Do you know anyone who absolutely pulverizes a high-intensity interval training (HIIT) circuit but can barely run a mile? How about a marathoner who can’t nail a box jump? The difference might lie in the makeup of their skeletal muscles.

Your body has two types of skeletal muscle fibers: slow-twitch (type I) and fast-twitch (type II).

Slow-twitch muscles give you the stamina to nail endurance tasks like distance running. People use fast-twitch muscles in explosive actions, like sprinting, jumping, or playing a cameo in a “Fast and Furious” movie. (If you have furious-twitch muscle fibers, seek medical assistance.)

We’ll break down how the two types of muscles might support different strengths and weaknesses when you’re training or competing. You’ll also learn how to work on fast- and slow-twitch muscles to work yourself up to peak performance. Eat it, second place!

During aerobic exercises like running or swimming, slow-twitch fibers are the first to contract. But when you’re throwing around heavy kettlebells or playing a sport, you’re using fast-twitch muscles.

Think of the muscles in terms of an excitable, thick chihuahua. We’ll call him John Cena (yes, like the wrestler) for the sake of the example.

The slow-twitching muscles are John’s way of trotting around, staring at you, and expecting treats. He may not be going berserk, but Mr. Cena will provide enough energy to complete the task of getting a cookie (or seven).

Fast-twitch muscles can be broken down into two categories: Moderate fast-twitch (type IIa) and fast-twitch (type IIb or IIx).

Moderate fast-twitch muscles are thicker, contract more quickly, and wear out the quickest. These are similar to Mr. Cena’s habit of borking himself into exhaustion over the course of 5 minutes.

Fast-twitch muscles are the most powerful but aren’t especially suited to endurance. The body activates these when it has worked the moderate fast-twitch muscles as hard as it can. These are Mr. Cena’s final bork before he dreams about exactly what he spent the last half-hour doing.

One group of muscles is not “better” than the other. Instead, you can help them work together to give you a performance boost.

Training to the point of near-exhaustion, such as when doing a quick HIIT workout or plyometrics, can help you make sure your fast-twitch fibers are showing out.

If you’re looking to get swole and improve your strength, using and developing fast-twitch fibers is the only way. Add another plate to that barbell and get another few reps in. We believe in you!

On the other hand, aerobic exercises that mainly use slow-twitch fibers can increase your stamina and your muscles’ absorption of oxygen, allowing you to burn energy for longer.

The folks you see spending 45 minutes on the elliptical or stair-climber are building their slow-twitch fibers. Same goes for those who regularly leave their phone upstairs and know they’ll have a few steps to climb that evening.

A 2019 study found that people with higher proportions of fast-twitch muscles tend to have higher blood pressure. The science bods behind this research also suggest that people with slow-twitch muscles recover more quickly.Hunter G, et al. (2019). Divergent blood pressure response following high-intensity interval exercise: A signal of delayed recovery?

A 2015 review looked at loads of studies on sex differences in several species. While it examined plenty of aspects of muscle behavior, it’s particularly interesting to note the differences in the ways muscles contract in males and females.Haizlip KM, et al. (2015). Sex-based differences in skeletal muscle kinetics and fiber-type composition. DOI: 10.1152/physiol.00024.2014

Testosterone is the male sex hormone, responsible for the bass filter on a male’s voice and the hair on their dangly bits. According to the review, animals with lower testosterone (including most females) tend to develop more slow-twitch muscles and lose fast-twitch muscles, and they have lower total muscle mass.

That means females typically don’t develop the same kind of muscle strength or mass as males, but they’ll recover more quickly from exercise — so don’t feel guilty about leaving your bae on the couch to hit another spin class.

Do feel a bit guilty if you run off with the instructor. It might explain the resting twitch face.

Can you change your muscle types? It’s a good question, and researchers don’t yet know for sure.

A 2012 study suggests it may be possible to change fast-twitch fibers to moderate fast-twitch through increases in endurance or resistance training. The findings also suggest that the opposite can happen.Wilson JM, et al. (2012.) The effects of endurance, strength, and power training on muscle fiber type shifting. DOI: 10.1519/JSC.0b013e318234eb6f

A more recent 2019 study of 22 older adults in good health also found the presence of a new type of muscle fibers: hybrid fibers.Moreillon M, et al. (2019). Hybrid fiber alterations in exercising seniors suggest contribution to fast‐to‐slow muscle fiber shift. DOI: 10.1002/jcsm.12410

The study found that 22 sedentary older adults had hybrid fibers that were partway between fast-twitch and slow-twitch. These findings suggest the fibers could be in the process of transitioning, but more research is needed.

In any case, keep in mind that fast-twitch muscle depletes much more with age than slow-twitch does, so increasing muscle power becomes more of a slog later in life. Definitely get those push-ups in before retirement.

And since there’s not yet any conclusive evidence that muscle fibers can transform from slow- to fast-twitch or vice versa, we recommend training the muscles separately. As long as you have varied workouts that mix up strength and endurance, the different muscle fiber types will stay relatively well-balanced.

Sidenote: If your bank account balance needs as much training as your fast-twitch muscle fibers, fear not! We put together a handy guide to making your own gym equipment at a much lower cost.

Generally, when you force your muscles to work differently and get out of your comfort zone, you get a better workout. Keep mixing up your workouts to excite all types of muscle fibers and fight your worst fitness enemy: Boredom in the gym.

And Mr. Cena? Keep chasing those pats. Good boy.