Muscle Fiber Types


Endurance (marathoner) vs power (sprinter)

There are generally three different muscle fiber types that can make up the individual skeletal muscles in the body.

Knowing the different muscle fiber types can help with understanding a certain muscle’s primary function and therefore how best to train the muscle for a particular goal such as strength, growth, endurance…

Muscle Fiber Types

Type I (Type 1) Muscle Fibers

Type I fibers are known as slow twitch (meaning slow contraction) or slow oxidative fibers. These slow muscle fibers are more efficient at using oxygen to generate fuel (primarily from fats) for continuous, extended muscle contractions over a long time.

They contract more slowly than fast twitch fibers so they can go for a long time before they fatigue. Therefore, slow twitch fibers are great for high rep lengthy activities like endurance running.

Theses fibers are red in color due to the presence of large volumes of myoglobin (stores oxygen until needed so that the muscle can maintain a high level of activity for a longer period of time), high numbers of Mitochondria (breaks down nutrients to create energy for the cells) and many blood capillaries (carry blood and oxygen).

Therefore, Type I fibers are very resistant to fatigue and are capable of producing repeated low-level contractions by splitting and producing large amounts of ATP (needed to transfer energy) over those long periods of time. However, because Type I fibers split the ATP slowly and have a slow contraction rate, they also generate low power/force/speed production.

The low force output means Type I muscle fibers have fewer motor units (responsible for contracting a group of muscle fibers).

Although Type I fibers can grow, their growth potential is less than the Type II fibers because of the fewer motor units.

Type IIA (Type 2A) Muscle Fibers

Type IIA fibers are known as fast twitch or fast oxidative-glycolytic fibers, meaning that glucose (from the body or carbohydrates) is also used for energy production.

Fast twitch fibers get their name because they are able to generate contractions rapidly. Type IIA muscle fibers also contain large amounts of myoglobin, mitochondria and blood capillaries so they are red in color similar to Type I.

Unlike Type I fibers, they manufacture and split ATP at a faster rate by using both aerobic (with oxygen) and anaerobic (without oxygen) metabolism making them better for intermediate bursts of strength and speed.

Type IIA fibers also have more overall motor units so they have greater potential for growth and are able to produce more overall force than Type I fibers; however, they are more prone to fatigue because of this.

Type IIB (Type 2B) Muscle Fibers

Type IIB fibers, also called fast twitch or fast glycolytic (anaerobic) fibers, contain a low amount of myoglobin, mitochondria, and capillaries so they are white in color.

Type IIB fibers create ATP at a slow rate through anaerobic metabolism but split it at a very fast rate. This results in a very fast contraction along with quick fatigue and long recovery time (needed to re-generate more ATP).

Type IIB fibers have the most motor units and greatest growth/force potential so they are best suited for strength/power activities that don’t last long.

Type IIX (Type 2X) Muscle Fibers

There are sometimes references to a possible fourth muscle fiber type called Type IIX fibers. Most times the references use this name as a direct replacement for Type IIB fibers. Other times they are shown as an actual fourth fiber that lie in between Type IIA and IIB in terms of function.



Most skeletal muscles of the body are a mixture of all three fiber types and the body as a whole generally has similar amounts of Type I and Type II fibers.

However, individual muscle groups may have different amounts of Type I vs. Type II fibers depending on the typical action of the muscle. For example, postural muscles of the neck, lower back, and legs generally have a higher proportion of Type I fibers. Similarly, muscles of the forearms and calves generally have a larger number of Type I fibers given their frequent use in everyday activities.

Genetics also determine the relative fiber make-up a person has. Olympic caliber sprinters, weightlifters, etc. may excel at their sports because they have a higher portion of Type II fibers. Accomplished long distance runners may genetically have more Type I fibers making them better for those long sustained low muscle contractions.

Training can also be used to develop specific muscle fiber types, hence the reason powerlifters use heavy loads and low reps opposite of an endurance athlete’s training.

All fiber types have the potential for growth, so you have to train all of them if you want maximum muscle.

Some also say that the type of training you do can even convert muscle fiber types from one type to another. But there is some debate as to whether you can actually change the ratio of slow to fast twitch fibers in a muscle by just training a certain way.

The body is very adaptive to its environment so it would certainly seem possible that a conversion of muscle fiber types could and would occur to adapt to the training.