What Are Fast-Twitch Muscle Fibers?

You may want to consider training your body by focusing on different muscle fibers—specifically your fast-twitch muscle fibers—if you want to get stronger and faster. From your biceps and triceps to your quads and glutes, your skeletal muscles contain slow-twitch (type I) and fast-twitch (type II).

Everyone is born with both muscle fiber types, though increasing, training, and strengthening your type II muscle fibers is possible. Here's what you need to know about your fast-twitch muscle fibers—including how to train them properly and why they're essential for your fitness routine and overall health.

What Are Fast-Twitch Muscle Fibers?

Fast-twitch fibers, also called white fibers or type II fibers, are skeletal muscles that help with power performance for short periods. Because so much power is used in a short time, these muscles become fatigued faster.

Fast-twitch muscle fibers are also broken up into two types: type IIa and type IIx, formerly known as IIb. These fibers play roles in activities such as:

• Jumping
• Powerlifting
• Sprinting
• Strength and agility training

Type IIa

Type IIa fibers are also known as intermediate muscle fibers. These fast-twitch muscles can be used longer since their time to fatigue is longer. Type IIa fibers are also used more for certain types of exercises. For example, you may engage them more during sprints or powerlifting sessions.

Type IIx

Type IIx muscle fibers are better at providing force. However, they fatigue faster, making them inefficient. This subset of type II muscle fibers is called "couch potato" fibers. Type IIx muscle fibers are necessary for quick, unexpected movements for all individuals regardless of their physical activity level.

Fast-Twitch vs. Slow-Twitch Muscle Fibers

In the simplest terms, fast-twitch (type II) muscle fibers are built for short, powerful bursts of energy. Slow-twitch (type I) muscle fibers are built for long-lasting activities. The following chart shows a comparison of the two types of fibers.

Why Can't You Use Fast-Twitch Muscles Longer?

Activity from fast-twitch muscles lasting longer than 30 seconds results in lactic acid accumulation. Lactic acid, or increased acid in muscle cells, can lead to pain and discomfort. You need adequate recovery between these exercises to clear out the lactic acid in your muscles.

You can get energy from food to help with muscle movement. That energy converts to a chemical compound called adenosine triphosphate (ATP), the immediately usable form of energy for the cells in their bodies.

The body stores a minimal amount of ATP within muscles already. However, most of the ATP becomes available when needed by one of three energy systems: phosphagen, glycolytic, and oxidative.

The phosphagen system, or the ATP-PC system, uses phosphocreatine (PC) or a high-energy phosphate. This system activates first when you use your fast-twitch muscle fibers and ultimately gets depleted first.

The ATP-PC system allows for up to 30 seconds of maximum effort. 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. After that, the oxidative system—closely associated with slow-twitch muscle fibers and which relies on fats and carbs—kicks in.

How To Know Which Muscle Fiber Type You Have

Everyone has both types of fast-twitch muscle fibers. Yet, 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," Eric Sternlicht, PhD, associate professor of health sciences and kinesiology at Chapman University in Orange, Calif., told Health. "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."

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," said Sternlicht.

Genetic testing and a muscle biopsy are the most accurate ways to test muscle fibers. However, since that isn't an option for most people, you can figure it out with other options. One test to try is a 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," Dan Giordano, PT, DPT, CSCS, co-founder of Bespoke Treatments Physical Therapy in New York City, told Health.

Another 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," said Giordano. 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.

How To Strengthen Fast-Twitch Muscle Fibers

Adding strength training into your routine can help build fast-twitch muscle fibers. Muscle fiber loss, also known as sarcopenia, usually starts to happen after age 30. 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 inactivity.

Sarcopenia can put you at greater risk for falls and fractures and can cause limited mobility. However, sarcopenia 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," said Dr. Sternlicht. "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," said Dr. Sternlicht.

"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," explained Giordano.

When a muscle fiber loses its nerve supply, Giordano explained 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. This means they begin to take on characteristics of slow-twitch muscle fibers.

"Data suggests that a 60-year-old has about 25 to 50% 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," said Giordano. "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."

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," said Dr. Sternlicht.

This also goes for people who stick to one mode of training. Doing interval work only on the treadmill and completely avoiding or under-training the upper body can result in the loss of fast-twitch muscle fibers in your upper body, said Dr. Sternlicht. "The bottom line is if you don't stress these muscle fibers through workouts, you eventually lose them," said Dr. Sternlicht.

Tips for Training Fast-Twitch Muscle Fibers

Giordano and Dr. Sternlicht recommended 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," said Dr. Sternlicht.

Plyometric exercises such as box jumps and burpees can ignite these power-producing fibers. "Work on explosiveness," said Giordano. "Think short bursts, go hard, and lift heavy."

Lifting heavy can be intimidating or new for some individuals. If that's the case for you, Dr. Sternlicht said that you can also use lighter weights and train at a faster velocity with them during the concentric phase—like pushing weights up—of each rep.

Resistance training involving 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," said Dr. Sternlicht. Just make sure you're keeping proper form.

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," said Giordano.

Giordano also recommended doing some mobility work, such as stretching, 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, said Dr. Sternlicht.

Don't forget about other muscle groups while focusing on your fast-twitch muscle fibers. This includes the anterior chains at the front of the body and posterior chains at the back of the body. "Training just one muscle group will lead to muscle loss," added Dr. Sternlicht.

A Quick Review

Fast-twitch muscle fibers are built for quick, powerful movements, compared to slow-twitch muscle fibers, which are more suited to endurance activities. The more you work your fast-twitch muscle fibers, the stronger they'll be. Activities that activate fast-twitch muscles include jumping, powerlifting, sprinting, and strength and agility training.

For overall well-rounded muscle health, opt for a variety of training techniques and muscle targets. Speak to a healthcare provider