Looking at Muscle Fiber Types

We’ve all heard the phrase that an actor has been typecast, or assigned the same type of role repeatedly. Typically, this is viewed in a pejorative light (although tell that to people who have carved out careers as character actors!). Muscles, too, are “typecast”, but this is all for the best.


The morphology of different types of muscular tissue is related to the roles they play within the body. Within the sphere of personal training, it is important to understand not only what activities certain muscle fiber types are geared towards, but what fuel sources are best for certain activities.

Traditionally, muscle fibers were categorized based on their color, which reflect the relative amounts of myoglobin (an oxygen binding protein). These terms are still useful when comparing the oxidative characteristics of the muscle types: Red fibers contain high levels of myoglobin and oxygen storing proteins, while white fibers have a comparatively low content myoglobin content and exhibit a whitish appearance.

Muscle Fiber Types

The main types of muscle fibers in humans are:

  • Type I. These red, slow-twitch fibers are relatively small in diameter and are best at endurance but poor for power and strength.
  • Type IIX. These are white, fast-twitch fibers and yield great force, but fatigue quickly and produce high amounts of lactic acid as a result of their anaerobic nature. It is worth mentioning that the designation Type IIb has been replaced by Type IIx, as no Type IIb tissues have been found in humans. These fibers contract quickly like the Type IIX, but exhibit the oxidative capacity of the Type I.
  • Type IIa. These are red, fast-twitch fibers exhibit qualities of the above types, but do not excel at either end of the performance spectrum.

Muscles are comprised of a combination of these three types. It is the action that a given muscle performs that dictates the proportions of each type.

The size of a given muscle fiber is associated also with the size of the motor neuron involved. The recruitment of the motor neurons follows the “size principle”, meaning that a smaller neuron is recruited before a larger. As it applies to the muscle types outlined above, the order of this recruitment would be:

Type I>Type IIa>Type IIX

What’s In a Name?

The names of the fiber types also reflect their metabolic features. Type I fibers are slow, oxidative fibers. Type IIx- are fast, glycolytic fibers. While the Type IIa- are fast oxidative, glycolytic fibers.

And, just as there are three types of muscle fiber types in humans, there are three types of fuel sources, or intracellular processes. While there might not be a one-to-one correlation between fiber type and fuel source, they are still related.

For example, the fuel sources for Adenosine-5′-triphosphate (or ATP, a coenzyme used in cells to transfer chemical energy) are:

  • Phospho-Creatine (PCr) hydrolysis (Phosphagen System). This occurs rapidly and without oxygen. The Phosphorylated Creatine (PCr) gives its phosphate to the ADP to yield an ATP. The PCr has a very limited supply however, which can be enhanced with supplementation.
  • Glycolysis (Glycolytic System) occurs relatively rapidly by means of the breakdown of glucose (anaerobic metabolism). Glucose can be furnished directly from the blood or from liver and muscle stores of glycogen.
  • Oxidative Phosphorylation-(Tricarboxylic Acid CycleTCA Cycle-Electron Transport System- ETS). Carbohydrates, fats and proteins are oxidized in the mitochondria to produce large quantities of ATP by means of the Tricarboxcylic Acid Cycle.

Just My Type(s)

This knowledge is particularly relevant when it comes to specialized training for athletes, who train for a specific type of activity and strive to enhance the efficiency of their movements and their metabolic pathways. For example, training an endurance athlete would place a focus on enhancing aerobic performance and efficiency. This would involve an effort to maximize the capacity to spare carbohydrates (by means of anaerobic threshold training) while maximizing the rate of fat metabolism (long-slow distance training). In preparation for long race, this could translate to the use of carbohydrate loading and ensuring that the athlete has ready access to a sufficient supply of glucose.


These resources are for the purpose of personal trainer growth and development through Continuing Education which advances the knowledge of fitness professionals. This article is written for NFPT Certified Personal Trainers to receive Continuing Education Credit (CEC). Please contact NFPT at 800.729.6378 or [email protected] with questions or for more information.