Interconversion of Muscle Fibers

A strength trainer, trainee, bodybuilder, Olympic runner is always looking for an edge or a way to maximize their performance. Now, while there are certainly many things one can do to improve one’s performance, the concept of muscle manipulation is likely high on everyone’s list. In this article, we will examine if it is possible to change the characteristics of our muscles to either favor our need or altogether change itself to fit our need. Let us jump in.

What are muscle fiber types?

We first have to understand that muscle is made up of different muscle fibers and those muscle fibers have varying characteristics. Without going too much in depth, we know of three dominant muscle fiber types, each with their own unique attributes.

The first is called Type I (original, right?), or oxidative (if you base it on substrate used to create energy), or slow-twitch (if you base it off neural innervation, and contractile speed and power); it is this fiber that aids runners in the fact that it does not fatigue easily and is a consistent force on which to rely on in any submaximal run, jog, walk, whatever. The next fiber type is called – wait for it – Type IIa; this muscle fiber is an intermediary fiber between the Type I and the next fiber we will discuss. Type IIa offers more power and is ideal for strength training, but does not resist fatigue quite as well as Type I. Finally, Type IIb is quite similar to Type IIa and that is why it remains in the “Type II” family instead of being called “Type III”. Type IIb is very much the opposite of Type I as it is not fatigue resistant at all, but it does provide explosive, powerful, quick contractions of the muscle – this lends itself perfectly for power exercises, and terribly for running long distances, or even moderate distances, at that.

Here is an example of three different mixes of muscle fiber types. Type I are shown in red due to their oxidative nature, and tend to be smaller. Meanwhile, the Type IIa and IIb are hard to distinguish from one another, but make up the lighter colored fibers. The instance furthest to the right is clearly well designed for burst, power movement. 

Is Interconversion Possible?

In short, yes; however, this is contextual and depending on the fiber type.

If you are trying to switch between Type IIb and Type IIa, absolutely it is possible [1][2][3]. This occurs if you strength train, you will usually see a shift from Type IIb to Type IIa [1][3]. I would render an educated guess that this shift from Type IIb to Type IIa is likely due to studies using 6-12 repetition models that are perfect for strength based lifting, but not specialized enough for power training – hence a drop in Type IIb and an increase in Type IIa. It does not hurt that Type IIa and Type IIb are similar in many regards – some of that will be explained in the following section.

Now, when we investigate Type I to Type II or vice versa, there is considerably more debate. In animal models we have seen changes in increases and decreases of both with an apparent decrease or increase in the other [2][4]. However, in human studies, these changes are far less common, but still seen in several investigations [4]. So, what do we make of this research? Likely, it is a matter of greater exercise specificity; only certain exercises, variations, or intensities and volumes lead to changes between the two major fiber types – more on why next.

Physiology Explained

Okay, just like the beginning of this article, we have to understand a bit more of the differences between these fiber types. First off, muscle fibers contract using two distinct proteins called myosin and actin, and we are able to identify fiber types based on their myosin content [5]. Not only that, an enzyme by the name of myosin ATPase plays a key role in contraction kinetics; so, what does this tell us about our fiber types?

Well, Type I fibers are much slower at contracting and do so less forcefully, so their myosin and myosin ATPase levels are low [5][6]. On the other hand, Type II fibers, especially Type IIb, are blisteringly quick at contracting and do so with exuberance, and that is due to their high myosin and myosin ATPase levels [5][6]. That squared away, there is also a neural difference between these fiber types as Type I fibers tend to be innervated by slower, smaller neurons [7]. Type II fibers, on the other hand, as you can likely guess, are innervated by blindingly quick, large neurons [7]. So, all in all, the change seen between the two Type II fibers is unsurprising as they share many of the same characteristics (myosin heavy chain, myosin ATPase, and even have similar neural innervation and structure). On the other hand, Type I fibers are considerably different from either of the Type II fibers, although they do share a few of the same characteristics to Type IIa (the more intermediate, less extreme of the two identified Type II fibers) as both have a higher oxidative capacity (aka, ability to use fat for energy).

As shown, fast twitch fibers (Type II) tend to have larger neurons innervating and communicating information. Slow twitch (Type I) tend, opposite, to have smaller neurons.

So, this means that it is relatively unsurprising that we see an easy interchange between Type IIa and Type IIb fibers, yet see a less definitive change from Type IIa to Type I or vice versa. However, in animals studies (mentioned earlier), when neuron innervation was artificially changed by switching the neural innervation of Type II fibers to Type I fibers, there was a dramatic shift as the Type I fibers rapidly became far more like Type II fibers [4]. This is especially fascinating, because it would imply that the biggest limiter is the neural signaling differences between the fibers and the rest can be overcome if this one aspect can be changed manually. Granted, this is not very practical for us humans, however, unless you want to have surgery to have all your neurons rearranged (good luck!). So, it is hypothesized that exercise that trains the neural system in an explosive fashion may have some benefit, especially over exceedingly long times (far more than studies typically investigate, >16 weeks) could remodel the way the neuron depolarizes (spatially and temporally) and subsequent increases or decreases in ATPase and myosin heavy chain content would follow suit [4].

However, that is not the end of the story. It seems that once the stimulus leading to an increase or decrease in any one of these muscle fibers ceases, then the muscle fiber, even if it assumes all of the characteristics of a different class fiber, will return back to its original fiber type. To clarify, if a person undergoes heavy resistance training for, say, 12 weeks and some of their Type IIb switch characteristics to Type IIa, if that person ceases resistance training, over time those previous Type IIb will switch back [4]. So, this implies that muscle fibers we are born with have some level of “memory” of what they were prior to the stimulus forcing them to adapt characteristics. Then, this might mean that no matter how much we may train, any fibers that interconvert may never be truly converted.


There we have it, between the three most recognized muscle fiber types there is possibility for conversion depending on the stimulus applied; this is certainly true for any changes between Type IIa and Type IIb, but is uncertain in investigations of Type I to Type II or vice versa. These changes are likely due to easy up or down regulation of myosin heavy chain proteins and myosin ATPase activity between Type II fibers; however, a similarity in neural innervation plays, likely, a large role in the seen change. This means, then, the likely difficulty in seeing changes between Type I and Type II may also be due to the large differences in neural innervation – this is, at least, part of the reason. Lastly, once the stimulus for change disappears, seemingly changed fibers seem to return to their original state, so this change is not permanent.

Writer: Nicolas Verhoeven

[1] Adams, G. R. (1993). Skeletal muscle myosin heavy chain composition and resistance training. Journal of Applied Physiology, 74(2), 911-915. Retrieved from

[2] Karp, J. R. (2001). Muscle Fiber Types and Training. Strength and Conditioning Journal, 23(5), 21. doi:10.1519/1533-4295(2001)023<0021:mftat>;2

[3] Andersen, J. L., & Aagaard, P. (2000). Myosin heavy chain IIX overshoot in human skeletal muscle. Muscle & Nerve, 23(7), 1095-1104. doi:10.1002/1097-4598(200007)23:7<1095::aid-mus13>;2-f

[4] 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. doi:10.1519/jsc.0b013e318234eb6f

[5] Muscle fibre types. (n.d.). Retrieved from National Heart & Lung Institute (NHLI) website:

[6] Muscle Physiology - Fiber Types. (2006, January 13). Retrieved from

[7] Knierim, J. (n.d.). Motor Units and Muscle Receptors (Section 3, Chapter 1) Neuroscience Online: An Electronic Textbook for the Neurosciences | Department of Neurobiology and Anatomy - The University of Texas Medical School at Houston. Retrieved from

"CLICK" for Most Recent