Changing Muscle Fiber Type and Improving Explosiveness

Is It Possible to Transform Your Muscle Fibers Into Different Types, for the Sake of Athletics?

A week ago, I wrote an article discussing the history and benefits of the deadlift. One detail I made about the deadlift was that this exercise is important for increasing explosiveness in athletics. This was a source of obscurity for those who found my article on the website Reddit.

To explain further, my point was that explosiveness in the hip extensors is important for athletes. I’ve always known that increased strength allows for increased power/explosiveness. Thus, I postulated that deadlifts will increase the power of the hips because they directly increase the strength of the hips.

To back up my point, I referenced a study that took participants, had them strength train and then stop training. Researchers found that trainees experienced a “shift” in their muscle fiber types, from type IIx (anaerobic fast-twitch, or explosive) to type IIa (aerobic fast-twitch), during training. After stopping training, the shift reversed, and even led to an increase in type IIx muscle fibers.

The Flaws with the Study

What caused a bit of discussion was the fact it required detraining to achieve this “surge” in explosive type IIx muscle fibers. It wasn’t a simple taper or a week off, but was three whole months of zero activity. Ask yourself this question… aside from those getting surgery, what athlete takes three months off from training “to get more explosive”?

Changing muscle fiber type.

“I’m becoming more explosive, bro.”

Not only is it counter-intuitive, it would serve against the athlete’s best wishes. Three months of inactivity not only leads to detraining of strength, but also of all of the fine motor-skills associated with their sport. Do I even need to link a study for this? Just look outside and look at all the fat folks who don’t do crap and how “un-athletic” they are.

The other issue with the study was that its participants were previously untrained. Well, magic seems to happen with untrained people, as seen with the famous “newbie gains”. Experienced and well-trained athletes may have very much seen different results in this study.

So, with that said, this study is all but useful. Is the transformation of fiber types possible? More importantly is it possible to become more explosive via muscle fiber type transformation?

Transforming to Type I Muscle Fibers – Possible

Type 1 muscle fibers are slow-twitch, aerobic, and commonly associated with endurance activities.

Two studies have observed the transformation of type 2 fibers to type 1 with copious amounts of endurance training (Howald H) (Smrkolj L, Škof B). Interestingly enough, both studies noted observed that explosive training leads to the hypertrophy of type II fast-twitch muscle fibers, and not a transformation. This may explain why sprinters are ripped to the bone, whereas marathon runners are thin and lanky.

Transforming to Type IIa Muscle Fibers – Possible

Type IIa muscle fibers are fast-twitch, aerobic muscle fibers associated with somewhat short but somewhat intense activities. Think: fighting, 400-meter runs, and soccer/football.

Two studies – performed in the same year and led by the same researcher – found that both powerlifters and Olympic weightlifters had a higher percentage of type IIa muscle fibers, compared to control subjects. Under the same token, there were less type IIx fibers in the lifters compared to control subjects, which may indicate that the type IIx fibers transformed into type IIa (Fry AC, et. al) (Fry AC, et. al). Additionally, another study observed that sprint-style training induced fiber transformations in males, from type I to type IIa (Jansson E., et. al).

Transforming to Type IIx Muscle Fibers – Not Possible

Type IIx muscle fibers are fast-twitch, anaerobic, and associated with explosive activities, such as sprinting and heavy lifting. Aside from the study discussed in my last article (Andersen JL, Aagaard P), there is very little evidence that observes the transformation of either type I or type IIa muscle fibers to type IIx fibers.

As we’ve discussed already, heavy lifting and sprinting, although explosive in nature, don’t actually increase type IIx fibers. Wouldn’t powerlifters, weightlifters, and sprinters benefit  from having more type IIx fibers? Why wouldn’t we a a changing of muscle fiber type towards type IIx? Is there a “preference” for type IIa fibers when the body is in a trained state?

It IS Possible To Become More Explosive without Changing Muscle Fiber Type

This lack of evidence, that changing muscle fiber types towards type IIx is possible, is a bit confusing indeed. It shouldn’t, however, be a source of concern for those simply looking to improve their explosiveness.

Researchers examined hammer throwers and determined that the hypertrophy of their type II fibers were responsible for their performance in the sport. It’s not their muscle fiber type proportions, but simply the size of their muscle fibers that make them more explosive (Terzis, G).

And how is muscular hypertrophy achieved? Through overload. Strength training and practicing hammer throws makes hammer throwers better at their sport. Referring back to the studies by Howald and Smrkolj, explosive training induced hypertrophy of type II fibers.

Why running sucks.

I had to be a jerk and post this.

When you practice something, you become better at it. Case in-point, one study took in soccer players and had them regularly train using 40-meter prints. Result? They got better at the 40-meter sprint (Tønnessen E, et. al). Nothing revolutionary here. You CAN become more explosive.

Now, if reaching an elite-level ability of sprinting is your goal, sweat and blood may not be enough.

Sprinting May Be the Red Herring of Improving Explosiveness

Whilst gathering resources when writing, I stumbled upon an interesting article that argues elite sprinting ability cannot be achieved solely through “hard work”. Essentially, the authors of this article claim that sprinting is truly a skill born out of innate talent (Lombardo MP, Deaner RO).

Innate talent is a vague term for what lay people call “genetics”. Genetics determine your chances of becoming an world-class sprinter, as determined by Lombardo and Deaner. To build upon their already well-researched article, let’s look at two more things:

First is the fact that a majority of awesome sprinters around the world are of African descent. In fact, there has been only one man, Christophe Lemaitre, of pure European descent to have broken the 10-second barrier in the 100-meter sprint. With race and descent being the theme here, genetics is a very suspicious culprit.

Second is that throwers (e.g., shot-put and javelin) have been observed to have varied muscle fiber type compositions, whereas sprinters have a very specific fiber type composition (Costill DL, et. al). Although it isn’t specified in the abstract, I believe this “specific fiber composition” is biased towards type IIx fibers.

Regular strength training and sprinting will sure develop your explosive ability as a sprinter, but unless you’re of African descent and hit the genetic (and fiber-type) lottery, don’t expect to become world-class.

In Conclusion

Changing muscle fiber types is a funky science, but it appears that changing fibers into type I and type IIa is possible. Additionally, changing fibers from type II into type I is not only possible, but established. Meanwhile, changing fibers into type IIx has yet to be observed under practical conditions.

The hypertrophy of type II fibers allows people to become stronger and more explosive. This explains why runners are tiny, and lifters/throwers/sprinters are muscular. The former transforms fibers into type I, while the latter uses hypertrophy of type II fibers on top of possibly transforming type I fibers into type IIa.

Unfortunately, the specific task of sprinting is more limited in its development than other explosive activities. “Genetics” is the cause for this, and may or may not be specifically related to the number of type IIx fibers someone has.


1. Howald H. Training-Induced Morphological and Functional Changes in Skeletal Muscle. International Journal of Sports Medicine 03.1 (1982): 1-12. Web.

2. Smrkolj L, Škof B. Factors of Success in Endurance Sports; Changing of Muscle Fiber Type. Acta Medica Medianae 52.4 (2013): 69-74. Ebscohost. Web.

3. Fry AC, et. al. Muscle fiber characteristics of competitive power lifters. Journal of Strength & Conditioning Research 17.2 (2003): 746-754. Web.

4. Fry AC, et. al. Muscle fiber characteristics and performance correlates of male Olympic-style weightlifters. Journal of Strength & Conditioning Research 17.2 (2003): 746-754. Web.

5. Jansson E, et. al. Increase in the proportion of fast-twitch muscle fibres by sprint training in males. Acta Physiologica Scandinavica 140.3 (1990): 359-262. Web.

6. Andersen JL, Aagaard P. Myosin heavy chain IIX overshoot in human skeletal muscle. Muscle & Nerve 23.7 (2000): 1095–1104.Web.

7. Tønnessen E, et. al. The effect of 40-m repeated sprint training on maximum sprinting speed, repeated sprint speed endurance, vertical jump, and aerobic capacity in young elite male soccer players. Journal of Strength and Conditioning 25.9 (2011): 2367-370. Web.

8. Costill DL, et. al. Skeletal muscle enzymes and fiber composition in male and female track athletes. Journal of Applied Physiology 40.2 (1976): 149-154. Web.

9. Lombardo MP, Deaner RO. You can’t teach speed: sprinters falsify the deliberate practice model of expertise. PeerJ 2:e445 (2014). Web.

10. Terzis G, et. al. Muscle fibre type composition and body composition in hammer throwers. Journal of Sports Science & Medicine 9.1 (2010): 104-109. Web.

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One Comment

  1. Dear Mark,

    great research on changing muscle fibers. 🙂

    Keep up the good work and teaching others to use their bodies properly and to become stronger.


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