Muscle Fiber Types – Overview and Application


Hello UESCA coaches, current and future! I hope your 2022 is off to a good start and you are excited about the upcoming opportunities coming up with the UESCA including new certifications, continued partnerships, and the overall growth of our UESCA coaching community. Today’s blog discusses the different muscle fiber types our muscular system consists of, and the application to working with athletes… and ourselves!


Muscle fibers are not created equally. This can be seen at looking at the physiques of a body builder or professional basketball player versus that of a marathoner or long-distance triathlete/cyclist. The training regimens of each athlete are a big contributor in determining the size of their musculature, but there are other factors as well including:

  • Diet
  • Strength training program
  • Cardiovascular program

While the athletic performance and muscle size of an individual is certainly affected by the above noted variables, absolute performance and muscle size of an individual is dictated largely by genetics.

There are two primary classifications of muscle fibers in the body, fast twitch (Type II-A and II-B) and slow twitch (Type I)

Type IIA (Fast Oxidative Glycolytic): is an intermediate fast-twitch fiber. This fiber type is sort of a hybrid between Type I and Type II, meaning that its contractile properties are faster than Type I, but not quite as fast as Type II-B. Because of this, Type II-A fibers have midrange endurance capabilities. As such, this muscle fiber type could also be termed, Intermediate Twitch.

Type IIB (Fast Glycolytic): these are the classic fast-twitch fibers, which means their contractile properties are the fastest of all fiber types but they lack the ability to fire repeatedly without fatiguing. As a result, Type II-B fibers are best suited for short, explosive efforts like sprinting.

Type I: Type I fibers are characterized by their ability to fire repeatedly with minimal fatigue. They fire more slowly and with less force than Type II fibers and, as a result, are better geared toward endurance events, such as distance running.

The chart below demonstrates the characteristics of the muscle fiber types across a wide range of criteria.

While muscle size can be increased with strength training, the degree of hypertrophy is largely determined by the fiber type.

Some muscles within the body are composed of the same fiber type, regardless of genetics. This is largely dependent on the function of the muscle or muscles. For example, the soleus (calf muscle) is predominately made up of slow-twitch fibers. The reason for this is that the soleus is responsible for stability of the ankle and therefore it is constantly active and must have exceptional endurance properties.

Fast-twitch muscles produce more lactate than slow-twitch muscles. Therefore, the intensity of exercise and the fiber type ratio of an individual influence their lactate threshold.


While the type of muscle fiber plays a large role in determining the size of the fiber, a protein in the body called myostatin regulates how large a muscle fiber can become. This occurs by the myostatin regulating the amount of resources the muscles consume to grow in size. Individuals or animals with mutations to the myostatin protein have abnormally large muscles due to the lack of muscle size regulation.

Muscle Fiber Recruitment

In order to understand how muscles respond to the demands placed on them, it is important to know how muscle fibers are recruited ‒ or engaged. Muscle fiber recruitment is the result of a neuromuscular reaction in which the central nervous system sends an electrochemical impulse to a particular muscle or muscles, at which point a contraction occurs. As noted previously, there are three types of muscle fiber types: 

Type I (oxidative)
Type II-A (fast oxidative glycolytic)
Type II-B (fast glycolytic)

When the intensity of an activity progresses from low or moderate to high, there is a hierarchy as to which types of muscle fibers are recruited. This is referred to as Henneman’s Size Principle. First, Type I fibers are recruited, and as the intensity level rises, Type II-A and then Type II-B are recruited. If the intensity is increased over a very short period and to a very high level (i.e., explosive-type movements), the central nervous system will recruit all three muscle types at the same time.

A muscle fiber cannot partially contract, or more specifically, a muscle fiber cannot contract with varying force depending on the effort required. A muscle fiber contracts either 100 percent or not at all. Therefore, the degree of muscle contraction is based on the number of muscle fibers recruited to perform an action, not the degree of contraction per muscle fiber.

Can Muscles Change Their Type?

Whether or not muscle fibers can change their type has long been debated, and to this day the debate has not been settled. In rare cases, muscles have demonstrated the ability to convert from Type I to Type II when the muscle was subjected to substantial deconditioning, in the case of injury. There has not been much evidence to support the theory that muscles can change from Type I to Type II through changes in training routines, although the lack of verification might be due to insufficient research in this particular area.

Regardless of a muscle fiber’s ability to change type, people can always improve their performance even if it is at odds with their genetic muscle fiber type composition. For example, let us say that an individual has predominately fast-twitch muscle fibers and wants to compete in an endurance event such as a marathon. Through development and application of the proper training regimen, they can become very proficient despite his or her genetic makeup to the extent that the individual can perform quite well in distance running events.

Muscle Hyperplasia vs. Hypertrophy

Hypertrophy refers to the increase in size of a muscle fiber whereas hyperplasia refers to the increase in the number of muscle fibers. While muscle hypertrophy has long been established as the method by which muscles increase in size, skeletal muscle hyperplasia is a much-debated topic. Skeletal muscle hyperplasia has been shown to exist in mammals as well as in human cardiac muscle.

However, evidence of skeletal muscle hyperplasia in humans is still largely inconclusive. A theorized reason for this is because the existence of skeletal muscle hyperplasia is impossible to detect via a standard muscle biopsy. Therefore, more research is required to determine with certainty if skeletal muscle hyperplasia exists in humans.

No Pain, No Gain?

It is widely accepted that muscles are damaged during intense exercise bouts and that they are rebuilt (i.e., remodeled, restructured) during the recovery phase, thus allowing for strength gains. However, detectable damage (i.e., plasma levels, pain, muscle soreness) to muscles is not a requirement for strength gains. In other words, just because an exercise does not cause pain does not mean that positive results aren’t realized in regard to strength/performance.

A 2011 study by Flann et al. put two groups (trained/untrained) through training programs that elicited muscle damage in one group (untrained) but not in the other (trained). It was found that both groups experienced similar increases in muscle rebuilding, strength gains, and muscle hypertrophy. Therefore, muscle damage is not a requirement for muscle rebuilding and strength increases to occur.

Can We Manipulate the Type of Muscle Fiber(s) Through Training?

Yes, to a degree. Around 50% of the muscle fibers we have are genetic, and 50% are influenced through the training environment. Every individual will be born with a certain percentage of each muscle fiber type. Elite sprinters have been shown to express up to 70-75% of FT type II fibers in their muscles, and endurance athletes 70-80% ST fiber. Therefore, there is a genetic limit to what you are born with.

You can, however, modify fiber type to an extent through training. FT type IIa fibers (the ‘intermediate’ fibers) are highly influenced by the training environment. If you complete a lot of endurance training, they will shift towards eliciting ST fiber characteristics. If you complete high intensity sprinting, they shift towards becoming explosive FT IIb fibers. 


The take home message? Your body adapts to the stressors you place on it. If you complete a heap of endurance work, you will both strengthen your current ST fibers, as well as manipulate your FT IIa (intermediate) fibers into expressing and/or transforming into ST fiber characteristics. 

Conversely, if you run intense, short duration sprint intervals with appropriate rest intervals you will further develop your FT IIb fibers which can also be useful for shorter events and/or the final kick towards the finish line.


Sean Begley is an advisor and contributor to United Endurance Sports Coaching Academy (UESCA), a science-based endurance sports education company. UESCA educates and certifies running, ultrarunning and triathlon coaches (cycling and nutrition coming soon!) worldwide on a 100% online platform.

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Rick Prince

Rick Prince

Founder/Director of United Endurance Sports Coaching Academy (UESCA).

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