• Introduction
  • The basics of muscle fibers
  • 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.


The musculoskeletal system consists of three muscle tissue types; cardiac, smooth, and skeletal muscle tissue. 

Cardiac muscle cells (cardiomyocytes) are found in the walls of the heart. Their main function is to contract the heart and pump blood throughout the body. Cardiac muscles are striated, have branched fibers, contain multiple mitochondria, and are under involuntary control. Each cardiac muscle cell contains a single, centrally located nucleus surrounded by a specialized cell membrane (sarcolemma).

Smooth muscle cells are found in the walls of hollow organs (the intestines, stomach, liver, respiratory tract, etc.). Smooth muscle tissue has a non-striated, fusiform (spindle-shaped, tapering at both ends) appearance and is under involuntary control and. Smooth muscle cells also contain a single centrally located nucleus.

Skeletal muscle cells or muscle fibers (myocytes), are long, cylindrical and have a striated appearance. These striations are caused by the regular arrangement of contractile proteins. Skeletal muscle cells also contain multiple nuclei. The main function of skeletal muscles is to control voluntary movement. The human body comprises of over 600 skeletal muscles and accounts for nearly 40% of total body weight.

This post focuses on the muscle fiber types found in skeletal muscle tissue, as well as their impact on physical performance.

The basics of muscle fibers

Muscle fibers are long, cylindrical cells that contain several mitochondria, nuclei, and sarcomeres. Each muscle fiber is surrounded by a thin connective tissue known as the endomysium. Approximately 20-80 muscle fibers are bundled together into fascicles and covered by a thin connective tissue sheath called the perimysium. Each muscle fiber comprises of hundreds of myofibrils (long contractile fibers that run parallel to the muscle fiber), which  are composed of overlapping thick and thin myofilaments (actin and myosin) organized into distinct, repeating units known as sarcomeres. An entire muscle is formed by a large number of muscle fascicles surrounded by a thick fibrous connective tissue that extends from the tendons – the epimysium. 

The motor neuron and all the muscle fibers it innervates is known as a motor unit. The number of muscle fibers innervated by a single motor neuron varies depending on the muscle and its function. Large motor units tend to produce broad movements with more force, whereas small motor units are responsible for movements that require dexterity. The amount of force exerted by a muscle ultimately depends on how many motor units are recruited and the rate at which they discharge action potentials (rate coding). 

Instead of recruiting all muscle fibers simultaneously, muscles conserve energy by altering the firing rate of motor units as well as recruiting motor units with different properties. Motor unit recruitment follows two principles; the all-or-none law, and the size principle. According to the all-or-none law, when a stimulus exceeds an activation threshold, all the muscle fibers innervated by the same motor neuron will fire. This means that the motor unit will always give a maximum response or none at all. The size principle states that motor units are activated according to size from smallest to largest. Slow motor units are naturally smaller in size and therefore have the lowest activation threshold. Fast motor units are larger in size and only fire when slow motor units are unable to produce enough force. 

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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 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 are not able to provide enough force for a certain movement, the 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. However, since fast muscle fibers lack the oxidative capacity of slow-twitch muscle 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 are unable to produce enough force for a specific movement. This means that if a sport requires maximal force production, it must be trained with heavier resistance to provide the correct training stimulus.

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 muscles are composed 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 they are used.

Genetics can also have a significant impact on muscle fiber proportion. On average, individuals 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. 

Sex can also have an impact on 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. 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 (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. Interestingly, slow-twitch muscle fibers (type I) retain much of their functionality throughout their lifespan. This age-related loss in lean muscle mass can be prevented with consistent physical activity, especially strength training.

Muscle fiber types and training background

Training background can have a significant effect on muscle fiber distribution and characteristics. 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. Muscle fibers can even convert from one type to the other to meet the physical demands. 

For example, endurance training increases all muscle fiber types’ ability to use oxygen for energy production. This is a result of an increased amount of mitochondria and capillary density in the muscle fibers. Fast glycolytic fibers (type IIb) can convert into intermediate muscle fibers (type IIa) and vice versa according to physical demands of the environment. 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. However, there is still inconclusive evidence whether muscle fibers can covert from fast to slow-twitch fibers. 

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, slow-twitch muscle fibers also adapt to contract faster and with more force.

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.

How to train both muscle fiber types

Because slow-twitch muscle fibers excel in aerobic (with oxygen) activities, they are best trained with low-intensity and high-volume exercises. For example, running, swimming, rowing and cross-country skiing are some of the best endurance training methods out there. These types of prolonged exercises are known to increase mitochondrial content, capillary density, and lactate buffering. Adding some higher intensity exercises into the mix can also significantly increase maximal oxygen uptake (VO₂max).

The heavier the resistance, or faster the repetition, the more fast-twitch muscle fibers must be recruited. Thus, increasing the training load will recruit more fibers to generate force and overcome the resistance. As a result of regular strength training, the body undergoes several hormonal, neural, and endocrine adaptations.

In order to have a specific outcome and training adaptation, workout programs must be periodized and utilize different training variables in a smart way. This ensures the long-term progression of a training program.

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 basketball player might have the same amount of fast muscle fibers as a weightlifter, but that does not mean that they could be proficient in both activities. Other physical factors such as weight, height, and muscle mass combined with training background, nutrition, recovery and overall healthy athletic lifestyle can have a bigger impact on athletic performance than just muscle fiber type. Furthermore, the only way to find out the exact proportions of muscle fiber types is to perform a biopsy of the muscle. Even then, how would this information affect the individual sports-specific training?

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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