These main types of muscle fibers can give you an insight into how your training affects your body.
Credit: Jonathan Knowles/Getty Images

Ever wonder how certain athletes—such as soccer all-star Megan Rapinoe or CrossFit champ Tia-Clair Toomey—perform the way they do? Part of the answer may lie in their muscle fibers. More specifically, the ratio between their fast-twitch muscle fibers and slow-twitch muscle fibers.

You've probably heard of slow- and fast-twitch fiber muscles, but do you really know what they are? Below, everything you should know about muscle fibers, including how they can help some athletes lift twice their body weight and others run sub-two-hour marathons, and whether or not you should train with your muscle fibers in mind.

The Basics of Muscle Fibers

Prepare for a flashback to your high school biology class. Skeletal muscles are the muscles attached to bones and tendons that you control and contract—as opposed to muscle you don't control, like your heart and intestines. They're made up of bundles of muscle fibers called myocytes. It's generally accepted that all the muscle fiber bundles can be broken down into one of two categories: slow-twitch (aka type I) and fast-twitch (aka type II).

Understand that muscle fibers exist on a super micro level. For instance, you couldn't look at a biceps muscle and say, that's a fast (or slow) twitch muscle. Rather, "every muscle has some fast-twitch muscle fibers and some slow-twitch muscle fibers," says Kate Ligler a certified personal trainer with MINDBODY. (The exact ratio depends on things like genetics and training regime, but we'll get to that later).

The main difference between the slow- and fast-twitch muscle fibers is 1) their "twitch speed" and 2) which energy system they use:

  • Twitch speed: "Twitch speed refers to how quickly a muscle fiber contracts, or twitches, when stimulated," says athletic trainer Ian Elwood, M.A., A.T.C., C.S.C.S., CF-1, founder of Mission MVNT, an injury rehab and coaching facility in Okinawa, Japan.
  • Energy systems: There are a few main energy systems at play in your body when exercising. Namely, the aerobic system generates energy with the use of oxygen and the anaerobic system generates energy without any oxygen present. The aerobic system requires blood flow to carry oxygen to the working muscles to create energy, which takes a little while—making it the preferred energy system for lower- or moderate-intensity exercise. Meanwhile, the anaerobic system pulls from the small amount of energy that's stored right in your muscle—making it faster, but not viable as an energy source long-term. (See more: What's the Difference Between Aerobic and Anaerobic Exercise?).

Slow Twitch = Endurance

You might consider slow-twitch muscle fibers to be Cardio Kings. Sometimes called "red fibers" because they contain more blood vessels, they're incredibly efficient at using oxygen to generate energy for a really long period of time.

Slow-twitch muscle fibers fire (you guessed it!) more slowly than fast-twitch fibers, but can fire over and over for a long period of time before tapping out. "They're fatigue resistant," says Elwood.

Slow-twitch muscle fibers are mainly used for lower-intensity and/or endurance exercises. Think:

  • A marathon

  • Swimming laps

  • Triathlon

  • Walking the dog

"These are actually the muscle fibers that your body turns to first, for any activity," says chiropractic doctor Allen Conrad, D.C., C.S.C.S. of Montgomery County Chiropractic Center in Pennsylvania. But if the activity you're doing requires more power than slow-twitch fibers are able to generate, the body will recruit the fast-twitch muscle fibers instead, or in addition.

Fast Twitch = Sprints

Because the body calls on your fast-twitch muscle fibers when it needs to apply additional force, you might nickname these Power Queens. What makes them more powerful? "The muscle fibers themselves are denser and larger than slow-twitch muscle fibers," says Elwood.

In general, "Fast-twitch muscle fibers use less or no oxygen, produce power much faster, and are more easily fatigued," he says. But to really understand this type of muscle fibers, you need to know that there are actually two types of fast-twitch muscle fibers: type IIa and type IIb.

Type IIa (sometimes called intermediate, transition, or moderate) muscle fibers are the lovechild of the other two types of muscle fibers (Type I and IIb). These muscle fibers can generate energy with oxygen (aerobic) or without oxygen present (anaerobic).

These are the muscle fibers we use for short-ish, but explosive activities like:

  • CrossFit WOD Fran (a superset of dumbbell thrusters and pull-ups)

  • 400m sprint

  • A 5x5 back squat

Because lactic acid is a waste byproduct of the anaerobic system (which these muscle fibers can use for energy), recruiting these muscle fibers can result in that hurt-so-good sensation of lactic acid build up in the muscles—when your muscles are burning and feel like they can't do another rep. (Related: How to Improve Your Lactic Acid Threshold).

Type IIb (sometimes called Type IIx or white fibers, because of their lack of blood vessels) might as well be called the fastest-twitch muscles fibers. "These muscle fibers have the fastest contraction rate," says Elwood. They aren't necessarily "stronger" than slow-twitch muscle fibers, they're simply able to produce more power because they contract so fast and frequently, explains Ligler.

Fueled exclusively by the anaerobic pathway, they also fatigue most quickly. So, what type of activities call on these muscle fibers?

  • 1 rep max deadlift

  • 100m row

  • 50yd dash

When trained (and we'll get more into this below), Type IIb fibers are known for increasing muscle size and definition. (Related: Why Some People Have an Easier Time Toning Their Muscles).

What Determines How Many Slow and Fast Twitch Muscle Fibers Someone Has?

Again, every muscle has some of each type of muscle fiber. Research shows that the exact ratio is somewhat determined by genes (and, fun fact: There are some DNA tests from 23andMe, Helix, and FitnessGenes which can show you if you're genetically predisposed to having more fast- or slow-twitch muscle fibers by testing something called your ACTN3 gene). But "activity level and your choice of sports and activities can make a huge difference," says Steve Stonehouse, NASM-certified personal trainer, USATF-certified running coach, and director of education for STRIDE, an indoor running studio.

Non-trained, non-active individuals typically have about a 50-50 mix of slow- and fast-twitch muscle fibers, according to Ligler. However, power-based athletes (sprinters, Olympic Lifters) typically have upwards of 70-percent fast-twitch (Type II), and endurance athletes (marathoners, triathletes) have been shown to have upwards of 70-80 percent slow-twitch (type I), she says.

There can even be huge variation in muscle fiber types within the same athlete! "There have been documented differences in fiber type ratios between dominant and non-dominant limbs in athletes," says Elwood, which is proof that the muscle-fibers adapt based on how they're trained, he says. Pretty cool, no?

Here's the thing: you're never losing or gaining muscle fibers, exactly. Rather, during marathon training, some of your fast-twitch muscle fibers can convert to slow-twitch muscle fibers to support your training efforts. Without getting too into the weeds, this can happen because "some of our muscle fibers are actually hybrid muscle fibers, which means they can go either way," says Elwood. "It's not exactly a shift in fiber type but more of a shift from these hybrid fibers into those three main categories." So, if after marathon training you ditch your miles for boot camp classes, those hybrid fibers can shift back to fast-twitch if you start training with plyometrics, for example.

It's a common belief that age plays a huge role in muscle fiber breakdown, but that's not actually true. As you get older, you'll likely have more slow-twitch than fast-twitch muscle fibers, but Ligler says that's because people tend to spend less time lifting as they get older, so their training efforts encourage the body to convert some of the fast-twitch muscle fibers into slow ones. (Related: How Your Workout Routine Should Change As You Get Older) .

ICYWW: Research on muscle fiber breakdown by sex is limited, but what's out there suggests that women have more slow-twitch muscle fibers than men. However, Ligler notes that the difference in exercise performance between men and women comes down to hormonal differences, not muscle-fiber ratio differences.

How to Train All Muscle Fibers

As a rule of thumb, Conrad says low-weight, high-repetition strength training (barre, Pilates, some boot camps), and lower-intensity, longer-duration cardiovascular training (running, biking, rowing, assault biking, swimming, etc.) will target your slow-twitch muscle fibers. And higher-intensity, heavier-weight, low-repetition strength training (CrossFit, powerlifting, weightlifting) and higher-intensity, shorter-duration cardio and power training (plyometrics, track sprints, rowing intervals) will target your fast-twitch muscle fibers.

So, including a variety of strength and aerobic exercises in your training regime is one way to target all types of muscle fibers, he says.

Does Training for Your Muscle Fiber Types Matter?

Here's where it gets tricky: While you can train with your specific muscle fibers in mind, experts aren't convinced that focusing on muscle fiber type is necessary.

Ultimately, "the fibers just do what they need in order to make you more efficient at whatever training you're doing," says Elwood. "Your goal should be to train for your specific health or fitness or sport goal, and trust that your muscle fibers will adapt as they need to help you get there." If improved overall health is your goal, you should be incorporating a mix of strength and cardio, he adds. (See: Here's What a Perfectly Balanced Week of Workouts Looks Like)

So, can thinking about your muscle fibers help #seriousathletes meet their goals? Probably. But is it necessary for most folks? Probably not. Still, knowing more about the body and how it adapts is never a bad thing.