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August 11, 2021 9 min read

Becoming the best version of yourself and crushing your fitness goals takes more than just going to the gym every day. If you want to get better, you must  understand your body. Part of understanding your body is getting to know your muscle fibers.

However, did you know that you have two different types of muscle fibers?

Not only that, but the two types of muscle fibers play different roles in your fitness performance? Those two types are called slow-twitch and fast-twitch.

To help you better understand your muscle fibers, we will break down the differences between the types and provide training tips.

By the end of this article, you will understand your muscle fibers and how you can better use them to develop maximum muscle mass!

What Are Muscle Fibers?

Muscle fibers are groups of muscle cells.

Also known as myocytes, there are three types of muscle cells including:

  1. Cardiac
  2. Skeletal
  3. Smooth

THREE TYPES OF MUSCLE TISSUE. Anatomy of muscular system

We are focusing on skeletal muscle cells because they form skeletal muscle fibers.  Skeletal muscle fibers are the type of fibers that initiate voluntary muscle contractions. Voluntary contractions are those that you consciously choose to make. Examples of voluntary contractions are exercise movements, such as running and lifting weights. The somatic nervous system controls voluntary contractions.

When grouped together, skeletal muscle fibers form muscles.

Moreover, muscles are groups of muscle fibers which are groups of muscle cells. The fibers connect to bones and tendons to form networks rooted in specific body parts. When the networks contract, they initiate a copious amount of different body movements.

Not all skeletal muscle fibers are the same, even though they are all made from the same kind of cells.

In fact, there are two different types:

  1. Slow-twitch muscle fibers (type I muscle fibers, red fibers)
  2. Fast-twitch muscle fibers (type II muscle fibers, white fibers)

Additionally, fast-twitch muscle fibers further break down into type iia and type iib (also known as type iix). While they are both fast-twitch fibers, they have enough differences to be in their own groups.  However, for the sake of time, we will group type iia and iib together in this article as they share a majority of the most important traits!

The two types of muscle fibers have different implications for your fitness. Therefore, if you are a regular gym-goer, it is essential to understand the difference between them.  That way, you can maximize your gym efforts!

Key Slow-Twitch and Fast-Twitch Muscle Fiber Differences

While they are both made up of skeletal muscle cells, the two skeletal muscle fiber types are vastly different. As we mentioned above, they have their own unique implications for your fitness. Understanding those differences can help you become a better lifter.

The primary differences between slow and fast-twitch muscle fibers have to do with:

  1. Their structure and environment
  2. How much oxygen they contain
  3. How long they can contract for
  4. The amount of force they produce
  5. The physical activities they activate for

If you want to reach your fitness goals faster, read on to understand more about these muscle fibers!

Structural Differences

The first key difference between slow and fast-twitch fibers is that they have structural differences.

The most evident include differences in:

  1. Mitochondria count
  2. Myoglobin count
  3. Blood vessels in their environment
  4. Myosin count

First off, mitochondria create the energy needed for your muscle fibers to contract, regenerate, and grow.

They combine sugar and oxygen and convert them into energy that your muscles can use. After the mitochondria create energy, they store it in adenosine triphosphate (ATP). Slow-twitch fibers have high levels of mitochondria. On the other hand, fast-twitch fibers have low levels of mitochondria. Therefore, slow-twitch fibers have more energy available to them.

Slow-twitch fibers also have more myoglobin.

Myoglobin is similar to hemoglobin, such that they are a red protein that transports oxygen in the blood. Unlike hemoglobin, myoglobin is only present in muscle cells.

Myoglobin transports oxygen to your mitochondria so that they can create energy for your muscles. 

Moreover, your muscle fibers need oxygen from the myoglobin so that the mitochondria can combine it with glucose for energy. Third, slow-twitch fibers have more blood vessels surrounding them. Specifically, they have more capillaries surrounding them. Capillaries are tiny blood vessels that carry oxygen-rich blood directly from the heart to the slow-twitch fibers.

Having many nearby capillaries further ensures that the mitochondria will have enough oxygen to create energy. Lastly, myosin is a type of motor protein that helps produce force in muscle contractions.  It is more abundant in fast-twitch fibers than it is in slow-twitch.  Thanks to having a lot of this protein, fast-twitch muscle fibers can generate strong forces. 

Because they don't have much of it, slow twitch-fibers can't generate those same strong forces.

In summary, slow-twitch fibers have higher levels of mitochondria, myoglobin, and surrounding blood vessels. These structural differences give slow-twitch muscle fibers an energy advantage when compared to fast-twitch fibers. 

While slow-twitch fibers have an energy advantage, they lag in being able to generate strong forces.  Fast-twitch muscle fibers are able to generate more forceful contractions thanks to being abundant in myosin.

Oxidative Capacity & Metabolic Differences

As mentioned above, slow-twitch muscle fibers have a higher concentration of mitochondria, myoglobin, and surrounding capillaries. Because of this, they have more readily available oxygen and the means to convert it into energy. We refer to this capability as having a high oxidative capacity. Therefore, slow-twitch fibers have a higher oxidative capacity than fast-twitch fibers.

Oxygen-rich muscle cells that convert oxygen into energy are aerobic metabolic cells. 

Therefore, slow-twitch muscle fibers are aerobic.  Aerobic respiration is an efficient way to generate energy sources.  It does not take a lot of work and takes little time to do.

On the other hand, oxygen-depleted muscle cells cannot do aerobic respiration because they have little oxygen.  Cells that do not use oxygen for energy are anaerobic.  Therefore, fast-twitch muscle fibers are metabolically anaerobic. While fast-twitch fibers do not have much oxygen, they still need to get their energy from somewhere. 

Instead of aerobically using oxygen, anaerobic fast-twitch fibers use glucose alone to create lactic acid for energy. 

Fast-twitch fibers take glucose and convert it into lactic acid without the help of oxygen.  This process is not as efficient as the aerobic process in slow-twitch muscle fibers.

Have you ever had a personal trainer tell you to roll out your muscles after an insane lifting session?  Part of the reason they instruct you to do that is to disperse the lactic acid built up in your muscles.  The next time someone tells you to roll out, you can thank your fast-twitch muscles cells and the anaerobic process!

In summary, slow-twitch muscle fibers have a high oxidative capacity and perform aerobic respiration thanks to having more capillaries, mitochondria, and myoglobin.

Fast-twitch muscle fibers have a low oxidative capacity and use anaerobic respiration thanks to having little available oxygen. 

Most importantly to remember, aerobic respiration is far more efficient energy-wise than anaerobic respiration. However, what impact does this have on your fitness abilities?

Muscle Fiber Impact On Fitness Performance

The two types of skeletal muscle fibers have different impacts on your fitness.  Thanks to them having distinguished structures and oxidative capacities, they make your muscles function very differently. 

They impact fitness and your muscles in three key areas, including:

  1. The amount of endurance they have
  2. The amount of force they can produce
  3. Which type of activities they are best for

Moreover, the two types of fibers are different in these three areas of your fitness. Here is more on each of them:


Endurance is how long you can sustain a muscle contraction for without having to take a break. Muscle fibers that can contract for long periods of time have a lot of endurance.  On the other hand, muscle fibers that contract for only short periods of time have little endurance.

Slow-twitch muscle fibers have more endurance than fast-twitch fibers, thanks to their aerobic metabolism.

The aerobic metabolism is more efficient with energy than the anaerobic metabolism.  Therefore, they can stay activated for sustained periods without having to stop and create more energy sources. 

As you exercise, your slow-twitch fibers are constantly replenishing themselves with more energy. Fast-twitch fibers do not have as much endurance thanks to their anaerobic metabolism.  When you exercise, your fast-twitch fibers can't stay contracted for extended periods of time. 

The reason is that they need to stop to replenish themselves with energy through the anaerobic process of creating energy. 

Unfortunately, they cannot continue to produce energy while your muscles contract. Therefore, you need to break the contraction for them to convert more energy sources.  In simpler terms, you need to stop exercising to replenish your muscle fibers with energy.

Muscle Force

Muscle force is how much pressure your muscles can apply to an object in a period of time. The more pressure your muscles can apply in a shorter amount of time, the more force your muscles have. For example, if you can push a heavy barbell in a bench press up to a lockout in a couple of seconds, you have a lot of muscle force.

However, if it takes you ten seconds to push a light barbell to a lockout, then you have less muscle force. Unfortunately, there is no basic standard for what is considered a good amount of muscle force.  Therefore, you only have yourself to compete against in the game of muscle force!

We already talked about how fast-twitch muscle fibers are highly abundant in myosin. Myosin is the protein that allows your muscle fibers to not only contract, but contract with great force. Therefore, fast-twitch muscle fibers can generate more muscle force than slow-twitch fibers!

While slow-twitch fibers have plenty of endurance, they cannot generate a substantial initial muscle force. Moreover, fast-twitch fibers have a force advantage, and fast-twitch fibers have an endurance advantage!  As a result, the two muscle fibers are more advantageous and disadvantageous in certain athletic areas than the other.

Activity Type

The bottom line is that slow-twitch and fast-twitch fibers are more or less active for certain activities in your athletic life. Given that slow-twitch fibers are aerobic, have a high-oxidative capacity, and have little myosin, they are best for activities such as:

  • Long-distance running
  • Endurance activities
  • Cardiovascular/aerobic activities

If you are a distance runner or even a marathon runner, then the odds are that your slow-twitch muscle fibers are incredibly strong and more abundant! Not only do runners utilize slow-twitch fibers, but also people training for muscular endurance.

Given that fast-twitch fibers are anaerobic, have a low oxidative capacity, and are high in myosin, they are best for:

 muscular bodybuilder handsome men doing exercises in gym

If weightlifting, sprinting, or doing quick bursts of activity are your primary forms of exercise, then the odds are that your fast-twitch muscle fibers are incredibly strong!

In summary, the type of training that you predominantly do determines whether you have more or less and stronger or weaker slow-twitch and fast-twitch muscle fibers!  Everyone has both types, but most people have one type that is stronger and more abundant than the other.

Training Your Different Muscle Fibers

One way to become a better athlete is to train according to your muscle fibers.

There are two ways you can do that, including:

  1. Structuring your workouts around activating the two types of fibers
  2. Optimizing your workouts by properly activating your fibers

First off, given that the types of muscle fibers are more or less active during certain activities, you can divide up your training according to muscle fiber type! Moreover, you do exercises for your slow-twitch fibers on certain days, then exercises for your fast-twitch fibers on the other days.

One way to do this is to alternate doing days of cardio or muscular endurance training and intense weightlifting. If you do that, then your slow and fast-twitch fibers will have enough time to recover in between workouts.  The more quality recovery time they have, the better your muscles will grow!

Second, there are correct ways to activate the two types of muscle fibers. When you activate them correctly, they function at their highest level!

The top tips to correctly activate your muscle fibers are:

  1. To train your slow-twitch fibers, do cardio at a low intensity for a long period of time OR do resistance training at a maximum of 60% of your possible effort for many repetitions
  2. To train your fast-twitch fibers, do weight training OR high-intensity interval training for only a couple of repetitions at 85% of your maximum effort

If you follow these rules of thumb, you will maximize the potential of your muscle fibers! With that, you will see the kind of results you want quicker!

Final Thoughts

Now that you know more about your muscle fibers, you are one big step closer to becoming the best version of yourself and crushing your fitness goals!  If your goal is to grow better and stronger muscles, it only makes sense that you get to know your muscles on a closer level.

Don’t forget to follow our top training tips to working out your muscle fibers!  If you follow them, you will further optimize your workouts and speed up the time it takes you to see results!

Bonus tip: Tabata is perhaps the most effective type of high-intensity interval training.  Therefore, it is one of the best ways to activate your fast-twitch muscle fibers.  Check out our  complete guide to Tabata to fire up your fast-twitch fibers!