3 Types Of Muscle Fibers – Anabolic.ca

3 types of muscle fibers

In the world of athletics and bodybuilding, the topic of muscle fiber type is often overlooked or misunderstood. People do not pay enough attention to muscle fiber type when considering training skeletal muscles. Ignoring the muscle fiber types may negatively impact their training and possible gains. Each muscle fibre type works slightly differently and, because of this, should be trained differently. If you are trying to grow your skeletal muscles, you should focus on training your fast twitch fibers, but some muscles contain more slow-twitch fibers than fast-twitch and thus should be trained differently to achieve muscle growth.

The 3 types of muscle fibers are as follows:

  • Type 1 
  • Type 2a
  • Type 2b

What are the different types of muscle fibers?

First, we need to clarify that when we say three types of muscle fibres, we are referring to the fast and slow-twitch muscle fibers. We are not talking about the different kinds of muscles such as cardiac muscle, smooth muscle and skeletal muscle.

There are 3 types of muscle fibers found in the human body, and each one of these muscle fibre types serves a unique purpose. Often when people talk about skeletal muscle, they do so with the notion that there is only one muscle fibre type, or as if all the fibers do the same thing. The fact is that these different fiber types do such different things within the body and serve very different functions.

Type 1 fibers are endurance muscle fibers as they produce weak muscle contractions but can maintain the contractions for long periods without getting fatigued. Compared to type 2a and type 2x muscle fibers, they can produce much more powerful muscle contractions but cannot maintain them for very long and quickly fatigue. Studies show that fast-twitch fibers can produce contractions between 3 to 4 times faster than slow-twitch fibers.[1]

Below you will find a breakdown of the 3 types of muscle fibers and their functions and differences.

Image showing different color of type 1 vs type 2 muscle fibers
Image showing different color of type 1 vs type 2 muscle fibers

Type I fibers (slow twitch)

Type 1 muscle fibers are often called slow-twitch or slow oxidative fibers. They are highly vascularized, which means they have a good blood supply, which you will see later is very crucial for their function. Slow-twitch muscles fibers have many mitochondria and a high concentration of myoglobin and aerobic enzymes. The concentration of myoglobin is so high that the color of fibers is red, leading to many to call type 1 muscle fibers red fibers.

Slow-twitch muscle fibers contract slowly and can only use aerobic respiration, which uses oxygen and glucose to produce ATP. ATP is the cellular energy source and what powers cells in the body

The highly vascularized anatomy and the high amounts of mitochondria, myoglobin, and aerobic enzymes contribute to slow-twitch muscles’ ability to contract and function for extended periods. You will find a high concentration of type 1 muscle fibers in areas of the body responsible for small and frequent movements such as maintaining posture and balance supporting the skeleton. Type 1 muscle fibers are not good a producing very powerful, forceful or fast muscle contractions, so you will not find a high density of slow-twitch muscle fibers in muscles needed for explosive, fast contractions.[2]

Because these fibers are adapted to produce prolonged and repetitive movements, people will often call them endurance muscle fibers. The endurance muscle fibers name is fitting because endurance athletes will usually have a higher amount of slow-twitch muscle fibers in their body than the average person. Studies show that endurance athletes have percentages of slow-twitch fibers ranging from 90 to 95 percent.[3] This phenomenon is common in endurance athletes of all different sports and disciplines. The bodies of these athletes have adapted and replaced some of the fast-twitch muscle fibers with endurance muscle fibers. This adaptation makes sense as endurance athletes do not need the fast-twitch as they do not train or use them in their sports. Because of this, the body has adapted and replaced those fibers with endurance muscle fibers that can work for long periods off of oxygen, glucose and ketones if needed.[4]

Activities that recruit type 1 muscle fibers

  • Long-distance running
  • Swimming
  • walking
  • sitting
  • Biking
  • Rowing
  • Yoga

Type IIa fibers (fast-twitch)

Type 2a muscle fibers are one of two types of fast-twitch muscle fibers. These fast-twitch skeletal muscle fibers are a hybrid between slow oxidative (slow twitch) and fast oxidative (fast twitch). They have the ability to use anaerobic respiration, also known as glycolysis, to produce Adenosine triphosphate (ATP). But they also can use aerobic respiration, which uses oxygen and glucose to produce ATP for energy.

Type 2a muscle fibers have a high mitochondria number, but they lack a high myoglobin concentration, giving them a lighter color than the slow twitch muscle. But the high mitochondria numbers like the type 1 muscle fibers allow them to contract for longer periods without fatigue.

Because they possess characteristics of both slow and fast-twitch fibers, they can do a little of both. They can contract relatively forcefully and do so for moderate periods before tiring. This group of fibers would be recruited in activities such as walking but would not be recruited for things like endurance training or explosive movements like sprinting or weight lifting.[5]

Type IIb fibers (fast-twitch)

Type 2b muscle fibers are pure fast-twitch muscle fibers and are sometimes referred to as type 2x muscle fibers. These fast-twitch skeletal muscle fibers are sometimes called fast oxidative because they rapidly produce energy (ATP) via glycolysis (anaerobic respiration) to generate rapid and forceful skeletal muscle contractions.

Type 2b muscle fibers have a large diameter and have high glycogen stores within them. They do not have many mitochondria compared to type 1 and 2a fibers, and myoglobin concentration is also low. This low myoglobin contributes to their white color.

Type 2b muscle fibers generate powerful, forceful, and rapid skeletal muscle contractions, but they fatigue quickly because of low vascularity, mitochondria, and myoglobin. Also, because they generate energy mainly from anaerobic pathways, they generate lactic acid as a byproduct of the anaerobic process. As the type 2b muscle fibers continue to contract and work, they create more and more lactic acid. The more the lactic acid builds up, the more it impairs the muscle’s ability to contract and work. If you have ever heard the term “burn” or “feel the burn,” it refers to the burning feeling within the working muscles caused by lactic acid buildup.

Just like with type 1, type 2b muscle fibers can be increased in number with proper training. researchers show that with as little as eight weeks of high-intensity weight training, you can increase your number of type 2b muscle fibers.[6] Athletes who do sports or exercise that requires fast and forceful contractions have roughly 60 to 80 percent of their skeletal muscle fibers as fast twitch[7]

Activities that recruit type 2b muscle fibers

  • Sprinting
  • Weight lifting
  • Powerlifting
  • Jumping
  • Boxing
Image of what a muscle fiber looks like
Image of what the contractile unit of a muscle fiber looks like

Summary of the types of skeletal muscle fibers

As you can see, there is a lot to know and understand about the 3 types of muscle fibers found in the body’s skeletal muscles. Each fibre type is unique and possesses unique characteristics and abilities.

The key takeaways about fiber types are that they are specialized to play a specific role, and you should think about this when developing your training regimen. If you are an endurance athlete, you need to understand your ratio of slow to fast-twitch muscle and adapt your endurance training accordingly. If you are a beginner, your number of type ii muscle fibers might be higher than someone who has been training for years.

The same is true if you are a beginner bodybuilder. Your number of type 2x muscle fibers will be lower than someone who has been bodybuilding for years.

But do not lose hope as your body will adapt to the training and the stress you put it through and increase the number of type 2x fibers.[8]


Do not take the information in this article as health or medical advice. The information contained within this article was not intended to be taken as health or medical advice.