Slow and Fast Twitch Muscle Fibers

How much do you know about muscle composition and how muscles adapt to certain movements and exercises? Learn more about slow and fast twitch muscle fibers.
Slow and Fast Twitch Muscle Fibers

Last update: 15 June, 2019

Did you know that your body has slow and fast twitch muscle fibers? Both of them play a role in exercise or certain activities. We’ll explain all about them in our post today.

What are muscle fibers?

First off, we need to go over muscle fibers. Muscle fibers are located in muscles and are composed of cells. They can contract or extend and they move their muscle tissue by using two fiber proteins: actin and myosin.

Muscle fibers are divided into three major groups: slow, combined and fast twitch fibers. Each group has a different capacity and stars in specific exercises or physical efforts.

The main difference between these three muscle fibers is the effect they have on physical performance. The number of slow or fast twitch muscles a person has will affect their speed and endurance.

Of course, you can train and work on these fibers. But genetics play a role in a person’s muscle make-up. These muscle fibers influence what kinds of exercise a person can partake in and the results he or she can achieve.

A muscle fiber is a special cell. Under a microscope, they show dark and light stripes that intersect each other.

But why is that important? It’s important in cases of muscular dystrophy– a common myopathy among athletes– that causes progressive muscle fiber degeneration.

Knowing how muscle cells work will help you better organize and plan a workout routine.

Characteristics of slow twitch fibers

Also known as type I muscle fibers, slow twitch fibers are smaller than fast twitch fibers– measuring half in diameter– and need three times as much time to contract after stimulation.

Slow twitch fibers allow us to keep moving or working for prolonged periods. With slow twitch fibers, muscle tissue has a large number of capillaries and a higher oxygen supply.

slow and fast twitch muscle fibers

As a result, slow twitch fibers get to work in exercises or sports that require a certain amount of endurance, like marathons.

You can identify them by their red color, which results from myoglobin pigment. The pigment is distantly related to hemoglobin, which is responsible for carrying oxygen through the bloodstream. Muscles that have more red slow twitch fibers are red in color.

Characteristics of fast twitch muscle fibers

These fibers are known as type II and are more numerous than the former. They can contract in 0.01 seconds or less after stimulation.

Unlike slow twitch fibers, fast twitch fibers have a bigger diameter and hold dense myofibrils inside to accumulate glycogen. However, they have less mitochondrion than slow twitch fibers.

When muscle fibers tense up, they quickly produce an explosive movement. As a result, they come into play in exercises or sports where speed matters, such as 100-meter races. These fibers are white in color.

Characteristics of combined fast twitch muscle fibers

The third group, also known as IIa fibers, are composed of a combination of slow and fast twitch fibers. However, these look more like white fast twitch fibers as they don’t contain much myoglobin and are lighter.

These fibers are surrounded by an extensive capillary network and are more resistant to fatigue than white fast-twitch fibers. These fibers are found in muscles that have been trained and conditioned.

In other words, when muscles go through exercises and conditioning, some fast twitch fibers convert into a combined fiber. As a result, the muscle doesn’t fatigue as easily.

Fibers for sprinters and marathon-runners

As we mentioned earlier, doing certain exercises and sports can influence muscle composition and the number of muscle fibers.

Sprinters and marathon-runners are clear examples. The former group runs at high speeds for short distances– such as 100 meters. Meanwhile, the latter group runs many more miles over a course of hours.

slow and fast twitch muscle fibers runners

Ninety percent of the muscle fibers in the calves of marathon-runners are slow-stitch fibers while more than 75 percent of them are fast-twitch fibers in sprinters. Of course, muscle fibers aren’t the only thing athletes need to run faster or endure, but they sure help them reach their goals.

To wrap up, these fibers show that the human body is designed to carry out all of the actions within its limits. It also shows that it can adapt as it can change according to its actions and needs or sport and training.

All cited sources were thoroughly reviewed by our team to ensure their quality, reliability, currency, and validity. The bibliography of this article was considered reliable and of academic or scientific accuracy.

  • Sachlos, E., & Auguste, D. (2008). Contracción Muscular. Biomaterials.
  • Minamoto, V. (2005). Classificação e adaptações das fibras musculares: uma revisão; Classification and adaptation of muscle fibers: a review. Fisioter. Pesqui12(3), 50–55. Retrieved from

This text is provided for informational purposes only and does not replace consultation with a professional. If in doubt, consult your specialist.