How Do Muscle Fibers Determine How Fast You Run?

The influence of genetics and training or activity is a subject of much controversy in the scientific community.

It seems almost obvious that if you plan to go very fast for a short amount of time, you’d want plenty of fast-twitch muscle fibers. And if you hope to run slower for a longer amount of time that would require slow-twitch muscle fibers. But, the logistics of how these muscle fibers work and how much control you have over them is actually far less obvious.

On average, people are born with about 50 percent slow-twitch muscle fibers and 50 percent fast-twitch. How much that varies depends primarily on genetics, said Scott Trappe, a professor of exercise science and director of the Human Performance Laboratory at Ball State University – and only a little on training. If you plan on being a world-class sprinter, you better “choose your parents wisely,” he joked.

Everyone is born with multiple types of muscle fibers: Type I (also known as slow-twitch), Type IIa (intermediate fast-twitch), and Type IIx (super fast-twitch). About one-third of the fast-twitch muscles are typically made up of the intermediate fibers, said Jorge Zuniga, an assistant professor in the Department of Exercise Science at Creighton University.

Fast-twitch muscle fibers are larger than slow-twitch fibers. “They actually look different,” said Zuniga. Those larger fibers are capable of more power and velocity, but are understandably not particularly efficient. They mostly utilize sugar for energy and the super fast-twitch fibers use a solely anaerobic energy process.

Slow-twitch fibers, on the other hand, are not as powerful, but far more efficient. They are needed to run long distances, primarily burning fat aerobically – meaning these fibers require oxygen to create energy.

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However, with 600,000 fibers in the average 25-year-old’s thigh, it’s hard to classify every muscle strand into those distinct categories. “It really exists on a continuum,” says Trappe, and there’s a significant amount of overlap, with many fibers having some of both characteristics.

It is those hybrid-like fibers that are easiest to change through training.

How much your muscle fiber make-up is determined by genetics and how much it is determined by training or activity is a subject of much controversy in the scientific community. While nearly everyone acknowledges that fibers can be converted within type (ie. from Type IIa to Type IIx), many argue that there’s simply no way to truly convert from one type to another type.

And, even if you can convert fast-twitch muscles to slow-twitch within a couple months through long slow runs (or simply through the process of aging), it’s less clear if you can convert slow-twitch muscles to fast-twitch. “It’s irreversible as far as I know,” said Zuniga.

But, Trappe said that multiple experiments have demonstrated that some of those hybrid slow-twitch muscles can convert to fast-twitch “within reason.” Extensive bed rest, as shown with astronauts and the injured, causes muscles to lose their endurance capabilities and change to fast-twitch fibers.

That doesn’t do the average athlete much good, but the principles have been used in training to similar effect. In the same way that long, slow runs convert fibers to slow-twitch, shorter, faster intervals interspersed with periods of rest (or inactivity following the workouts, up to a week) can convert some hybrid fibers to fast-twitch.

The conversion possible through training, however, is probably only about 10 percent of the total muscle fibers, said Trappe.

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Since world-class sprinters typically have 65-85 percent fast-twitch muscle fibers – and vice versa for world-class distance runners and slow-twitch fibers – it’s unclear how much they were able to create that biological advantage and how much they were simply born with it. Elite runners typically have far fewer hybrid fibers, said Trappe, because they’ve often trained them one direction or the other through repeated activity. But, if you happen to be born with more slow-twitch muscles, it’s likely that you’d be better at long-distance races, and then choose to do those and train for them.

Similarly, if you happen to be born with more fast-twitch muscles, you might naturally find yourself speedier and lean toward sprinting. “You can still run a marathon, but you’re probably not going to be the winner,” said Trappe.

Of course, that doesn’t mean you should simply curse your genetic fate and throw in the towel based solely on your muscle make-up. “It’s only one component,” said Trappe.

A study done in the mid-1970s, said Zuniga, took two groups of high-level runners – one group with 70 percent slow-twitch muscle fibers and the other with 85 percent slow-twitch – and had them run a 10K. On average, the two groups performed equally well.

“Fiber type is only a percentage of your performance,” said Zuniga. That percentage is approximately 40, as some studies have shown.

The takeaway: Don’t forget about the other 60 percent.

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About The Author: 

Kelly Dunleavy O’Mara is a journalist/reporter and former professional triathlete. She lives in the San Francisco Bay Area and writes for a number of magazines, newspapers, and websites. You can read more about her at www.sunnyrunning.com.

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