Fast Twitch Fibres

Fast Twitch Fibres

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Our muscles are made up of different types of fibres. Fast twitch fibres and slow twitch fibres are the two main types. Each type has its own unique characteristics. Have you ever wondered if a boxer's arm muscles have the same fibres as a marathon runner's leg muscles? We'll answer that question in this article, so keep reading to learn more about fast twitch fibres!

The characteristics of fast and slow-twitch muscle fibres

Our muscles are made up of two main types of skeletal myofibers, which are muscle cells: slow-twitch fibres and fast-twitch fibres. The proportion of these different myofibers varies between individuals and even between different muscles in the same person.

Slow-twitch fibres

Type I fibres, also known as slow-twitch fibres, contract more slowly than fast-twitch fibres and produce weaker contractions. They are red in colour because of the high amount of myoglobin and a large number of mitochondria. Myoglobin is a protein that binds with oxygen more efficiently than haemoglobin, helping to transfer oxygen from the blood to the muscle. This allows type I fibres to produce large amounts of ATP through aerobic respiration and resist fatigue, making them ideal for endurance activities such as running marathons.

Slow-twitch fibres have high myoglobin content, a rich blood supply, and a large number of mitochondria. These adaptations help them generate energy without building up lactic acid, making well-suited for low-intensity, sustained activities. Postural muscles, such as those in the neck and spine, and endurance athletes, like marathon runners, have a higher proportion of type I fibres. However, both genetics and training influence muscle fibre composition.

Slow-twitch fibres are also found in the calf muscles of the lower leg and the back muscles, which are involved in maintaining posture.

Equilibrium dissociation curve

Oxygen dissociation curve
Oxygen dissociation curve

Myoglobin and hemoglobin are both cytoplasmic proteins that bind oxygen on a heme group. Myoglobin has a higher affinity for oxygen than hemoglobin, meaning it binds oxygen more tightly and at lower partial pressures of oxygen (pO2) than hemoglobin. This is due to the fact that myoglobin has only one globulin group, whereas hemoglobin has four. As the partial pressure of oxygen gas increases, the oxygen saturation of both myoglobin and hemoglobin increases until they are both saturated.

Fast-twitch fibres

Type II fibres, also known as fast-twitch fibres, contract rapidly and generate more powerful contractions than type I fibres. However, they are not adapted for prolonged contractions and are only suited for short bursts of intense activity. Type II fibres are further divided into two types:

Type IIa fibres are a hybrid of type I and II fibres and possess large numbers of mitochondria and myoglobin, making them red in colour. They generate ATP rapidly through aerobic and anaerobic metabolism, allowing them to produce fast, intense muscle contractions with some resistance to fatigue. Type IIa fibres are recruited for high-intensity activities such as sprinting and weightlifting.

Type IIb fibres, also known as fast glycolytic fibres, contain fewer mitochondria and little myoglobin, making them white in colour. They generate ATP mainly through anaerobic respiration and can produce short, rapid bursts of power but are prone to fatigue. Type IIb fibres can be trained to become more resistant to fatigue and turned into type IIa fibres. Activities such as heavyweight training, powerlifting, and 100m sprints predominantly require type IIb muscle fibres.

Fast-twitch muscle fibres are found in bicep muscles and the muscles responsible for eye movement.

A comprehensive comparison between different muscle fibre types is in Table 1.

Comparison of different skeletal muscle fibres
Comparison of different skeletal muscle fibres


Determinants of the muscle fibre composition

Our muscles are composed of both fast-twitch and slow-twitch muscle fibres, and the composition of these fibre types depends on one’s genetics, physical activity level, and age. Fast-twitch muscle fibres are used for quick, intense activities, and are fatigue-resistant. They contain fewer blood vessels and mitochondria than slow-twitch fibres, and less myoglobin, resulting in a paler color. Slow-twitch muscle fibres are used for sustained, smaller movements and postural control, and contain more mitochondria and myoglobin.

Muscle fiber type composition is largely genetically determined and has important muscle-specific training implications.  Fast-twitch fibers respond best to low volume, long rest intervals, high intensity and low reps. Slow-twitch fibers respond best to higher volume, shorter rest intervals, and higher reps.


The genes we inherit from our parents determine many things including our muscle composition. Further details about this are beyond the scope of the course specification.

Physical activity level

Individuals who do not have an athletic occupation tend to have a relatively even distribution of fast-twitch and slow-twitch muscle fibres. However, the muscle fibre type composition of elite athletes can differ depending on their sport. Power athletes such as weightlifters and sprinters tend to have a higher ratio of type II muscle fibres, which are better suited for short bursts of intense activity. Endurance athletes such as marathon and long-distance runners, on the other hand, have more type I muscle fibres, which are better suited for sustained, lower-intensity activity. Research has shown that genetic factors play a significant role in determining an individual's muscle fibre type composition. However, training can also affect the relative proportion of fast-twitch and slow-twitch muscle fibres in an athlete's muscles. For example, endurance been shown to of slow-twitch muscle fibres, while strength training can the of fast-twitch muscle fibres. It is important to note that while muscle fibre type composition can influence athletic performance, it is not the only factor that determines an athlete's success. Other factors such as technique, nutrition, and mental preparation also play a crucial role in achieving athletic excellence.


As we age, we begin to lose lean muscle mass, which is accompanied by a decline in our fast-twitch fibre content. This loss of muscle mass can contribute to age-related pathologies such as metabolic dysfunction, increased risk of falls, and decreased functional capacity.

The three types of muscle fibres include slow-twitch or type I, fast oxidative twitch fibres or type IIa, and fast glycolytic twitch fibres or type IIb. Type I fibres have high numbers of mitochondria and myoglobin, making them fatigue-resistant and best suited for endurance-type activities. Type II fibres have moderate numbers of mitochondria and my, making them moderately resistant to fatigue and suited for both endurance and strength activities. Type IIb fibres, on the other hand, have few numbers of mitochondria and myoglobin, making them easily fatigued but with high glycolytic capacity, making them best suited for strength-type activities.

Various factors come together to determine the composition of muscle fibres in a muscle, including genetics, physical activity level, and age. However, resistance training has been shown to be effective in preventing the loss of lean muscle mass, promoting muscle hypertrophy, and maintaining or increasing fast-twitch muscle fibre content. In summary, while our muscle fibre type composition is influenced by genetic factors, physical activity level, and age, resistance training can help prevent the loss of lean muscle mass and maintain or increase our fast-twitch muscle fibre content, which is important for maintaining our functional capacity as we age.


Fast Twitch Fibres

Can fast-twitch type IIb fibres convert into other fibres?

Yes, you can change your muscle fibre type with training. Type IIb fibres can be converted to type I or type IIa through low resistance with high repetition or long duration with low-intensity training. 

How big are fast-twitch fibres?

Fast-twitch fibres are the largest of the three muscle fibre types. Their diameter is between 50 and 100μm.

How to improve fast-twitch fibres?

High resistance training, such as lifting heavy weights, deadlifts, and sprinting can help develop fast-twitch muscle fibres. 

What are the 2 types of fast-twitch muscle fibres?        

Fast oxidative (IIa) and fast glycolytic (IIb) fibres.

Where are fast and slow-twitch muscle fibres?  

The calf muscles in the lower leg and the back muscles involved in maintaining posture mainly contain slow-twitch muscle fibre. Bicep muscles and muscles that move the eyes are made up of fast-twitch muscle fibres.

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