Everything to Know About Slow- and Fast-Twitch Muscle Fibers

December 02, 2019 by Shape

Ever wonder how certain athletes—such as soccer all-star Megan Rapinoe or CrossFit champ Tia-Clair Toomey—perform the way they do? Part of the answer may lie in their muscle fibers. More specifically, the ratio between their fast-twitch muscle fibers and slow-twitch muscle fibers.

You've probably heard of slow- and fast-twitch fiber muscles, but do you really know what they are? Below, everything you should know about muscle fibers, including how they can help some athletes lift twice their body weight and others run sub-two-hour marathons, and whether or not you should train with your muscle fibers in mind.

The Basics of Muscle Fibers

Prepare for a flashback to your high school biology class. Skeletal muscles are the muscles attached to bones and tendons that you control and contract—as opposed to muscle you don't control, like your heart and intestines. They're made up of bundles of muscle fibers called myocytes. It's generally accepted that all the muscle fiber bundles can be broken down into one of two categories: slow-twitch (aka type I) and fast-twitch (aka type II).

Understand that muscle fibers exist on a super micro level. For instance, you couldn't look at a biceps muscle and say, that's a fast (or slow) twitch muscle. Rather, "every muscle has some fast-twitch muscle fibers and some slow-twitch muscle fibers," says Kate Ligler a certified personal trainer with MINDBODY. (The exact ratio depends on things like genetics and training regime, but we'll get to that later).

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