When Do You Use Your Type I Muscle Fibers?

By Jenessa Connor

If your goal is to PR a marathon, climb a mountain, or crush your coworkers in the annual office plank competition, you need to understand type 1 muscle fibers and how to build them.

Of the two primary types of muscle fibers (type 1 and type 2), type 1 muscle fibers are more endurance-oriented, and are crucial to steady-state exercise, high-rep strength-training sets, and isometric holds.

But before we get into the specifics of how to target type 1 fibers in your workouts, let's first take a closer look at what they are and how they function.

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What Are Type 1 Muscle Fibers?

Compared to type 2 muscle fibers, which are larger and and more powerful, type 1 muscle fibers have long contraction times (hence their “slow-twitch" moniker). As a result, they generate less force, but are more fatigue-resistant. That's why they're your body's go-to fibers for longer-duration, steady-state activities like running and cycling, and for continuous-effort, higher-rep strength workouts like circuit training.

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What Do Type 1 Muscle Fibers Look Like?

Type 1 muscle fibers require more oxygen to produce energy than type 2 muscle fibers, so they are red. Their counterparts, type 2 muscle fibers, which are more anaerobic (oxygen-independent), are white.

"Type 1 fibers are also a bit smaller," says Breanne Celiberti, adjunct instructor in the Human Performance Department at the University of Tampa. “Upon close examination, type 1 muscle fibers have higher capillary density and oxidative capacity, as well as a smaller diameter than type 2 fibers."

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How To Train Type I Muscle Fibers

Every muscle contains both type 1 and type 2 muscle fibers, and everyone is born with their own unique ratio of them, says Celiberti. But with targeted training, you can focus and build one fiber type over the other.

To develop your type 1 muscle fibers, focus on aerobic and endurance-oriented activities and training modalities, such as:

Steady-State Training

Maintaining a steady pace for longer distances will hammer your type 1 fibers (as opposed to sprinting and interval training, which target type 2 fibers).

Resistance Training

Weave light-weight/high-rep sets into your routine to target your type 1 fibers. Research shows that performing both light-weight/high-rep and heavy-weight/low-rep sets can help maximize muscle building by working both of the primary muscle fiber types.

Circuit Training

Back-to-back sets of plyometric, bodyweight, and weightlifting exercises will tax both your cardiorespiratory system and your muscles — especially your type 1 fibers.