Muscle Fibers

What are muscle fibers?
Muscle fibers are muscle cells. If you look down at your arm or leg, or any part of your body, you are looking at millions of interwoven cells making up the larger, visible muscle structure. These cells tend to be long like fibers, so the names are used interchangeably, but they are one and the same. If you would like to know more about the inner workings of the muscle cell, feel free to check out my article detailing the topic in my Muscle Contraction article.

What are slow and fast twitch muscle fibers?
There are a few different types of muscle fibers which are broken up between slow twitch and fast twitch. You may have heard of them as type I and type II fibers, or even as red and white fibers, respectively. All those names relate to the distinction between two main types of fibers, and then one further distinction which we will cover in a bit. First, why all the names? What do they mean?

Slow Twitch, Type I, Red Fibers

These fibers are the muscle cells we use for long duration movement, be it through exercise or through our daily activity (ex. standing, walking, moving your hand over your computer mouse, jogging, etc) [1]. Anything that can be sustained for a long period of time is powered through these muscle cells.

How do they maintain long duration movement?
These cells have high density of mitochondria, which, if you are aware of cell biology is the “powerhouse” of the cell. So, the more mitochondria we have in the cell, the more energy that cell can create for use. However, you should also know that the mitochondria depend on oxygen to create energy, and while this process can last forever, it is slow. So, a lot of mitochondria, each producing endless energy at a slow rate; that means that as long as there is enough oxygen coming in through the lungs and then being carried via the blood system to be absorbed by these cells, these cells will continue to create energy to be used, and this energy is used to power long term functions like standing in line at the register of a Walmart with only one cashier on duty, long duration exercise (jogging, walking), and so on.

Why are they named “slow”, “red”?
There are multiple reasons why these muscle cells are called “slow”, but one of them has already been discussed. They function by generating endless energy, but at a relatively slow rate (slow comparable to the “fast twitch” discussed next). They are also called “slow”, because of their contractile speed. This means that the speed at which they contract or “tense” is slower than – well – “fast twitch”. The contractile speed of slow twitch muscle fibers is around 110 milliseconds [2].

As for them being interchangeably called “red” fibers, this is because they have the highest density of mitochondria. How does that apply? Well, the more mitochondria (and myoglobin), the more oxidative a cell is, meaning the cell relies more readily on oxygen for consistent, slow energy production. Now, the density of mitochondria determines the darkness of the cell, and as you will soon learn in more detail, fast twitch (white) fibers do not contain nearly as many mitochondria, so the more mitochondria located in the cell, the darker the cell; as such, researchers have called mitochondria rich (slow twitch) fibers as “red” for their comparable appearance to their counterpart.

Fast Twitch, Type II, White Fibers

We have just familiarized ourselves with slow twitch muscle fibers, and in many ways it makes the description of fast twitch fibers much easier as they are, in many respects, complete opposites. These muscle cells are mostly used for short term, high intensity movements like sprinting, lifting heavy objects; anything that puts you in a position in which the body is forced to make you stop, because the intensity is too high to maintain.


If you were to cut your leg or arm in half and look straight into the inside of that section with a microscope, you would see many millions of muscle fibers from the top-down. The images here show exactly that (a bit less gruesomely), and show the difference between "red", slow twitch fibers (typically dark colored), and "white", fast twitch fibers (typically lighter colored). 

How do they produce energy for high intensity movement?
As we know with slow twitch fibers, one source of energy is the mitochondria and the more mitochondria, the more energy can be created at any given point, but this energy system is dependent on oxygen delivery and the body needs a system capable of generating energy at a faster rate than the one dependent on oxygen. You may be familiar with the term “anaerobic” (meaning, “without oxygen”), and it is this system that offers enough energy to fuel massive bouts of power demand from some high intensity movement, if only for a short period of time. Well, these fast twitch muscle cells depend more readily on this energy system than the oxidative, mitochondria rich system found in slow twitch fibers. This system is run outside of the mitochondria in the sarcoplasm of the cell and it is likely for this reason that there are far fewer mitochondria in fast twitch muscle cells [3].

Why are they named “fast”, “white”?
Similar to why slow twitch are called slow, fast twitch are called fast, because they offer a far more rapid production of energy. Not only that, their contractile speed is double that of slow twitch fibers creating tension at a blistering speed of 50 milliseconds [2].

Also, as already explained, because the dramatically lower amount of mitochondria and myoglobin, the fast twitch fibers, relative to slow twitch fibers, do not appear dark.

Type IIa and Type IIb?
There are two variations of fast twitch (type II) muscle fibers. They are sometimes referred to as intermediate and power as type IIa is known for having some mitochondria density like type I, slow twitch, and type IIb being almost exclusively non-mitochondria, sarcoplasmic related [2].

The difference, in applicable terms, would be something like a sprint being more related to type IIa fibers and a 1 repetition maximum being type IIb related; a large amount of power for a minute or less versus a maximal amount of power for a few seconds.

Size differences between slow and fast twitch muscle fibers?
If you are interested in if there are size differences between types of muscle fibers, then here is your answer - yes, there are differences. Slow twitch (Type I) fibers tend to be smaller than their fast twitch (Type II) counterparts, because as they are focused on oxidation and therefor mitochondrial density, they tend to increase capillary density and efficiency to better intake oxygen, which requires little change to the actual muscle fiber size [6]. Meanwhile, fast twitch are larger, because the fiber changes with hypertrophy (growth) of the muscle fibrils (components of the muscle fiber).

The actual size differences are typically around 5000 um (micrometer) for a type I fiber and 7000 um for a type II fiber (about 40% bigger according to that calculation)[6]. Differences between type IIa and type IIb are not nearly as large and rather insignificant [6].


What muscles are fast twitch and slow twitch?
All muscles are made up of a variation of all fiber types. However, all muscles favor a greater percentage of one fiber type over another. For example, it would make sense for certain muscles in the calves to be slow twitch, type I considering the amount of time we spend standing, walking, and the calf muscle must stabilize even in a static stance [2].

In general, people are made up, throughout their body, of 50% slow twitch and 50% fast twitch muscle types [2]. However, a person’s genetics are most telling of their body’s favoring one or the other. Certain individuals are genetically gifted to be great marathoners, in which case their slow twitch fibers are likely proportionally higher in their legs than most others, even compared to average, non-genetically gifted competitors [2].

Can I increase the number of muscle fibers in my body?
No. The current literature seems to say that the only thing a person can do is grow the muscle fibers they were genetically set with (hypertrophy), but one cannot grow new fibers (hyperplasia) [4]. This is still being researched and although there is some doubt, typically it is assumed that hypertrophy is the only option in improvements of one or the other fiber type performance [4].

What does training do, then?
Well, while the number and type of fibers is likely set at a particular number and percentage at birth, it is perfectly possible to make a series of different improvements via efficient training methodology. First, one can improve neural development. Second, fibers can increase in size and efficiency. Third, there is some evidence of fibers changing their biological make up to change their type. However, this is not to say that type II fibers will become type I if trained as such, but more so that intermediate fibers like type IIa can become more like type IIb and vice versa [2][5]. This would conclude in an increase in performance, but the changes would be minimal.

Training to improve fibers?
It should go without saying at this point that training for long distance events would likely enhance slow twitch fiber activity via improved neural pathways, blood supply, possible slight enlargement (hypertrophy), efficiency, and making some type IIb fibers become slightly more oxidative and look more like type IIa which have a slight oxidative ability (some mitochondria). On the other hand, more strength and sprinting practice would yield the same changes in favor of type IIa and type IIb, depending on exactly what one would focus on in the field of resistance training.


Activation Sequence of Fiber Type
There is a ramp-like sequence of activation for muscles. This means that as force increases, muscle fiber recruitment changes. This makes sense considering what we know now about each muscle fiber type. As more force is needed, all slow twitch are activated and more fast twitch are activated until maximal force is needed, in which case all fibers are contracting [2].

How is force developed through the muscle fibers?
Force is created via activation or no activation; this means that fibers are either activated/contracting or not contracting. For individual fibers, there is no way to gradate contraction amount. So, this should make you wonder how force is not maximal during any and all activities. It is because the number of fibers activated to contract dictates force produced, as well as number of contractions, not the actual calibration of the individual fiber itself [2].


How does age impact fiber type?
While it is common to see a decline in fast twitch fibers with age, this is likely not due to their disappearance, but rather atrophy (decline in size) due to lack of use [2]. Likely, if, as we age, we trained those particular fibers, the atrophy would be non-existent or minimal. On the other hand, we are in constant use of our slow twitch muscle fibers and therefor see little to no negative change as we age.


This is difficult to summarize, but essentially there are two main “types” of muscle fiber between type I and type II. The differences between the two are many, but mainly in their function as type I is smaller and tends to be used for repetitive movement (oxygen based) while type II tends to be larger and for more explosive, strength based movement (oxygen-less based). The body uses each type interchangeably and does not gradate activation of each fiber, but rather achieves gradation of activation (equaling certain power output) by activating a certain number of fibers rather than a certain amount of activation per fiber.

Writer: Nicolas Verhoeven


[1] McCall, P. (n.d.). Slow-twitch vs. Fast-twitch Muscle Fibers. Retrieved from


[2] Courtright, R. (n.d.). Muscular Control of Movement Part B [Powerpoint]. East Carolina University.

[3] Beashel, P., Taylor, J., & Alderson, J. (1996). Advanced studies in physical education and sport. Walton-on-Thames: Nelson.

[4] Taylor, N. A., & Wilkinson, J. G. (1986). Exercise-Induced Skeletal Muscle Growth. Sports Medicine, 3(3), 190-200. Retrieved from


[5] Wilson, J. M., Loenneke, J. P., Jo, E., Wilson, G. J., Zourdos, M. C., & Kim, J. (2012). The Effects of Endurance, Strength, and Power Training on Muscle Fiber Type Shifting.Journal of Strength and Conditioning Research, 26(6), 1724-1729. Retrieved from


[6] Van Wessel, T., De Haan, A., Van der Laarse, W. J., & Jaspers, R. T. (2010). The muscle fiber type–fiber size paradox: hypertrophy or oxidative metabolism?. European Journal of Applied Physiology, 110(4), 665-694. Retrieved from

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