They're responsible for high-intensity exercise like sprinting and heavy lifting—but only for short periods of time.

By Tiffany Ayuda
Updated June 15, 2020
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Maybe you're a long-distance runner who struggles with sprints; or you're a regular on the elliptical who avoids heavy lifting at all costs—both are fantastic ways to maintain fitness, but if you want to get stronger and faster (and keep your body healthy in the long run), you may want to consider training in a new way, by focusing on different muscle fibers—specifically your fast-twitch muscle fibers.

From your biceps and triceps to your quads and glutes, your skeletal muscles contain two different types of muscle fibers—slow-twitch (type I) and fast-twitch (type II)—which differ in the type of energy they produce. Here's what you need to know about your fast-twitch muscle fibers—including how to properly train them, and why they're essential not only for your fitness routine but overall health.

What are fast-twitch muscle fibers?

In the simplest terms, fast-twitch (type II) muscle fibers are built for short, powerful bursts of energy—that's in contrast to slow-twitch (type I) muscle fibers, which are built for endurance activities like long-distance running or biking. "Type II fibers are needed for high-intensity work, such as heavy lifting or sprinting," Dan Giordano, PT, DPT, CSCS, co-founder of Bespoke Treatments Physical Therapy in New York City, tells Health. "They contract and fatigue quickly, synthesizing energy through the anaerobic [without oxygen] process." That's different than slow-twitch muscles, which rely on aerobic [with oxygen] respiration, meaning through the steady intake of oxygen, exercise can be sustained for a longer amount of time.

To know why you can't sustain high-intensity exercises that fire up your fast-twitch muscle fibers for very long, we have to get into a little of the nitty-gritty as to how those muscles work in general. Overall, we get energy from the food we eat, but it's not a direct relationship—that energy we get from food is converted to a chemical compound called adenosine triphosphate (ATP), which is the immediate usable form of energy for the cells in our bodies, according to the American Council on Exercise (ACE). While the body stores a minimal amount of ATP within muscles already, most of the ATP available to our bodies is made when needed by one of three energy systems: phosphagen, glycolytic, and oxidative.

The phosphagen system—aka, the ATP-PC system, which uses phosphocreatine (PC) or a high-energy phosphate—is activated first when you use your fast-twitch muscle fibers, and ultimately gets depleted first. Per the ACE, the ATP-PC system allows for up to 30 seconds of maximum effort, and once that's depleted, the glycolytic system kicks in, which uses the energy in glucose to form ATP. The glycolytic system can supply ATP for a longer amount of time, from 30 seconds to three minutes, and after that, the oxidative system, which relies on fats and carbohydrates, kicks in (this system is more closely associated with slow-twitch muscle fibers).

You can only maintain high-intensity activity for so long because activity from fast-twitch muscles that lasts longer than 30 seconds results in the accumulation of lactic acid, or an increase of acid in the muscle cells, which can lead to pain and discomfort, says Eric Sternlicht, PhD, associate professor of health sciences and kinesiology at Chapman University in Orange, CA. You need adequate recovery between these exercises to clear out the lactic acid in your muscles.

But that's not all: fast-twitch muscle fibers also broken up into two different types: type IIa and type IIx (formerly known as IIb). "Type IIa fibers are used for more sustained power activities, such as repeated heavy lifts below maximum weight or a 400-meter sprint," Giordano says, noting that type IIx muscle fibers—used for max lifts and shorter sprints (about 40 yards)—fatigue faster than type IIa. Sternlicht likes to think of type IIa muscle fibers as the intermediate between slow-twitch and type IIx fast-twitch muscle fibers. “Type IIa muscle fibers are more fatigue-resistant than type IIx, but more fatigable than slow-twitch muscle fibers. They also produce more force than slow-twitch muscle fibers but less than type IIx,” he says.

Everyone has both types of fast-twitch muscle fibers, but some people may have more of one than the other, depending on how you train and the type of activities you do. "For example, if you do more endurance training, then you’ll have more type IIa muscle fibers than type IIx, but if you play sports and do more power-based exercises with explosiveness, then you’ll have more type IIx muscle fibers,” Sternlicht says. “Having more of one type versus the other isn’t a bad thing. It just means it allows you to compete in a certain way,” he explains.

How do you know if you have fewer fast-twitch muscle fibers?

Most people are born with 50% slow-twitch muscle fibers and 50% fast-twitch muscle fibers. “Only elite strength or power athletes might have 80% type II muscle fibers and endurance athletes have about 90% type I. They have more homogeneous fiber distributions from birth and that is what allows them to excel in their respective sports,” Sternlicht says.

Giordano says genetic testing and a muscle biopsy are the most accurate ways to test muscle fibers, but since that isn’t an option for most people, there are two tests he refers to for muscle fiber composition. One is the vertical jump test. “Tell someone to perform a max vertical jump without taking a step. If they have a short range and explode, they probably have more type II muscle fibers, and if they have more type I, then they might dip lower and have a slower transition,” Giordano says.

Another test is the Dr. F. Hatfield test: “Determine the one-rep max, then rest 15 minutes and perform a bench press at 80 percent of the one-rep max,” Giordano says. If you complete less than seven reps, you are probably fast-twitch dominant; seven or eight reps means you have mixed fibers; and completing more than eight reps indicates slow-twitch dominant.

Also important: While there are no differences in fiber distribution between men and women, hormonal differences do give men more of an anabolic environment that allows them to have larger and more powerful muscles than women, Sternlicht says. “Women can certainly train their type II fibers to get stronger and firmer by recruiting them in workouts. This gives their muscles appear more toned and firm than flaccid and flabby. That is not to say that it is fat but rather, untoned and untrained muscles,” he says.

Should you work on strengthening your fast-twitch muscle fibers?

Yes; just like the way your skin wrinkles and sags with age, your muscles take a hit too. They become weak, shrink, and are fatigued easily. That’s why it’s so important to incorporate more strength training into your routine. Muscle fiber loss, also known as sarcopenia, usually starts to happen after age 30, where you can lose as much as three to five percent of muscle mass per decade, according to Harvard Health.

While there are many contributing factors for muscle loss due to aging—hormonal changes, nutrition deficiencies, and chronic illnesses—the biggest cause of muscle loss is due to inactivity. Sarcopenia can put you at greater risk for falls and fractures, and can cause limited mobility, but what is particularly unique about sarcopenia is that it greatly affects your fast-twitch muscle fibers more than your slow-twitch ones. “Muscle fiber loss from aging is primarily in the type IIx fiber population, as these fibers are only used with high-intensity exercises or explosive activities,” Sternlicht says. “Since older people tend to either be sedentary or train inappropriately, they seldom recruit their type IIx muscle fibers and with time, they are lost,” he says, adding that this muscle loss can begin as early as around age 25.

Muscle fiber loss in general is due to a loss in motor neurons, or communication cells that send signals to muscles, making them perform. This results in the loss of nerve supply to the muscles, which is often referred to as denervation,” Giordano explains. When a muscle fiber loses its nerve supply, Giordano says it goes through a process called apoptosis. Because of this, these fast-twitch muscle fibers start to receive nerve supply from different motor neurons, usually from slow-twitch muscle fibers, meaning they begin to take on characteristics of slow-twitch muscle fibers. “Data suggests that a 60-year-old has about 25 to 50 percent fewer motor neurons than a 20-year-old. It’s important to train fast-twitch muscle fibers in order to slow down the denervation of the fibers,” Giordano says. “A decrease in these fibers not only decreases your strength and power, but it also increases your risk of injury and negatively affects your body composition,” he explains.

In fact, a May 2013 review in Current Opinion in Clinical Nutrition and Metabolic Care shows that fast-twitch muscle fibers are more vulnerable than their slow-twitch counterparts for atrophy and degeneration. “If you’re not recruiting your fast-twitch muscle fibers, then you’ll eventually lose them. With age, most people aren’t doing enough power-based exercises that recruit these muscles,” Sternlicht says. This goes for people who stick to one mode of training too. For example, if you do interval work only on the treadmill and completely avoid or under-train the upper body, then you’ll lose fast-twitch muscle fibers in your upper body, Sternlicht says. “The bottom line is if you don’t stress these muscle fibers through workouts, you eventually lose them,” he says.

How can you train your fast-twitch muscle fibers?

Giordano and Sternlicht both recommend sprinting and powerlifting, or resistance training with max repetition and weight, to strengthen and build fast-twitch muscle fibers. “If a person prefers running or other cardiovascular activities, then doing intermittent sprints, hill repeats, fartlek runs (in which you sprint to a pole or tree in intervals), can help recruit the type II fibers,” Sternlicht says. Plyometric exercises, such as box jumps and burpees can also ignite these power-producing fibers. “Work on explosiveness," says Giordano. "Think short bursts, go hard, and lift heavy."

If lifting heavy is a little intimidating or new for you, Sternlicht says that you can also use lighter weights and train at a faster velocity with them during the concentric phase (think: pushing weights up—in a bicep curl, that would be lifting the dumbbell toward your shoulder) of each rep and completing eight to 12 reps. According to an August 2019 study in The Journal of Strength and Conditioning Research, resistance training that involves performing reps at faster velocities can help older adults maintain and strengthen their type II muscle fibers. “Limit eccentric loading, or exercises in which you slowly move against the gravity of the weight,” Sternlicht says. Just make sure you're keeping proper form, Sternlicht says.

It's also important to take your rest days when training fast-twitch muscle fibers, since high-intensity work can take a toll. “Take 48 to 72 hours in between training type II muscle fibers to allow for the repair phase of the muscle to recover,” Giordano says. He also recommends doing some mobility work, such as stretches, and using a foam roller or percussive therapy to decrease stress on the tissue and optimize recovery. Two to three days a week of strength training fast-twitch muscle fibers is sufficient, Sternlicht says.

And while you're focusing on your fast-twitch muscle fibers, don't forget about other muscle groups, including the anterior (front of the body) and posterior (back of the body) chains. “Training just one muscle group will lead to muscle loss," Sternlicht says, so for well-rounded muscle health, opt for a variety of training techniques and muscle targets.

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