• Introduction
  • Anatomy of a skeletal muscle
  • How do muscle fibers contract?
  • Slow-twitch muscle fibers
  • Physiological factors that affect muscle fiber distribution
  • Slow-twitch muscle fibers and training background 
  • Slow-twitch muscle fibers respond well to training
  • 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.
  • 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.


Skeletal muscles consist of two different muscle fiber types; slow-twitch muscle fibers (type I) and fast-twitch muscle fibers (type II). Type II fibers can also be further divided into fast oxidative muscle fibers (type IIa) and fast glycolytic muscle fibers (type IIb/IIx). 

Each of these muscle fibers has its distinct characteristics, properties, and responses to physical activity. Generally speaking, slow-twitch muscle fibers have a better endurance capability with lower strength, whereas fast-twitch fibers produce more force, but fatigue more easily. 

It is also important to remember that all muscles consist of both muscle fiber types. However, the proportion varies between individuals as well as according to the function of a specific muscle. For example, postural muscles (tonic muscles) usually have more slow-twitch fibers than muscles responsible for movement (phasic muscles), because they must constantly remain active.

This post explains the basics of slow-twitch muscle fibers and what makes them so important for physical performance. If you want to learn more about fast-twitch muscle fibers, we’ve written an in-depth article about it here.

Anatomy of a skeletal muscle

Skeletal muscles are composed of thousands of tubular muscle fibers (myocytes) that run through the length of a muscle. These myocytes are a combination of thousands of myofibrils that are bundled together into fascicles, which are connected together via a thin layer of connective tissue (fascia).

Myofibrils are comprised of repeating rows of sarcomeres – the basic contractile units of muscle fibers. These, on the other hand, are made up of two protein filaments called actin (thin filament) and myosin (thick filament). These filaments are active structures that are ultimately responsible for muscle contraction according to the sliding filament theory. This refers to the protein filaments’ ability to slide past each other to contract a muscle – much like interlocking your fingers.


Muscle fibers (myocytes)



Actin & Myosin filaments

How do muscle fibers contract?

Muscle fibers are activated by a motor neuron, which controls muscle contraction by transferring a signal from the brain to the muscles. A single motor neuron and the muscle fibers it innervates is called a motor unit. Motor units are activated by two main principles; the size principle and the all-or-none law.

The size principle describes the motor units’ tendency to activate from smallest to largest. Because slow-twitch muscle fibers are the smallest of all muscle fiber types, they also have the lowest activation threshold. Thus, slow muscle fibers are always recruited first, whereas fast muscle fibers are activated when slow fibers are unable to produce enough force. Additionally, the more muscle fibers are activated during contraction, the more force is produced

The all-or-none law means that the strength of a muscle fiber’s response is not dependent on the strength of the stimulus. Simply put, if a stimulus exceeds the activation threshold, all muscle fibers that are innervated by the same motor unit will be recruited. Thus, there will be a full response or none at all. 

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Slow-twitch muscle fiber (Type I)

Low force productionHigh endurance capabilityAerobic energy productionResponds well to training

Slow-twitch muscle fibers

Slow-twitch muscle fibers, also known as type I muscle fibers, are fatigue-resistant and focus on sustained muscle contraction. This is also why slow muscle fibers are found in higher proportion in muscles that are responsible for postural control. 

Unlike fast-twitch fibers, slow-twitch muscle fibers contain a high number of mitochondria (the powerhouse of the cell). This means that they are able to produce adenosine triphosphate (ATP) aerobically (using oxygen). Slow-twitch muscle fibers also contain a significant amount of capillaries, which help move nutrients into the muscle while removing lactate. 

Slow-twitch muscle fibers also contain myoglobin (a protein that binds iron and oxygen), giving type I fibers their signature red color. This is also why slow muscle fibers are sometimes referred to as red muscle fibers. 

Last but not least, slow-twitch muscle fibers are also smaller in size compared to fast-twitch fibers. Therefore, their activation threshold is also lower, and they will be recruited first during contraction. However, slow muscle fibers contract with significantly less force than fast muscle fibers. This means that when slow-twitch muscle fibers are unable to produce enough force to complete a task, your nervous system recruits fast fibers for extra help. 

Muscle 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)




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




Muscle Fiber Diameter




Muscle Fiber Color

Dark Red

Dark Red

Pale Red

Motor Neuron Size
(larger neurons provide faster activation)



Very Large

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




The average human has roughly the same amount of fast and slow-twitch muscle fibers.

Physiological factors that affect muscle fiber distribution

Muscle fiber distribution is dependant on a few physiological factors. These include:

  • Genetics
  • Sex
  • Age
  • Training background

Genetics have a significant effect on muscle fiber distribution. Although most people tend to have an even distribution of slow and fast muscle fibers, some elite athletes may have up to 80% of a certain muscle fiber type. For example, a marathon runner may have significantly more slow-twitch fibers whereas a sprinter may have a higher proportion of fast-twitch fibers. These differences are due to hereditary and environmental factors. 

Sex is another factor affecting muscle fiber distribution. Some studies have hinted indicated that females may have a higher proportion of fatigue-resistant slow-twitch muscle fibers, which is also why women tend to perform better in certain muscular endurance tasks. There is also some evidence that women may be able to utilize fat for fuel more efficiently than men. This is most likely due to hormonal differences.

Aging also causes some significant changes in muscle mass, strength, endurance, and even flexibility. Due to natural loss of lean muscle mass (sarcopenia), your strength declines around 5% a year after the age of 45. This effect is mostly due to the loss of both muscle fibers, although some studies have indicated that aging may have a bigger impact on fast muscle fibers. Whatever the case, muscle mass can still be maintained with consistent strength training and other forms of physical activity.

Slow-twitch muscle fibers and training background 

Your training background has a tremendous effect on muscle fiber characteristics. Although there is little scientific proof that training increases the number of muscle fibers, their proportions, size, and characteristics do adapt according to physical activity. 

For example, endurance training increases the number of mitochondria and capillary density of both muscle fiber types, leading to improved endurance capability. On the other hand, high-intensity and resistance training increase the size of both muscle fibers as well as the volume of their contractile proteins. However, studies still state that fast-twitch muscle fibers show greater growth in cross-sectional area and actin and myosin filaments due to training. 

Although there are signs that fast muscle fibers can convert from type IIa to type IIb and vice versa, there is little evidence suggesting that slow-twitch muscle fibers can convert to fast-twitch muscle fibers. 

Slow-twitch muscle fibers respond well to training

Because slow-twitch muscle fibers have the ability to produce energy aerobically (with oxygen), they excel in endurance activities that last a long time. Because they are smaller in size compared to fast-twitch muscle fibers, they are also recruited first during movement. Thus, a lower stimulus is needed to specifically train them. 

Due to their increased endurance capability, the best training methods for training your slow-twitch muscle fibers are running, swimming, cycling, rowing, etc. In a gym setting, you can also perform a high number of repetitions (>15) with lower weight (<50% of maximum) and shorter rest periods (<30s) for similar muscular endurance benefits. Different bodyweight and isometric exercises (no movement or change in muscle’s length) also work well here.

However, keep in mind that slow muscle fibers also respond well to strength training, and grow in size for improved force production. 

Slow-twitch muscle fibers are specialized in sustained and energy-efficient contraction.

Final thoughts

Even though the average person has an equal amount of fast and slow-twitch muscle fibers, some elite athletes may have a higher proportion of either muscle fiber type. But, it is important to remember that this is far from being the only determining factor for a successful athletic career. 

For example, a professional swimmer may not have the same amount of fast muscle fibers as an NFL player, but that doesn’t mean either is inherently better than the other one. Other factors such as height, weight, sports-specific skill, combined with proper nutrition and recovery are just as essential for athletic success. Additionally, there is little benefit in doing a biopsy of a muscle to determine the muscle fiber ratio, because that would not change the most effective training methods for a specific sport. 

Furthermore, people tend to gravitate towards activities that they are naturally good at, which is often a good indicator of muscle fiber distribution and body type. For example, a lean individual with more slow muscle fibers may find long-distance activities easier and more motivating whereas a heavier-built person might prefer strength-related activities like weightlifting. 

Although genetics plays an inevitable role in muscle fiber distribution and body type, it is important to remember that it is not a shortcut to athletic success. Otherwise, this would undermine years of hard work, dedication, and perseverance towards your goals. Truthfully, reaching the elite level is a combination of several everyday actions, the correct nutrition, the right training, and sometimes luck.

Did you learn anything new about slow-twitch muscle fibers? Let us know in the comments. 


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