Your Guide to Fast-Twitch Muscle Training

When you’re in the gym, you want to be all those things. But by nature, you’re better at some athletic activities than others. You see this in children too: The children who do better in the mile run aren’t the same ones who crush a sprint where they have to change direction often. (The opposite is often true as well.)


Why? Because of muscle fibers, particularly the concentration of fast-twitch muscle fibers. Your fast-twitch muscle fibers, also known as Type II fibers, are the fibers responsible for explosive movements, things like vertical leaps, 40-yard sprints, and exercises like power cleans and dumbbell snatches.

Your body has a limited number of Type II muscle fibers, and research has shown that that’s somewhat determined by your genetics. With smart training, however, you can, to some extent, alter some of your other muscle fibers to essentially function as fast-twitch muscle fibers.

Looking to beef up your bench press, improve your sprint, or even dominate the cardio row? Then you want to create hone your explosive ability by developing as much fast-twitch muscle response as possible. And I’m going to show you how to do exactly that. Like Liam Neeson in Taken, your muscles were born with a particular set of skills. You can’t change those, but smart training can push your body where you want it.

What’s in a muscle?

A muscle is essentially a bundle of . . . bundles. Let’s start with muscle fibers. A bunch of muscle fibers are bundled into fascicles, which are then bundled together. A bundles of fascicles essentially comes together to form, say, your biceps, or any other larger muscle you can think of.

The best way to think about muscle composition is to think of a ponytail. That’s hundreds of thousands of tiny strands bundled together, but, by and large, all those hairs move together. The job of any one muscle (say, flexing your elbow, which is what your biceps do), is completed by those smaller muscle components working together.

Fast-twitch fibers

SWe’ve been over these, and they’re the reason you’re reading this story. These are your explosive fibers, and they generally anaerobic power, without using oxygen.

There are two types of fast-twitch fibers: Type IIA and Type IIX. Type IIA fibers are intermediate, and help accommodate more intermediate capacity. They can also replicate some aerobic qualities of slow-twitch muscle fibers, although they still deliver higher power output than your slow-twitch fibers. Type IIX fibers are more powerful, but they’re found more frequently in animals, such as lions, than in humans.

Anything explosive is driven by your fast-twitch fibers. Think of a bench press, where you fire the weight up as quickly as you can (even if it’s moving slowly), or a power clean, where you’re exploding the weight up swiftly, or the explosive first steps of a sprint or speedy first minute on the Airdyne bike.

In general, you can’t change the quantity of Type II fibers, so you can’t hone your explosive ability that way. But stick with me for another few paragraphs, because you can train to become a more “fast-twitch athlete." And that will make you better at exercises like the classic dumbbell bench press.

Slow-twitch fibers

Slow-twitch fibers, also known as Type I fibers, have a high resistance to fatigue, and a high aerobic capacity. Each slow-twitch fiber has more oxidative enzymes than fast-twitch fibers, as well as more mitochondria and more capillaries. That means higher aerobic function, which can power you through miles upon miles of long-distance run or an hour-long spin class. Any endurance event is relying on Type I muscle fibers.

Hybrid muscle fibers

A good percentage of your muscle fibers are hybrid muscle fibers, meaning they sit between Type I fibers and Type II. Muscle fibers can gradually change, based on your activity level. If you’re inactive, these hybrids tend to lie between slow-twitch and Type IIA fast-twitch, or between the two brands of fast-twitch, Type IIA and Type IIX.

Your Fast-Twitch Muscle Training

Research shows that it’s easier to train fibers toward Type II (fast-twitch) than Type I. However, there isn’t much research into the specific training protocols that can push you toward one type of fiber. Trainers are still figuring that out.

Your best bet, very often, is to train as explosively as possible. The trick, however, is that your body can only train explosively so often, so you have to space out that brand of training properly. Think of taking at least two days of rest between explosive workouts geared specifically toward shifting hybrid fibers into Type II territory.