• Introduction
  • Anatomy of a skeletal muscle
  • How do muscle fibers contract?
  • Fast-twitch muscle fibers
  • Type IIa muscle fibers
  • Type IIb/IIx muscle fibers
  • What physiological factors affect muscle fiber distribution?
  • Fast-twitch muscle fibers and training background
  • Fast-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 are comprised of two different muscle fiber types; slow-twitch muscle fibers (type I) and fast-twitch muscle fibers (type II). The latter can also be further divided into type IIa (fast oxidative) and type IIb/IIx (fast glycolytic) muscle fibers. 

Each of these muscle fibers types have their own specific properties and responses to physical activity. Fast-twitch muscle fibers generate more force but fatigue faster, whereas slow muscle fibers produce less force while excelling in sustained physical performance.

An important thing to note is that all skeletal muscles consist of both muscle fiber types. However, this also varies between individuals and where the muscle is located. For example, postural muscles (tonic muscles) often contain more slow muscle fibers than muscles responsible for movement (phasic muscles). This is because you must maintain good posture at all times. 

This post explains the basics of fast-twitch muscle fibers and what makes them so integral for athletic performance. If you’re looking to know more about slow-twitch muscle fibers, we’ve written a dedicated article about it here. 

Anatomy of a skeletal muscle

Muscles are made up of thousands of tubular muscle fibers known as myocytes that run through the length of a muscle. These myocytes are comprised of thousands of myofibrils that are bundled together into fascicles and held together by a thin layer of connective tissue, known as fascia. 

Myofibrils consist of repeating rows/sections of sarcomeres, which are the basic contractile units of a muscle. Sarcomeres are comprised of two protein filaments, actin (thin filament) and myosin (thick filament). These active structures are able to slide past each other, causing the muscle to contract – much like interlocking your fingers. This phenomenon is known as the sliding filament theory.


Muscle fibers (myocytes)



Actin & Myosin filaments

How do muscle fibers contract?

Muscle fibers are activated by a motor neuron, which controls muscle contraction by delivering a signal from the brain to the muscle. A single motor neuron and all the muscle fibers it innervates is known as a motor unit. These motor units are activated according to two principles; the size principle and the all-or none-law.

The size principle refers to the way motor units are activated from smallest to largest. Because slow-twitch muscle fibers are naturally smaller, they are also recruited first. On the other hand, fast-twitch muscle fibers are only activated when slow fibers are unable to produce enough force. The more muscle fibers are recruited, the more force is ultimately 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. If a stimulus exceeds an activation threshold, all muscle fibers innervated by the same motor unit will contract. In short, there will be a full response or no response at all.

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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

Fast-twitch muscle fibers

Fast-twitch muscle fibers, or type II fibers, are known for contracting rapidly with a lot of force. Thus, making them better suited for sports that require strength, power, and speed.  Although fast muscle fibers do not contain as many mitochondria (the powerhouse of the cell) as slow fibers that help with aerobic energy production (with oxygen), fast muscle fibers have significantly higher glycogen storages. This provides improved anaerobic energy production (without oxygen) during high-intensity exercises. However, keep in mind that this also produces lactate as a side product.

Fast-twitch muscle fibers are also larger in size than slow-twitch fibers. So, according to the size principle, they are recruited only when slow fibers are unable to produce enough force. Thus, you must use heavy resistance or high intensity if you want to activate your fast-twitch fibers. 

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

Type IIa muscle fibers

Type IIa muscle fibers are also known as intermediate muscle fibers or fast oxidative muscle fibers. These muscle fibers rely mainly on aerobic energy production while still contracting relatively fast. However, type IIa fibers can also switch to anaerobic respiration when needed. In a sports context, this means that fast oxidative fibers generate more tension than slow-twitch muscle fibers, but also fatigue quicker. Thus, type IIa muscle fibers can be described as a combination of both fast and slow muscle fibers.

These characteristics are a result of a few physiological factors. Like slow-twitch muscle fibers, type IIa fibers also contain a relatively high number of mitochondria. On the other hand, they only contain a moderate amount of myoglobin (a protein that binds iron and oxygen) which gives fast oxidative muscle fibers a lighter red color. These muscle fibers also contain a higher amount of glycogen, which can be readily used in sudden strenuous activity. 

Type IIb/IIx muscle fibers

Type IIb/IIx muscle fibers are often referred to as fast glycolytic muscle fibers. These muscle fibers contain the smallest amount of mitochondria, myoglobin, and capillaries out of any muscle fiber type. Because of their small myoglobin content, they are also pale in color, which is why they are sometimes called white muscle fibers.

These aforementioned properties mean that these muscle fibers have a low capacity for aerobic energy production. However, type IIb/IIx fibers are the largest in size and contain the highest amount of glycogen. Thus, they rely on anaerobic energy production and produce the most force out of any muscle fiber type, albeit at the expense of endurance capability. 

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)




Fast-twitch muscle fibers contract with a lot of force but fatigue easily.

What physiological factors affect muscle fiber distribution?

There are several factors affecting your muscle fiber distribution. These include;

  • Genetics
  • Sex
  • Age
  • Training background

Genetics have a significant impact on physical traits such as height, weight, muscle mass, and muscle fiber distribution. Often times leaner individuals tend to have a higher proportion of slow muscle fibers, making them better suited for endurance activities – and vice versa. These factors are a result of both hereditary factors as well as environmental variance. 

Sex can also have a small effect on muscle fiber distribution. In fact, some studies have stated that women may have more slow-twitch muscle fibers on average. There are even indications that women may be able to utilize fat for fuel more efficiently than men. This is most likely due to estrogen’s (female sex hormone) effect on metabolism. However, this claim is still disputed and requires further research. 

Aging also causes some inevitable effects on strength, muscle mass, endurance, and flexibility. This is due to a phenomenon called sarcopenia – a natural age-related loss of muscle mass. In fact, recent studies have stated that muscular strength tends to reduce 5% every year after the age of 45. Interestingly, sarcopenia seems to have a bigger impact on fast-twitch muscle fibers, whereas slow fibers maintain much of their functionality as you grow older. This is also one of the reasons why contraction and relaxation times become longer as you age. Luckily these effects can be prevented with strength training and an overall active lifestyle.

Fast-twitch muscle fibers and training background

Training background has a significant effect on muscle fiber properties. Although there is little evidence showing that the number of muscle fibers change due to training, their size, properties, and proportions adapt according to the way you use them. There is even evidence that fast oxidative fibers (type IIa) can convert to fast glycolytic fibers (type IIb/IIx) and vice versa. However, there is no proof to support conversion between fast and slow muscle fibers.

While endurance training improves all muscle fibers’ aerobic capacity due to increased capillary density and mitochondria content, high-intensity and heavy resistance training increase the actual size of both muscle fibers and the volume of their contractile proteins. This leads to faster and stronger force production. 

One thing to note is that fast muscle fibers are naturally larger and show greater growth in both the cross-sectional area and actin and myosin filaments as a result of consistent resistance training. Thus, they play a more important role in muscle mass and strength development.

Fast-twitch muscle fibers respond well to training

Because strength, power, and speed are directly related to your ability to recruit fast-twitch muscle fibers during exercise, they also require heavier resistance or higher intensity to properly train them. The faster the repetition or heavier the resistance is, the more fast-twitch fibers will be activated during contraction. This is due to the fact that not all motor units need to be activated for any given movement. Thus, increasing the load leads to recruiting more muscle fibers in order to overcome the resistance. This recruitment process is also the reason why it takes up to 0,5s-2,5s to reach maximum force production. 

Strength training is a great way to teach your body to recruit more motor units with better efficiency. This motor unit synchronization means that motor units learn to fire simultaneously with less effort. As a result, you’ll see strength gains very quickly after starting regular weight training. 

"Fast-twitch muscle fibers show greater growth in size due to consistent resistance training."

Because fast-twitch muscle fibers tend to tire quickly, you must recover for at least two minutes between sets. This gives your muscles enough time to replenish CP and ATP storages while also providing your motor units time to recover. If the recovery time is too short, you will not be able to recruit as many motor units during the exercise, which is especially important for maximum strength training.

Some studies have found that tapering (lowering training intensity and volume) improved the strength capabilities of type IIa fibers while maintaining the endurance capacity of type I muscle fibers. Although this requires further research, it is common knowledge that a well-rounded training routine with varied stimuli helps prevent plateauing while keeping things interesting for the athlete. 

Fast-twitch muscle fibers are activated when slow muscle fibers are unable to produce enough force.

Final thoughts

Although an average person tends to have an equal amount of both muscle fibers, some elite athletes may have up to 80% of a certain muscle fiber type. But remember, this is not even close to being the determining factor in athletic success as it overlooks a tremendous amount of variables between individuals. 

Athletic performance is extremely multifaceted, with different sports relying on different skills and fitness components. For example, top triathletes require significant endurance capability and may therefore have more slow muscle fibers than weightlifters. 

However, most sports rarely rely purely on one specific physical trait. In most cases, athleticism is a combination of skill, training background, as well as physical traits like weight, height, and muscle mass. These factors, combined with proper nutrition, sufficient recovery, and well-designed training have much bigger of an impact on physical performance than muscle fiber type. Additionally, it is important to note that taking a biopsy to measure one’s muscle fiber distribution has little effect on the way you should train for your sport. After all, your body adapts to the way you use it. 

Usually, people tend to gravitate towards activities where they are naturally good at. This can also be a good indicator of muscle fiber distribution – having more fast-twitch muscle fibers often make strength and speed-related activities more enjoyable and vice versa. While there is evidence showing that some people may be better suited for certain activities, this also overlooks the years of hard work and preparation needed to reach their full potential. This is what really makes top-level athletes. 

Did you learn anything new about fast-twitch muscle fibers? Leave a comment down below.


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