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Fast-Twitch Muscle Fibers Are Behind All Your Speed and Power. Here’s How to Train Them

These little strings of tissue are to thank for all your workout prowess.

By Lauren MazzoMarch 8, 2024

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“Humans have the incredible ability to demonstrate physical feats across the entire speed, strength, and endurance continuum,” says Peloton Instructor Andy Speer. “The physical demands on the body of an elite wide receiver sprinting to catch a ball and an ultra-marathon runner are at the extreme opposite ends of this continuum. What gives these athletes the ability to excel at their respective sports?”

The answer: muscle fibers. These small but mighty strands of tissue inside your muscles are the key to every movement you make. And just like athletes themselves, different fibers specialize in different activities.

Learning more about the different types of muscle fibers can give your workouts a deeper meaning and help you tailor your training to your goals. If you’re dabbling in powerlifting, want to get faster at sprints, or working on higher box jumps, for example, then fast-twitch muscle fibers, especially, are about to become your best friend. Here’s everything you need to know about fast-twitch muscle fibers and how to tap in.

First, What Are Muscle Fibers?

To start, we’re putting the microscope on skeletal muscles, which are the type of muscle responsible for moving your body. Every skeletal muscle is comprised of hundreds or thousands of individual muscle fibers, which are unique in size, shape, and arrangement compared to other muscles in the body, per the National Institutes of Health (NIH). Each fiber is a cylindrical muscle cell that can contract and relax; think of it as a piece of spaghetti that’s attached to bone at either end. It’s bundled with many other spaghetti pieces and wrapped in connective tissue to form a muscle.

How Are Muscle Fibers Different From Each Other?

Not all muscle fibers are the same. There are two main types: slow-twitch (Type I) and fast-twitch (Type II), Andy explains. They differ in size, function, and where they get their energy, which makes each type suited to different kinds of athletic training. At the most basic level, slow-twitch fibers excel at endurance activities and slow, low-intensity contractions, while fast-twitch fibers provide quick, more powerful contractions and fatigue faster.

Everyone has Type I and Type II muscle fibers in their body, according to the National Academy of Sports Medicine (NASM), and all the muscles in your body contain a mix of both.

What Are Fast-Twitch Muscle Fibers?

Fast-twitch muscle fibers (Type II) create short, powerful muscle contractions like those seen in explosive exercises such as sprinting, weightlifting, or box jumps, explains Scott Cheatham, a board-certified orthopedic physical therapist on the NASM Scientific Advisory Board. “Fast-twitch fibers do what their name implies; they contract quickly and create a lot of force,” Andy adds.

There are two types of fast-twitch muscle fibers: Type IIa and Type IIx (or IIb).

Type IIa muscle fibers are your “in-between fibers,” Andy says. “They produce more force than Type I fibers, but still less than Type IIx.” They’re bigger in size than Type I and smaller than Type IIx (if we stick with the pasta analogy, consider this classic spaghetti) and they can use two different processes for energy (but more on this below). 

Type IIx (IIb) muscle fibers are the “big movers,” Andy says. “Type IIx fibers are responsible for producing the highest-intensity contractions. This means lifting heavy weights, sprinting, and jumping. Anything that requires a lot of force in a short amount of time.” They use anaerobic glycolysis to create energy without the help of oxygen, which means they can produce energy very quickly but also run out of energy quickly. They’re also the biggest of all the fiber types (think: linguine).

The Differences Between Fast-Twitch and Slow-Twitch Muscle Fibers

“Slow-twitch muscle fibers (Type I) are designed for endurance and slow muscle contractions,” explains Cheatham. They generate low-force muscle contractions but can go for a long time. “Think of your postural muscles or your soleus (the lower part of your calf)—these muscles keep you upright and moving (relatively) slowly for a long time before fatiguing,” Andy says.

Type I muscle fibers are generally smaller in diameter than Type II fibers (think: angel hair pasta), but receive more blood flow, according to NASM. That blood contains oxygen, and the continued oxygen supply allows these muscle fibers to use a process called aerobic respiration for energy. It helps provide continued energy to the muscle over extended periods.

Type II muscle fibers are generally larger in size than Type I fibers, but receive less blood flow, so they’re quicker to fatigue compared to slow-twitch fibers, according to NASM. They don’t have lots of oxygen available to use aerobic respiration for energy, so they rely on a process called anaerobic glycolysis instead. It’s a faster source of energy, but doesn’t last as long as aerobic respiration, Andy says.

Why You Should Train Your Fast-Twitch Muscle Fibers

If you’re not an elite athlete or weightlifter, you might be inclined to glaze over the importance of fast-twitch muscle fibers. But everyone can benefit from training their Type II fibers—not just for performance reasons, but for life.

“Fast-twitch muscle fibers help with activities that require strength and power,” Cheatham says. And even if you’re not powerlifting or training to sprint down a track, you should have some strength and power in your arsenal. “Preserving these muscle fibers may help individuals to perform quick, explosive movements such as quickly running across the street, quickly lifting a box, or hopping onto another surface,” he says.

“Maintaining your ability to move quickly—think about jumping out of the way of a bicycle or car, catching your fall if you trip, running with your kids in the yard—[that’s all] fast-twitch,” Andy says.

This is especially important as you age, since you naturally lose fast-twitch fibers as you get older, Andy adds. “In the same way, if you don’t ‘use’ your fast-twitch fibers even in your younger years, you lose them, or they become more slow-twitch dominant. Think about walking and sitting at your desk; slow-twitch is all you need to carry out these tasks, so you have to ask your fast-twitch fibers to work if you want to keep them around.”

How To Train Fast-Twitch Muscle Fibers

Fast-twitch fibers help with power, strength, and explosive movements, so that’s exactly what it takes to train them. That said, tapping into your Type II muscle fibers isn’t as simple as picking up a dumbbell or going for a tempo run.

“Slow-twitch fibers are always recruited first,” Andy says. Once your body realizes the physical need surpasses what Type I fibers can do, then the Type II fibers kick in. But, “until the demand of strength, intensity, or speed on the muscle is high enough, you won’t recruit your Type II fibers.”

So how do you train them? “The quick and easy answer is to go heavy and move fast,” Andy says. “This doesn’t mean you need to lift maximum weight like a powerlifter. In fact, for most of us, that’s probably not the most functional or safe way to train our fast-twitch muscles.” Here are a few ways he recommends training to tap into those Type II fibers instead:

Lift Moderate Weight with Speed

“Think of a weight you can squat for eight reps,” Andy says. On the upward portion of the movement, focus on moving as quickly as you can, especially on the last two or three reps when your muscles are tired. “This will recruit your fast-twitch fibers,” Andy says. “Even if you’re not moving as fast as you did on the first rep, if your intent is to move as fast as possible, your fast twitch fibers take the brunt of that work.”

Light Weights Moved Fast

Moving with the intent of creating velocity is a great way to tap into your fast-twitch muscles even when moving small amounts of weight (or none at all). For example, throwing a medicine ball at the wall or floor as hard as possible, Andy says.

Plyometrics, Like Jumping and Sprinting

Plyometric training involves quick, powerful movements that require an eccentric contraction followed immediately by an explosive concentric contraction, per NASM. Jump training (think: squat jumps, burpees), sprinting on a treadmill or bike, or even throwing fast punch combinations during a boxing class can all be plyometric training, Andy says. The key is to make these short (10-20 second) bouts of max effort with recovery between sets.

Conditioning Work

For example, a treadmill class that involves sprint intervals, or a Peloton Bike or Peloton Tread Bootcamp. Sprints and circuit training can train your Type IIa fibers and depending on the actual movements and speed, your Type IIx, too, Andy says. 

How To Determine Your Muscle Fiber Type

Untrained individuals have around a 50/50 split of fast-to-slow-twitch fibers, per a research review published in the Journal of Strength and Conditioning Research. Otherwise, “everyone is born with a unique ratio of muscle fibers,” Cheatham says, and it’s primarily determined by genetics.

Each muscle in your body will have a different breakdown, too. “For example, your calves and postural muscles will tend to have more slow-twitch, while your quadriceps and pecs tend to have a higher ratio of fast-twitch,” Andy explains.

Here’s where it gets interesting: “This congenital ratio can change over time due to different types of physical activity and physical training,” Cheatham says. Research has shown that different muscle fiber amounts are common in different athletic populations. “For example, Type II muscle fibers may be more prominent in power athletes (e.g. football) or weightlifters, and Type I may be more prominent among endurance athletes such as long-distance runners or cyclists,” he says. The aforementioned research review found, for example, that endurance athletes can have anywhere from 60 to 95 percent slow-twitch muscle fibers, while elite power athletes like sprinters or weightlifters have 60-80 percent fast-twitch.

Depending on your athletic history, you may be able to estimate your muscle-fiber ratio from that alone. If you want to get more exact, the gold-standard method is to get a muscle biopsy—but it’s invasive, expensive, and inaccessible for the average person.

Research suggests you can roughly judge your dominant muscle fiber type by performing an exercise using a weight that’s 80 percent of your one-rep max (1RM, the heaviest weight you can lift for one single rep of a given exercise). For example, one study had participants do back squats using 80 percent of their 1RM, and found that if you can do five to eight reps with that weight, you’re likely to have more fast-twitch muscle fibers in your quads than if you can do 11–15 repetitions. If you can do 9 or 10 repetitions, you likely have an equal proportion of fast- and slow-twitch muscle fibers. If you’re used to lifting heavy, you may be able to try this strength test on your own; otherwise, seek out an expert to make sure you’re staying safe.

Does Training Change Your Muscle Fiber Type?

Yes, to an extent. “By training endurance sports, your slow-twitch muscles will be used more, and therefore develop the capacity to function at a higher level and go for longer,” Andy says. “The same is true for your fast-twitch fibers.”

The truth is, in addition to Type I, IIa, and IIx/b muscle fibers, research shows that our muscles also contain hybrid fibers, which express characteristics of multiple types. “This is where the magic happens,” Andy says. “If you train for strength and power, the hybrid fibers will become more fast-twitch dominant. The same holds true for slow-twitch. So even if you have a higher ratio of fast or slow, the hybrid fibers react to the type of training you do.”

The Role of Fast-Twitch Muscle Fibers In Fitness

Training Type II muscle fibers can help you become stronger, faster, and more explosive—it’s a pretty sexy promise. But that doesn’t mean they’re “better” than Type I fibers or that they should become the focus of all your training. You need both Type I and II fibers to move through your life, and there’s a lot more that goes into a muscle’s function and performance than the fiber type. 

“At the end of the day, the point is not to train muscle fibers, it is to train your strength and power,” Andy says. “You’re training for your ability to move weights and move yourself quickly! So keep lifting, throwing, jumping, sprinting as best you can and you’ll keep your fast-twitch fibers firing for you.”


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