Before we dive into the answer, there are a few things you need to know about fiber types. First, researchers generally classify human muscle fibers as Type I (slow twitch), Type IIa (fast twitch), or Type IIx (super fast twitch). As methods of fiber typing have become more precise, however, scientists now know that fiber types exist on a continuum. “We have slow fibers and fast fibers, but we also have another category called hybrid fibers that express qualities of both slow and fast,” says Scott Trappe, a professor of exercise science and director of the Human Performance Laboratory at Ball State University.
While researchers generally agree that fibers can change within their own type—IIa can convert to IIx and vice versa, for example—they’re still squabbling about whether or not we can, through training, change between Type I and Type II muscle fibers. Case in point: When Outside contacted the Journal of Strength and Conditioning to get a copy of a recently published article discussing this very question, editors said sure, we could have it, as long as we “make sure the answer is right, and the answer is NO, one cannot change inherent fiber types I to II, only within the I or II subtypes.”
But Trappe believes otherwise. “With training, you could probably change your fiber type about 10 percent,” he says. Most people are born with about a 50/50 split of slow and fast twitch muscles, he says, and it’s clear, without taking out a chunk of your muscle for a biopsy, that if you’re a gifted endurance athlete or sprinter that you were probably born with more slow-twitch or fast-twitch fibers, respectively.
The article cited above points out that researchers still have a lot of questions to answer. We don’t know whether muscle types are more malleable early in life; whether certain muscles, like the biceps, are more adept at changing fiber types than others; or, if muscles can truly change between Type I and Type II, how long it takes to do so. Most studies thus far have examined the effects of exercise on muscle fiber types over just five to six months. In addition, we still don’t know the exact mechanism behind fiber type conversion, though some researchers believe it has to do with the nerves that activate the muscles.
What does all of this mean for you? Probably what you’ve already figured out through training: You can improve your speed and endurance, but if you were born with an average amount of fast-twitch fibers, you’ll probably never sprint as fast as Allyson Felix.
THE BOTTOM LINE: Yes, you can change your muscle fiber type to become a better endurance athlete or sprinter. Researchers are still debating how much you can change through training, and whether or not the changes occur purely within the categories of slow and fast fibers (e.g. fast twitch to super fast twitch), or across slow twitch and fast twitch fibers.