There’s a common misconception that you’re doomed to a certain muscle fiber type for life, but the opposite is true. You can get jacked or be diesel even if you have slow-twitch or fast-twitch muscle fibers right now.
For the uninitiated, your muscles are made up of thousands of individual muscle fibers. “Think of it as a ponytail,” says Dr. Andy Galpin, an exercise physiologist who has spent more than 20 years studying human performance. Just as a ponytail is made up of thousands of tiny hairs, he explains, a single muscle is made up of thousands of muscle fibers.
Physiologists separate muscle fibers into categories based on their appearance and the speed with which they contract. Slow-twitch muscle fibers (also known as “type I,” or “MHC I”) are responsible for longer-duration, lower-effort activities like jogging and cycling. MHC I’s are more fatigue resistant, but they’re not as strong and don’t grow as much or as easily as other fibers. By contrast, fast-twitch muscle fibers (also known as “type-2,” or “MHC II”) are primarily responsible for short-duration, high-effort activities like strength training and sprinting. MHC II’s burn out fast, but they grow like crazy.
For a long time, gym bros and science types alike have believed that fiber type is fate: you have primarily one or the other, and you’re stuck with the fibers you were born with. Someone with a long, stringy physique will never build much muscle and someone with a thick, burly body will never achieve a high level of endurance. But recent research—much of it coming from Galpin’s lab at CSU Fullerton, has amended some of these assumptions and flipped others entirely on their heads.
First, there are, in fact, at least three distinct types of muscle fibers: slow, fast, super-fast (“MHC IIx” fibers), and three additional types of intermediate, or hybrid, fibers that are part slow and part fast. This research goes back as far as 1971, which shows how slow gym culture can be to catch up with science.
Second: Every muscle is a mix of both fast, slow, and all manner of intermediate fibers, and that mix varies not only from person to person but from muscle to muscle as well. A college high-jumper’s quads—which she’s been using to blast herself over the bar in track meets since she was 12—may contain 50 percent fast-twitch fibers. But her soleus muscles (located in the calf), which fire daily when she stands, jogs, and walks around, may only be 20 percent fast-twitch. As even newbie lifters know, individual muscles respond and adapt to the specific tasks demanded of them. That seems to be true of individual muscle fibers as well.
Which brings us to the final point: “All muscle fiber types change with training—and rather quickly,” Galpin says. Remember those hybrid fibers we were just talking about? They are particularly susceptible to change. The more sedentary you are, the more of your fibers idle in hybrid mode; the more you train, the more of those hybrid fibers hone in on specific tasks.
Research on astronauts showed that their muscles reverted to hybrid state in just 14 days in zero gravity. And in a striking 2018 case study on identical twins—one an avid long-distance runner, the other mostly sedentary—Galpin found that nearly all of the runner’s muscle fibers were slow twitch. By contrast, 25 percent of his genetically identical twin brother’s fibers remained unassigned.
So, how do you make those hybrid fibers jump into the action, either as big-and-strong fast-twitch fibers, or lean and enduring slow-twitchers? Easy enough: Get those fibers in the game and make them figure it out. Want to be big and strong? Lift. Want to be a runner? Run.
This isn’t to say that genetics have no role in muscle building: There will always be people who gain muscle size, strength or endurance more easily than others. But if you want to make a change—from thin to muscular, or from chubby to lean—your muscle fibers, millions of them, are lying in wait, like eager cadets, willing and able to help you make the switch. All you have to do is lead the charge.
