Muscle Fibers Explained: Type I and Type II (Slow & Fast Twitch)

In this article we will explain muscle fibers, what they are, the different types and how they impact our performance.

What are muscle fibers?

To help understand the role of muscle fibers, it is useful to be aware of the structure of skeletal muscle itself. Each muscle is wrapped in a thick connective tissue called the epimysium. 

Within this is a number of muscle fibers which are bundled together to form a fascicle, which are held in place by the perimysium. Within each muscle fiber we have a number of myofibrils which are aligned in parallel, each individual fiber is surrounded by endomysium – the smallest of the connective tissue. 

Skeletal muscle is therefore made up of hundreds, if not thousands of muscle fibers. These fibers are singular protein dense cells which contain many nuclei, mitochondria and endoplasmic reticulum. Each fiber is surrounded by a plasma membrane called the sarcolemma, and inside this sits the sarcoplasm, a gelatin-like substance which separates myofibrils and helps maintain the shape of the cell. 

Myofibrils contain functional units called sarcomeres which are defined by Z-discs and contain thick (myosin) and thin (actin) myofilaments which gives muscle its striated appearance. It is these myofilaments that slide across one another to allow the muscle to contract and generate force.

Muscle fiber diagram

Breakdown of muscle fibers with bone, tendon and sub-sections of muscle fibers

image looking at muscle fiber myofibril with Z-discs, H zone and M line

Types of Muscle Fiber: 

There are three main types of muscle fiber:

  1. Type I (Slow-twitch)
  2. Type IIa (Fast-twitch oxidative)
  3. Type IIb (Fast -twitch glycolytic)

Did you know?

We can classify muscle fibers due to the presence of myosin heavy chain (MHC) isoforms, the three original isoforms identified are MHC I, IIa and IIb. 

However, muscle fibers can actually contain multiple myosin heavy chain isoforms. And because of this feature we now recognise that a number of hybrid fiber types also exist.

Type Ic, IIc, Iac, IIab fibers have all been identified, consequently meaning that there is a total of 7 different types of muscle fiber found in human skeletal muscle. If this sounds interesting, then you can learn more about this in the recommended further reading. 

Slow-twitch muscle fibers are the most resistant to fatigue, but also produce a lot less force than fast-twitch fibers. Therefore, having a higher percentage of slow-twitch fibers would suit endurance activities such as running or cycling. Whereas, having more fast-twitch fibers definitely helps Usain Bolt break 100m world records. 

Fast-twitch, type II muscle fibers are further divided into type IIa and type IIb. Whereby type IIb (Fast-twitch glycolytic) fibers are more powerful but less resistant to fatigue than type IIa (Fast-twitch oxidative) fibers. 

For an image of muscle fibers from following paper: Muscle fiber type diversity revealed by anti‐myosin heavy chain antibodies – Schiaffino 2018. Click this link. Due to copyright we can’t place the image in this article.

Differences between muscle fiber types: 

Characteristic Type IType IIaType IIb 
Contraction speedSlow Fast V. Fast
Force producedLowHighV. High
Fiber size (diameter)SmallestLargeLargest
Fatigue ResistanceHighMediumLow
Motor neuron sizeSmallLargeV. Large
Oxidative CapacityHighMediumLow
Mitochondrial densityHigh MediumLow
Capillary densityHighMediumLow
Metabolic TypePrimarily aerobic/oxidative Primarily aerobic/ oxidativePrimarily anaerobic/glycolytic

When assessing and comparing muscle fibers it is important to consider both their function and structure (morphology), and from this information we can start to explain how they can underpin our sporting performance

Slow-twitch / Type I Muscle Fibers: 


Type I muscle fibers are typically the smallest fiber type, this is assessed by measuring their diameter, not length! They also have a higher oxidative capacity (they consume more oxygen) than type II fibers. This is because they have more myoglobin and capillaries that surround them which help deliver more oxygen, but also because they have more mitochondria which helps to utilise this oxygen in order to provide energy. Having a higher percentage of these fibers would make a muscle more red in colour. 


Consequently, with a structure that helps deliver and utilise oxygen, type I muscle fibers unsurprisingly are more resistant to fatigue. However, type I fibers are not very powerful, they produce much less force and take longer to produce tension than their type II counterparts. 

Impact on performance

Their ability to support aerobic metabolism as well as the fatigue resistance shown by type I muscle fibers makes them crucial for prolonged periods of exercise, such as swimming, cycling or long-distance running.

Fast-twitch oxidative / Type IIa Muscle Fibers: 


Because of their features type IIa fibers are often labelled as the intermediate muscle fibers. The best way to think about these guys is that they are the middle ground between type I and IIb fibers, sharing characteristics of both. For example, they sit between the other fibers in terms of their size, oxidative capacity, mitochondrial content and capillary supply. 


Considering their structure, It will come as no surprise to hear that type IIa fibers produce more force than type I fibers but less than type IIb fibers. They are also able to sustain their force for longer periods of time than type IIb fibers, but not quite to the same extent as type I fibers, which are the most resistant to fatigue.

Impact on performance

Given their position as an intermediary, type IIa fibers are able to bridge the gap between aerobic and high-intensity exercise. They ultimately enable us to work at high-intensities for a slightly longer duration. 

Fast-twitch glycolytic / Type IIb Muscle Fibers: 


The largest of all human muscle fibers are the type IIb fiber. Type IIb fibers have a lower oxidative capacity, and also don’t have as many mitochondria, myoglobin or capillaries supplying them. Having a higher proportion of type IIb fibers would make a muscle look white in colour. 


Type IIb muscle fibers are without doubt the most powerful, they produce the most force and are faster at getting to their peak force. However, they are easily fatiguable, meaning this high-level of force cannot be sustained for as long as type I fibers.

Impact on performance

Due to their ability to produce high amounts of force very quickly, these fibers play a vital role in power-based activities such as sprinting, jumping and weightlifting. 

Did you know? 

Because a lot of research into muscle fibers has been conducted in animals, we have actually falsely assumed that as humans we contain type IIb muscle fibers. 

When we look at small mammals like rats, we find that they have four myosin heavy chain isoforms – type I, IIa, IIb, but also a type IIx. 

It has been discovered that the fibers in humans that we have labelled as type IIb fibers actually are the type IIx fiber type found in rats and humans do not actually contain the myosin heavy chain isoform IIb.

So to be technically correct, what we have referred to throughout this article as type IIb muscle fibers are actually type IIx fibers. So keep this in mind when you find yourself reading further about muscle fibers!

What affects muscle fiber composition

Muscles are rarely just one fiber type. Instead, what we see is that human muscles contain both type I and II fibers, however the mixture between these fiber types are often different between muscles. 

For example, approximately 80% of the soleus in humans is made up of type I fibers, however, this varies between individuals. This is because muscle fibers are plastic, meaning they are capable of changing their size and shape. Even more interesting is the fact that they can also convert from one type to another! These changes to muscle fibers are due to a number of factors, these include your age but also your activity level. 


As part of the natural ageing process we all demonstrate a loss of muscle mass as we get older. It is estimated that after our 50th birthday we tend to lose around 1-2% of our muscle mass every year! 

Older adults show a decrease in the number of muscle fibers, but also those fibers that remain also get smaller – particularly the type IIb fibers. As we previously mentioned, the type IIb fibers produce the most force, which explains why muscle weakness and falls pose a risk for elderly populations. 

Activity level 

Evidence suggests the amount of exercise we do and the type of exercise can have an impact on our fiber type make-up. It has been shown that untrained individuals show a more even split of type I and II muscle fibers, but as we begin to look at more specialised athletes we can start to see that one fiber-type becomes more dominant than the other.

For example, those who complete high-velocity and high-intensity actions such as sprinting, show more type II muscle fibers. Whereas athletes who complete endurance training tend to show a shift towards type I fibers. Interestingly, when we stop doing exercise altogether we can see a drastic decrease in type I fibers, and some increase in type IIb fibers. 

What is your muscle type? 

As we have shown, muscle fiber types are varied between individuals, so it is highly likely that you show differences between your friends and family. Sadly, the only true way of finding out what your muscle fiber types is through a muscle biopsy – this involves taking away a tiny portion of the muscle for testing in the labs. 

Now, while this is extremely interesting and has been particularly useful for research into elite athletes and certain chronic diseases, I would argue for the majority of us it is probably best to remain in the dark about our muscle make-up! While knowing our muscle fibre type may be interesting, in practice taking a biopsy causes damage to the muscle tissue, there are plenty of tests that are far cheaper and less intrusive that we can use to understand what athletic qualities we need to develop. 

Why understanding fiber type is important for coaches, athletes and practitioners

As athletes and coaches, why do we need to be aware of the different types of muscle fibers? Well, as previously mentioned each fiber type has different characteristics and therefore is better suited to certain sporting demands. 

For example, if you are a long distance runner then the higher percentage of type I fibers you have, the better. Therefore it is important to assess the sport you play. Ask yourself, what are the key qualities of this sport and what specifically would you need to improve as an individual, this should help identify what fiber type we would want to see more of.

You know what you want, but how do we get there? 

We know muscle fibers change, and importantly, we also know that the type and amount of exercise we do seems to influence these alterations. As coaches and athletes it is therefore important to ensure the training we are doing is causing the changes we want for our sport, and not harming us in the pursuit of our goals! 

Without going into too much detail, I have highlighted some key training variables and what  fiber type they help to promote, helping to ensure your training hits the mark. 

Table explaining how training affects development of slow and fast twitch muscle fibers

Further Reading: 

Scott, W., Stevens, J. and Binder–Macleod, S.A., 2001. Human skeletal muscle fiber type classifications. Physical therapy, 81(11), pp.1810-1816.

Wilson, J.M., Loenneke, J.P., Jo, E., Wilson, G.J., Zourdos, M.C. and Kim, J.S., 2012. The effects of endurance, strength, and power training on muscle fiber type shifting. The Journal of Strength & Conditioning Research, 26(6), pp.1724-1729.

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Josh Parker headshot
Josh Parker
BSc Sport and Exercise Sciences, Level 2 Strength & Conditioning Coach | Website | + posts

S&C coach who has provided support for both amateur and professional athletes working across cricket, netball and boxing programmes.

Also completed an Industrial placement year within a professional football club as a Sports Scientist and is particularly interested in physiology, athletic development and maturation.