Whether your goal is to run a marathon, climb a mountain, or crush your co-workers in the annual office plank competition, you need to understand type 1 muscle fibers and how to build them.

Of the two main types of skeletal muscle fibers, type 1 muscle fibers (aka “twitch” muscle fibers) are more endurance-oriented and are essential for steady-state exercises, high-repetition 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 look at what they are and how they work.

What are type 1 muscle fibers?

Compared to type 2 muscle fibers, which are larger and stronger, type 1 muscle fibers have long contraction times (hence the nickname “slow twitch”). As a result, they generate less force, but are more resistant to fatigue.

That’s why your body’s fibers are good for longer-duration, steady-state activities like running and cycling, and for sustained effort and high-repetition strength training, like circuit training.

How to train type 1 muscle fibers

Every muscle has type 1 and type 2 muscle fibers, and each is born with its own unique ratio, says Breanne Celiberti, MS, former assistant professor in the University of Tampa’s Department of Human Performance. But with targeted training, you can improve and build one type of fiber over another.

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

1. 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).

2. Resistance training

Incorporate light weights and high rep sets into your routine to target your type 1 fibers. Research also shows that performing light/high-rep and heavy/low-rep sets can help maximize hypertrophy (muscle building) by working both primary muscle fibers.

3. Circuit training

Plyometric exercises, such as bodyweight and weightlifting, will tax your cardiorespiratory system and muscles, especially type 1 fibers.

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, are more anaerobic (oxygen independent), white.

“Type 1 fibers are also a bit smaller,” says Celiberti. “When examined manually, type 1 muscle fibers have a higher capillary density and oxidative capacity, as well as a smaller diameter than type 2 fibers.”