more of.

If you have long, thin muscles, you may have a higher percentage of type I muscle fibers.

If you have short, thick muscles, you may have a higher percentage of type II muscle fibers.


How to Train Your Muscle Fibers

Now that you know the difference between the two types of muscle fibers, let's discuss how to train each type.


Type I Muscle Fiber Training

To increase the capacity of your slow-twitch muscle fibers, you’re going to be better suited for endurance and performance-based training.

This type of training involves sustained muscle contractions for long periods of time as well as specific skill-focused exercises.

Endurance training can be done with either aerobic or anaerobic activities.

Aerobic activities are any activity that uses oxygen to generate energy, such as running, cycling, and swimming.

Anaerobic activities are any activity that doesn't use oxygen to generate energy, such as weightlifting and sprinting.

To see the greatest results, you should mix both types of activities into your training regimen.

Our Elite Operator V2 training program is the ideal workout for anyone who is Type I dominant.


Type II Muscle Fiber Training

To increase the size and strength of your fast-twitch muscle fibers, you need focus on resistance and power-based training.

This type of training involves short bursts of muscle contractions.

Power training can be done with either explosive or plyometric exercises .

Explosive exercises are any exercise that uses quick, powerful movements that emphasize speed, not necessarily the amount of weight you can move. Wall balls are a great example.

Plyometric exercises involve high impact, jumping exercises. Box jumps and jump squats are two examples.

To see the greatest results, you should mix both types of activities into your training regimen.

If you want to see fast results in your size AND strength, try our Tactical Monster program, which focuses on building functional muscle mass.