Different types of muscle fibers

The purpose of this article is none other than to highlight the main differences between the three types of muscle fibers and the reason why, even being part of the same muscle, they differ in their structural and functional characteristics.

I am sure that most runners, cyclists, etc., during their training have challenged each other at some point “Let’s see who gets up this hill first” or “Shall we do a sprint?” If they know each other a little, assuming their fitness level is similar, they already know who is going to win and who is not. “It’s because you are more explosive, but in long distances, I would beat you.”

What we perceive at first glance has its origin in the predominance of one of the types of muscle fibers. And it is that the fibers of the muscles contract and relax at different speeds.

Muscle fibers can be divided into 3 types: Slow (or red), intermediate, and fast (or white). They are also called type I, type IIA, and type IIB respectively.

Structural and functional differences of the different types of muscle fibers

We proceed to differentiate the 3 types of muscle fibers, as well as their main differences (2):

Slow muscle fibers or type I

First, we have the types of slow oxidative muscle fibers, also called Type I. These have a smaller diameter, and therefore, lower levels of strength.

Their color is dark red because they contain large amounts of myoglobin and blood capillaries, as well as numerous mitochondria that allow them to obtain large amounts of energy aerobically.

Their contraction speed is slow, but this provides them with a high resistance to fatigue, thus maintaining exercise for hours, typical of endurance sports (cycling, marathon, ultratrail).

Fast muscle fibers or type IIx

Secondly, the types of fast glycolytic muscle fibers or Type IIx, have a larger diameter and number of myofibrils, so they generate greater strength.

Unlike Type I, their content in myoglobin, capillaries, and mitochondria is low, giving them a white color, and glycolytic is their main way of obtaining energy.

Their contraction speed is very high, typical of intense anaerobic activities (powerlifting, sprinters) so they fatigue quickly.

 

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Fast muscle fibers type IIa

Finally, we find the types of fast oxidative-glycolytic fibers or Type IIa, which are in an intermediate point between the two previous ones, because they present traits of both.

The diameter is intermediate, and like Type I, they present a red (more pinkish) hue due to their large amounts of myoglobin, capillaries, and mitochondria.

But at the same time, they are capable of contracting at a fast speed, with moderately high fatigue resistance.

Below are the different types of muscle fibers:

Where is each type of muscle fiber found?

Once the classification of the type of muscle fibers is seen, it is easy to think that there will be fast and slow muscles, but it is not entirely correct.

A large part of the muscles are composed of a mixture of the three types of muscle fibers (2), of which almost half are slow oxidative. The proportion of one type or another varies depending on:

• Genetic factors: it is believed that the percentage of fast or slow muscle fibers is already genetically predetermined, so a sprinter like Usain Bolt has become the fastest man in the world, but he already had part of the way done.
• Muscle action: the postural muscles of the neck and back, with continuous activity, have a predominance of Type I muscle fibers. On the other hand, the muscles of the arms, with intermittent action, present type IIx muscle fibers, therefore they hypertrophy more easily. And the legs, which support body weight but are also used for running, show a majority of IIa fibers.
• Training method: Those endurance athletes who want to have a high percentage of slow muscle fibers should perform low-intensity, long-duration training. Conversely, athletes who need to generate high levels of strength for short periods of time should include high-intensity, short-duration exercises (1). In part, when an effort requires low levels of strength, only the motor units (motor neuron + muscle fibers it innervates) of Type I fibers are activated, in greater efforts the IIx join, and finally, in maximum efforts the IIa.

How can I know my fiber profile?

After knowing how, when, and in what way the types of muscle fibers are distributed throughout the muscular system, it is normal to feel curious about what our fiber distribution should be.

Unfortunately, it is not an easy process to carry out because it is invasive. It is a muscle biopsy in which a sample of the muscle is removed for later analysis.

So if we do not have a laboratory at hand, we will be left wanting to know it exactly, but as I said at the beginning of the article, at first glance we can already get an idea of who has more fast muscle fibers and who has more slow ones.

In conclusion, the percentage of types of muscle fibers is a fact that is difficult to modify, but on which it is possible to slightly influence through training.

The arrangement in our musculature of the type of muscle fibers is variable, as it depends on the muscle group and the person, that is, it is a genetic issue. Also, the type of training and age influence it to some extent.

Furthermore, it can be useful to know it when specializing in a sports modality, or within a sport, such as cycling, to know what role I can play within a team, whether sprinter, climber, etc.

Bibliography

1. Wilson, J. M., Loennecke, J. P., Edward, J., Wilson, G. J., Zourdos, M. C., Jeong-Su, K. (2011). The Effects of Endurance, Strength and Power Training on Muscle Fiber Type Shifting. J Strength Cond Res. Doi: 10.1519/JSC.0b013e318234eb6f.
2. Tortora, G. J., Grabowski, S. R. (2005). Principles of anatomy and physiology 9th edition. Mexico: Oxford university press.