There are two different schools of thought when it comes to working out. One says that you should go fast and get as many reps in as possible (HIIT workouts, for example). The other says that low-and-slow is the name of the game (like Pilates). With fast, explosive movements in HIIT, you activate fast-twitch muscle fibers, but slow-twitch muscle fibers activate when you’re doing a Megaformer workout, distance running, or lifting heavy weights, and these slow-twitch muscle fibers are the ones in charge of your endurance.

“Fast-twitch muscles fire quickly, exerting a lot of power, but can also tire very quickly,” says Abbie Rosser, co-owner of Urban Lagree fitness studio. “Slow-twitch muscles take a longer time to burn out, so building on these and making them more efficient will significantly improve endurance.” Keep scrolling to learn why slow-twitch-activating exercises are important to improve your endurance.

The connection between slow-twitch muscle fibers and endurance

First up, some biology. While fast-twitch muscles work in anaerobic situations (aka you’re holding your breath to get 60-second squat series o-v-e-r), slow-twitch muscles are responsible for oxygen generation within the muscle fibers. “This oxygen is fuel for continuous exercise, so that there’s less lactic acid buildup, meaning that you can go for extended muscle contractions in a longer amount of time,” says Amoila Cesar, a Beachbody trainer and NBA strength coach. The fibers are actually a deep red in color, which Cesar explains is from oxygenated blood within the muscles and a high density of capillaries. “This is compared to fast-twitch muscle fibers, which are more pink,” he says.

Because your slow-twitch muscles contract more slowly than your fast-twitch muscles, they have a “very high aerobic capability,” says Edward Laskowski, MD, a physiatrist and co-director of Mayo Clinic Sports Medicine. And that means that they’re better equipped for the long haul. “[Slow twitch muscles] are more resistant to fatigue, which is where long distance and endurance runs come in,” says Cesar.

The benefits of slow-twitch muscle fiber workouts

While it’s important to work both fast- and slow-twitch muscles, if you’re looking to better your endurance, focusing on slow-twitch muscles will pay off in a big way. “You can build up your endurance by doing slow-twitch workouts—you’ll be more efficient,” says Cesar. “Even if you’re an endurance runner, it’s important for you to get into activating slow-twitch muscles through weight training, which gives you the most performance out of these muscles.”

What happens with any type of slow-twitch muscle work is that it gets your muscles acclimated to endurance. “For example, once you get past 10 reps with weights, you’re then fighting through lactic acid, which is the burn that you feel,” says Cesar. “This activates your aerobic energy system, which requires longevity to push through. Even when you’re running, you’re fighting through that lactic acid burn—that’s half of the battle.” So that’s all the more reason to sign up for Megaformer-style classes and other slow burn-style workouts.

Improve your endurance through slow-twitch muscle building movements

1. Long distance running: “If you run over three miles, you’ll start to activate those slow twitch muscles,” says Cesar. So start to increase your distance.

2. Isometric workouts: Cesar explains that with isometric contractions, your muscles are working against gravity. Do exercises like the hollow hold, a wall sit, a plank, or simply pause at the bottom of strength training exercises (like squats).

3. Circuit training: “Alternate from exercise to exercise with little to no rest,” says Cesar. This is something you can do with weights or strength training. The lower your rest time, the more your endurance will be challenged.

4. Light weights, high reps: According to Cesar, heavy lifting activates your fast-twitch muscle fibers because they require more force. “That’s why we recommend lighter weights for your endurance work,” he says. Then you can rep them out for longer.

You could also try this lower body slider workout to build endurance: 

This is what a corrective exercise specialist wants you to know about eccentric exercise (which gets you stronger). And here’s how compound exercises increase your strength, too.