Keeping your fast-twitch muscle fibers well-conditioned can help you stay injury-free as you age.
Your muscle fibers may not be top of mind when you’re programming your weekly workouts. Instead of focusing on the tiny cells in your body, you likely consider big-picture elements such as your fitness goals, favorite movements, desired intensity, and time available.
But taking your muscle fibers, specifically the fast-twitch variety, into account when developing your workout plan can help you prevent injury, perform with more power, and support your metabolism as you get older. Here’s why training your fast-twitch muscle fibers is so important — and what you can do to keep them healthy and strong throughout your lifetime.
Muscles Fiber Types, Explained
All of the muscles throughout your body are made up of multiple types of fibers that contract at different speeds. Slow-twitch muscle fibers (aka type I fibers) contract slowly and are fatigue resistant, so they support you during endurance workouts such as long-distance running and cycling, according to 2021 research published in the journal Sports. On the other hand, fast-twitch fibers (aka type II fibers) contract quickly and produce greater force than slow-twitch fibers, so they come into play during plyometrics, sprints, and heavy lifting, but they tire fast, per the journal.
Fast-twitch fibers can be further broken down into type IIa fibers — which contract faster than type I but aren’t so fatigue-resistant — and type IIx fibers, which are the fastest muscle fibers and tire the quickest Some muscle fibers are also hybrid varieties of multiple fiber types (think: type I and IIa, type IIa and IIx), according to the Sports research.
Research findings suggest that muscle fibers can shift to different fiber types (e.g. from slow- to fast-twitch) or from hybrid to pure fibers, depending on your training. But in general, certain muscles will have a greater proportion of one fiber type, depending on their function, says Michael Crandall, CSCS, PN2, a Tier X trainer at E by Equinox - Hudson Yards.
The gastrocnemius muscle in your calves, for example, helps generate force for jumping and running — activities that require quick, powerful contractions. In turn, it tends to be composed mostly of fast-twitch muscle fibers, he explains. On the flip side, the muscles that help you maintain an upright, healthy posture (such as the rhomboids, levator scapula, and serratus anterior) tend to be predominantly made of slow-twitch muscle fibers, as they need to work for long periods of time. “They're endurance muscles,” says Crandall. “You're using them all day just to stand up, so they’re going to be really strong.” Generally, slow-twitch muscle fibers tend to be smaller in size than fast-twitch muscle fibers, he adds. That’s why people who frequently recruit their gastrocnemius muscles in their workout or sport will often have large, defined, and round calves, says Crandall.
The Importance of Training Your Fast-Twitch Muscle Fibers
Fast-twitch muscle fibers help generate power, or the ability to produce the most amount of force in the shortest amount of time, says Will Waterman, PT, DPT, CSCS, the head of performance and sports science at Proteus Motion. But after age 30, the amount of power you can generate declines about 9 percent per decade, largely due to a loss of muscle mass and a decreased nerve conduction velocity (how fast electrical signals move through your nerves), according to results from Proteus’ research. Research from 2022 also shows that the size and contractile function of fast-twitch muscle fibers declines with age.
Essentially, a loss of fast-twitch muscle fibers hampers your ability to jump, sprint, and stay injury-free and fully functional, says Crandall. “Power is what’s going to allow you to react fast enough so that you don't hurt yourself,” adds Waterman. If you were to trip on a curb, power is what enables you to quickly step your foot in front of you and stop yourself from toppling over, he explains. “Fast-twitch muscle fibers even help you with little things like opening up a jar or going up the stairs,” says Crandall. “In order to maintain your ability as a functional adult, you need your fast-twitch muscle fibers.”
Plus, strong fast-twitch muscle fibers can keep you safe and help you perform your best during your sport of choice. Their resulting power can help you find your footing and stay upright when an opponent runs into you during a soccer game — and react fast and forcefully enough to steal the ball and kick it to a teammate.
Recruiting your fast-twitch muscle fibers can also impact your metabolism. Slow-twitch muscle fibers are generally aerobic, meaning they use carbohydrates and/or fat as fuel, while fast-twitch fibers are anaerobic, so they use only carbohydrates as fuel, says Crandall. Research shows that during prolonged exercise at low intensities (about 35% of VO2 max), skeletal muscle gets most of its energy from the oxidation of fatty acids; carbohydrates are utilized more as the intensity increases, and glucose becomes the main fuel source when you’re working at 95% of your VO2 max.
As you age, training your fast-twitch muscle fibers can help you stay metabolically balanced, says Crandall. “Your body switches into an aerobic machine over time, and part of that is because the slow-twitch muscle fibers will remain intact,” he explains. “[But] we want to be able to burn fat consistently and we want to use carbohydrates [through fast-twitch training].”
How to Train Your Fast-Twitch Muscle Fibers
Building up and maintaining your power — and thus your fast-twitch muscle fibers — as you age isn’t too daunting of a task, as it’s fairly responsive to training, says Waterman.
No matter your age, do a workout that incorporates power-focused exercises once a week, says Crandall. Practice medicine ball throws in every direction: toss it up over your head, slam it to the ground, and hurl it against the wall at your side, he suggests. Try to jump as high or far as possible and hop out to your sides, and run through ladders with your feet moving lightning-fast, he adds. With his own clients, Crandall has people squat as fast as they can (with proper form and under supervision, of course).
Outside of power training, aim to perform resistance training with relatively heavy loads two to three days a week, targeting major muscle groups such as your pecs, lats, quads, and deltoids, Crandall recommends. “For the most part, resistance exercises are going to train your fast-twitch muscle fibers,” he notes. “A good resistance training program is largely going to be the same 20 to 30 exercises where you kind of work with different weights.” You might perform weighted squats, lat pull-downs, lunges, and pull-ups, for example.
Performing these movements, particularly the power-focused exercises, may feel awkward at first if you’ve neglected your fast-twitch muscle fibers for a while. But soon enough, says Crandall, you’ll re-establish those mind-muscle connections and be able to move quickly and with ease.