• Introduction
  • Anatomy of a skeletal muscle
  • How do muscle fibers contract?
  • Muscle fiber types
  • Slow-twitch muscle fibers
  • Fast-twitch muscle fibers
  • Type IIa muscle fibers
  • Type IIb/IIx muscle fibers
  • What physiological factors affect muscle fiber distribution?
  • Muscle fiber types and training background
  • How to train your slow-twitch muscle fibers
  • How to train your fast-twitch muscle fibers
  • Final thoughts
  • Sources
  • Actin: an important contributor to the contractile property of a muscle.
  • Capillaries: small blood vessels that form networks throughout the bodily tissues.
  • Mitochondria: the powerhouse of the cell.
  • Motor unit: a motor neuron and all muscle fibers innervated by it.
  • Motor pool: a collection of motor units.
  • Myocyte: a muscle cell.
  • Myofibril: long filaments that run parallel to each other to form muscle fibers.
  • Myoglobin: an oxygen-binding protein located primarily in muscles.
  • Myosin: motor proteins interact with actin filaments to contract a muscle.
  • Sarcomere: the basic contractile unit for both striated and cardiac muscle.


You may have heard people say that some athletes are just born faster than others. While this might sound far fetched, there could actually be some truth behind it. The reason behind this that some people may be born with a higher ratio of a certain muscle fiber type, which can significantly affect your performance. 

You see, slow-twitch muscle fibers are specialized in energy-efficient contraction with relatively low force production. On the other hand, fast-twitch muscle fibers contract rapidly with a lot of force and relatively poor efficiency, making them more suitable for short and powerful performances. 

This post explains the basic physiology of skeletal muscles, different muscle fiber types, how they contract as well as what kind of effect they can have on your performance. 

Anatomy of a skeletal muscle

Each skeletal muscle consists of thousands of tubular muscle fibers called myocytes that run through the length of a muscle. These muscle fibers are a combination of thousands of myofibrils that are bundled together into fascicles and connected via a thin layer of connective tissue, called the fascia. 

Myofibrils are composed of repeating sections of sarcomeres, which are the basic contractile units of a muscle fiber. It is made by two main protein filaments called actin (thin filament) and myosin (thick filament). They are also the active structures responsible for muscle contraction via the sliding filament theory, which describes these protein filaments’ tendency to slide past each other and contract the muscle – almost like interlocking your fingers. 


Muscle fibers (myocytes)



Actin & Myosin filaments

How do muscle fibers contract?

Muscle fibers are activated by a motor neuron, which sends signals from the brain to the muscles and controls their actions. A motor unit, on the other hand, describes the functional combination of both the motor neuron and the muscle fibers it innervates. There are two main principles in how these motor units are recruited; the size principle and the all-or-none law.

The size principle means that motor units are activated in order from smallest to largest. And, since slow-twitch muscle fibers are naturally smaller in size, they also have a lower activation threshold. This means that they are recruited first during movement. As the resistance increases and your slow-twitch fibers are unable to produce enough force, your nervous system starts recruiting fast-twitch fibers for extra force production. In fact, the more muscle fibers you can activate simultaneously, the more force you are able to produce. 

The all-or-none law states that the strength of a muscle fiber’s response is not dependent on the strength of the stimulus. If a stimulus exceeds a certain threshold, all muscle fibers innervated by the same motor unit will contract. Simply put, there will either be a full response from a motor unit or none at all.

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Slow-twitch muscle fiber (Type I)

Low force productionHigh endurance capabilityAerobic energy productionResponds well to training

Fast-twitch muscle fiber (Type IIa)

High force productionModerate endurance capabilityAerobic & anaerobic energy productionResponds well to training

Fast-twitch muscle fiber (Type IIb)

Very high force productionLow endurance capabilityAnaerobic energy productionResponds well to training

Muscle fiber types

Skeletal muscles consist of two different muscle fiber types; slow-twitch muscle fibers (type I) and fast-twitch muscle fibers (type II). Fast-twitch muscle fibers can also be further categorized into type IIa and type IIb/IIx muscle fibers. Each muscle fiber type has its own specific characteristics and responses to physical activity, but generally slow fibers are considered better for sustained low-intensity activity, while fast fibers excel in short high-intensity exercises. 

An average adult has roughly the same amount of both fast and slow-twitch muscle fibers, whereas some elite athletes may have up to 70% of a specific muscle fiber type. For example, sprinters and weightlifters may have a greater amount of fast-twitch muscle fibers while endurance athletes have more slow-twitch fibers. 

It is also important to remember that all of your muscles are comprised of both muscle fiber types, although their proportions may differ individually. Interestingly, the muscles responsible for movement (phasic muscles) often contain more fast-twitch muscle fibers whereas postural muscles (tonic muscles) have a greater amount of slow-twitch muscle fibers. This is due to the fact that postural muscles must constantly remain active to maintain an upright position. 

Slow-twitch muscle fibers

Slow-twitch muscle fibers, or type I muscle fibers, are fatigue resistant and focus on postural control as well as sustained physical performance. Slow-twitch muscle fibers contain mitochondria (the powerhouse of the cell) that use oxygen to create adenosine triphosphate (ATP), which is used as an energy source during exercise. They also have a significant amount of capillaries, which helps provide oxygen and nutrients to the muscle while removing unwanted byproducts. On top of this, slow-twitch fibers also contain myoglobin, which is a protein that binds iron and oxygen, giving slow-twitch muscles their signature red color. As a result, slow-twitch muscle fibers have a higher oxygen capacity than fast muscle fibers, making them more beneficial for sustained aerobic exercises.

Slow muscle fibers are also smaller in size and have a lower activation threshold. This means that they are the first muscle fibers to contract during any specific movement. However, they also generate significantly less force than fast-twitch muscle fibers. Therefore, if slow-twitch muscle fibers aren’t able to provide enough force for a certain movement, your body recruits fast-twitch muscle fibers to produce a stronger muscle contraction. 

Fast-twitch muscle fibers

Fast-twitch muscle fibers, also known as type II muscle fibers, contract rapidly with a lot of force. This makes them essential for explosive sports that rely on strength, power and speed in shorter bursts such as football, ice hockey, weightlifting, wrestling and track-and-field. However, since fast muscle fibers aren’t as good at using oxygen for energy production as slow fibers, they have greater glycogen storage that can be used for anaerobic (without oxygen) energy production. This produces lots of energy in a short amount of time, but also creates lactate as a side product. 

Fast-twitch muscle fibers are also larger in size, which means they have a higher recruitment threshold. Thus, they are only activated when slow-twitch muscle fibers aren’t able to produce enough force. So, if your sport relies on maximum force production, you must train with heavier resistance to activate those specific muscle fibers. 

Fast-twitch muscle fibers can also be divided into two categories; type IIa and type IIb/IIx fibers. 

Type IIa muscle fibers

Type IIa muscle fibers are often referred to as intermediate muscle fibers or fast oxidative muscle fibers. They primarily use aerobic energy production while still contracting relatively fast. However, they can also switch to anaerobic respiration when needed to produce lots of energy in a short amount of time. This also means that they produce more tension than slow fibers but also fatigue quicker. In a way, they are a combination of both slow and fast muscle fiber types.

Much like slow fibers, intermediate muscle fibers also have a high number of mitochondria which is needed for aerobic energy production. However, they only contain a moderate amount of myoglobin, giving type IIa fibers a lighter red color. On the other hand, they also have large amounts of glycogen that is used for energy production under high tension.

Type IIb/IIx muscle fibers

Type IIb/IIx muscle fibers, or fast glycolytic muscle fibers, mainly utilize anaerobic glycolysis for energy production. They are also the largest in size and contain the highest amount of glycogen. As a result, they can produce the highest amount force out of any muscle fiber type, albeit at the expense of endurance capability. 

Because fast glycolytic muscle fibers do not primarily rely on aerobic energy production, they also have the smallest amount of capillaries, mitochondria and myoglobin of any muscle fiber type. As a result, type IIb/IIx fibers are pale and therefore often referred to as white muscle fibers. 

Here’s a quick comparison chart of different muscle fiber types.

Fiber Type




Contraction Speed



Very Fast

Fatigue Resistance




Force Production



Very High

Mitochondria Content
(powerhouse of the cell)




Myoglobin Content
(a protein that binds iron & oxygen and gives blood its red colour)




Capillary Content
(capillaries provide muscles with oxygen and nutrients while removing unwanted byproducts)




Oxidative Capacity
(ability to use oxygen for energy production)




Movement Efficiency




Motor Neuron Size
(larger neurons provide faster activation)



Very Large

Glycolytic Capacity
(ability to store and break down glycogen for intense exercises)




ATPase Level
(enzyme that controls glycogen breakdown and ATP synthesis)




All of your muscles are comprised of both muscle fiber types.

What physiological factors affect muscle fiber distribution?

The amount of different muscle fiber types is a combination of a few physiological factors, including genetics, sex, age and training background. Although different muscle fiber types have certain characteristics that may make them seemingly more suitable for certain sports, studies have shown that both fiber types adapt to the way you use them.

Genetics can also have a significant impact on your muscle fiber proportion. On average, people tend to have roughly the same amount of both fast and slow-twitch muscle fibers. However, some elite athletes may have up to 80% of a specific muscle fiber type. For example, sprinters and weightlifters may have a greater amount of fast-twitch muscle fibers while endurance athletes have more slow-twitch fibers. These individual differences between muscle fiber proportions are a result of both hereditary factors and environmental variance. 

"Slow fibers are considered better for sustained low-intensity activity, while fast fibers excel in short high-intensity exercises."

Sex can also have an impact on your muscle fiber distribution. Some studies have shown that women may have more fatigue-resistant slow-twitch muscle fibers. Because of this, some female athletes can outperform men in certain muscular endurance exercises. On top of having a greater proportion of type I muscle fibers, some studies have stated that women may be able to burn fat for fuel more efficiently. This may be a result of estrogen’s (female sex hormone) effect on overall metabolism. However, there is still insufficient evidence supporting the theory that women have a greater proportion of type I fibers.

Aging causes some inevitable changes in our strength, muscle mass, endurance and even flexibility. In fact, studies have concluded that your muscular strength declines up to 5% every year after the age of 45, due to a natural loss of lean muscle tissue, also known as sarcopenia. While these age-related effects are primarily due to the loss of both muscle fiber types, it seems that aging has a bigger effect on fast-twitch muscle fibers (type IIa & type IIb). This is also one of the reasons why older muscles have a slower contraction and relaxation times. On the other hand, slow-twitch muscle fibers (type I) retain much of their functionality as you grow older. Although these effects may sound daunting, the good thing is that lean muscle mass loss can be prevented with consistent physical activity, especially strength training.

Fast-twitch muscle fibers activate when slow fibers aren’t able to produce enough force.

Muscle fiber types and training background

Training background can have an enormous effect on your muscle fiber distribution. While there is little proof that the amount of muscle fibers changes due to training, there is significant evidence that both muscle fiber types adapt in size, properties and proportions according to physical activity. In fact, they can even convert from one type to the other to meet the demands of your performance. 

For example, endurance training increases all muscle fiber types’ ability to use oxygen for energy production, which is due to an increased amount of mitochondria and capillary density in the muscle fibers. This can have an enormous benefit on your steady-state performance (highest workload without increasing blood lactate) and even increase your lactate threshold. 

Additionally, fast glycolytic fibers (type IIb) can convert into intermediate muscle fibers (type IIa) and vice versa according to the way you train. This means that endurance exercises can decrease the overall percentage of type IIb fibers while increasing the amount of more fatigue-resistant type IIa muscle fibers. On the other hand, there is still inconclusive evidence whether muscle fibers can covert from fast to slow-twitch fibers. 

"Training adapts your motor units to synchronize and contract with better accuracy."

High-intensity training causes similar adaptations to muscle fibers as endurance training. While endurance training improves the oxidative capacity of a muscle, heavy resistance training increases the actual size of both muscle fiber types and the volume of their contractile proteins. Although this effect is often considered to be a result of your fast muscle fibers becoming bigger and stronger, your slow-twitch muscle fibers also adapt to contract faster and with more force.

One thing to remember is that fast-twitch fibers are naturally larger and therefore contract with more force. As a result of consistent resistance training, they also show greater growth in both the cross-sectional area of the muscle and their actin and myosin filaments. Therefore, fast muscle fibers also play a bigger role in muscle mass development.

Interestingly, some studies have stated that slow muscle fibers grew more in size through low-weight and high repetition exercises. While fast-twitch fibers have shown greater growth via high-resistance and low repetition exercises. However, these results are relatively new and need further research.

Endurance training also improves the endurance capability of your fast-twitch muscle fibers.

How to train your slow-twitch muscle fibers

Because slow-twitch muscle fibers can produce their own energy aerobically (with oxygen), they also have to be trained with lower intensity for a prolonged amount of time. The key here is to keep resistance low enough to specifically utilize slow muscle fibers. During low-intensity exercises, the stimulus is not strong enough to recruit fast-twitch muscle fibers, which means that you will not be experiencing any strength or muscle mass gains.

For example, running, swimming, rowing and cross-country skiing are some of the best endurance training methods out there. However, you can also focus on slow-twitch muscle fibers in the gym as well. In this case, you need to perform low-resistance exercises with more (>15) repetitions and shorter (30s) rest periods between each set. Additionally, different isometric (involves no movement or change in a muscle’s length) bodyweight exercises such as planks are great at utilizing a specific muscle group for an extended amount of time. All of these aforementioned exercises are essential for improving your slow-twitch muscle fibers’ ability to produce energy with oxygen. Naturally, this will result in enhanced endurance capability and better athletic performance. 

How to train your fast-twitch muscle fibers

Since strength, power and speed are heavily related to your ability to recruit fast-twitch muscle fibers, it is easy to see why heavy weight training is the most effective way to train them. And, the only way to ensure that you are using fast-twitch fibers is to work out until temporary fatigue. In fact, the heavier the resistance, or faster the repetition, the more fast-twitch muscle fibers need to be recruited during a specific movement. This is due to the fact that the body only has a limited amount of motor units, which means that fast-twitch fibers are not activated unless they are needed. Thus, increasing the load will recruit more fibers to generate force and overcome the resistance. This is also the reason why it takes a relatively long time (0,5s-2,5s) to reach maximum force production.

Regular strength training teaches your body to recruit more motor units with less effort. This phenomenon is called motor unit synchronization, and it means that your motor units learn to fire at the same time with less effort. Exercises like cleans, power cleans, snatches and even medicine ball throws are great in engaging your fast-twitch muscle fibers.

While fast-twitch muscle fibers can produce a significant amount of force, they also tire relatively quickly. That is why you need to rest up to two minutes between sets especially if training with heavy resistance. This offers you some time to replenish your ATP storages and gives motor units some time to recover. If your recovery time is too short, you may end up just training your slow-twitch muscle fibers, which won’t provide the results you are looking for.

Interestingly, some studies have stated that tapering, or reducing volume and intensity, during a training program can improve the strength of type IIa muscle fibers while maintaining the endurance capability of type I muscle fibers. Whether this holds true for every athlete is controversial, but we do know that a versatile training program with varied stimuli ensures that both muscle fiber types challenged. This helps prevent plateauing in your performance while also keeping training sessions interesting.

Both muscle fiber types respond well to training.

Final thoughts

Although most people have an equal proportion of both slow and fast muscle fibers, it is true that some people are genetically predisposed to having a higher amount of a certain muscle fiber type. However, this is not the only determining factor of a successful athletic career and people claiming this often tend to forget that there is a huge amount of other variables between individuals. 

Athletic performance is extremely multifaceted. For example, a professional NBA player might have the same amount of fast muscle fibers as a weightlifter, but that doesn’t necessarily mean that they could be proficient in both activities. In this case, your other physical factors like weight, height and muscle mass combined with your training background, nutrition, recovery and overall healthy athletic lifestyle will surely have a bigger impact on your athletic performance than your muscle fiber type. Furthermore, the only way to truly find this out is to do a biopsy of the muscle itself, which may not even be useful in most cases. After all, how would that affect your sports-specific training anyway?

Oftentimes athletes tend to gravitate towards sports that is more suitable for their body type. In most cases, this is actually a good indicator of your muscle fiber distribution. Having more slow-twitch muscle fibers makes endurance sports easier and more motivating whereas a greater amount of fast-twitch muscle fibers can make strength-related activities and sprints more enjoyable.

While genetics and muscle fiber distribution may some performance benefits, it would be wrong to say that genetically gifted individuals have an easier time to become professional athletes. The fact is, what really separates elite athletes from the rest is hard work and dedication to become a better athlete every single day. Thus, the right training, proper nutrition and sufficient rest are what athletes truly need to focus on in order to maintain healthy and effective athletic progress.

Did you learn anything new about muscle fiber types? Let us know in the comments.


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    Daniel Kiikka

    Daniel Kiikka holds a Master’s Degree in sports science, with a focus on sports pedagogy. After graduating from the University of Jyväskylä in 2015, Daniel worked nearly a decade within the world-renowned Finnish educational system as a physical education and health science teacher. Since 2021, Daniel has worked as a Lecturer at the Amsterdam University of Applied Sciences.

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