Fast-Twitch Muscle Fibers: What They Are and How To Build Them

Have you ever sprinted at full speed or caught yourself from a fall? If so, you’ve used your fast-twitch muscle fibers, also called type II muscle fibers. These cells in your muscles allow you to make rapid, explosive movements.

But your fast-twitch muscle fibers can weaken and disappear over time if you don’t keep them active. Exercise physiologist Katie Lawton, MEd, has some ideas to help you keep those important muscle fibers strong.

The importance of fast-twitch muscle fibers

Human skeletal muscles — the ones you use to move your body — are pretty complex structures. Each muscle cell gives you endurance or power, and some do a little of both. Your fast-twitch muscle fibers are important because you don’t have lightning-fast reflexes without them.

“Fast-twitch fibers aren’t just for elite weightlifters and football players,” says Lawton. “When you trip, your fast-twitch fibers can help you catch yourself before you hit the ground.”

People are born with a mix of slow-twitch and fast-twitch muscle fibers, but our fast-twitch muscle fibers tend to decline as we age.

“Starting in your 30s or 40s, you can start to lose muscle through a process called sarcopenia,” Lawton explains. “Sarcopenia tends to affect fast-twitch muscle fibers more than slow-twitch fibers. But you can help prevent sarcopenia by doing activities that call your fast-twitch fibers into action.”

As sarcopenia targets your fast-twitch muscle fibers, that helps explain why we’re more prone to falls as we age. “Research shows that building fast-twitch fibers is an effective way to help prevent falls, which can be serious in older adults,” she adds.

Fast-twitch muscle fibers vs. slow-twitch muscle fibers

For most things you do, you use slow-twitch muscle fibers. These are your endurance muscle cells. “You need slow-twitch fibers for walking, jogging and other moderate-intensity activities,” says Lawton. “These muscle fibers can work for a long time without getting fatigued.”

Your slow-twitch muscle fibers, also called type I muscle fibers, need blood and oxygen to work. And, as the name suggests, they utilize energy more slowly than their fast-twitch counterparts.

In contrast, fast-twitch muscle fibers aren’t really involved in endurance activities — they’re built for speed and power. They’re essential for activities such as high-intensity interval training (HIIT), sprinting, jumping, powerlifting and football.

“Sports and activities that involve sudden, powerful movements require your fast-twitch muscle fibers,” reiterates Lawton.

Your fast-twitch muscle fibers also need fast fuel. But oxygen moves a bit too slowly through your system to meet that need. So, your body turns to anaerobic glycolysis to power them up.

During anaerobic glycolysis, your body utilizes carbohydrates stored in the muscles to generate adenosine triphosphate (ATP) for energy. ATP is often regarded as “energy currency” for your body’s cells.

“The production of ATP is speedy but limited in the amount that can be produced in the timeframe when it’s needed,” explains Lawton. “That’s why you can only use your fast-twitch fibers for a few seconds or, at best, a couple of minutes.”

How many fast-twitch muscle fibers do you have?

In general, your inventory of fast-twitch muscle fibers depends on what you arrived with at birth. After that, three main factors affect the muscle fibers in your body.

• Age: Muscles age like the rest of our bodies. Most people reach their peak muscle mass around age 30.
• Fitness: If you regularly lift weights or play sports requiring explosive movements, you probably have larger fast-twitch muscle fibers.
• Genetics: Some studies show that genes can affect athletic performance and overall muscle composition. “Some people are genetically more likely to have more type II muscle fibers than others,” says Lawton.

It’s difficult to know your proportion of fast-twitch to slow-twitch muscle fibers, though. Doctors can only measure it through a biopsy, which only measures one muscle in your body. But knowing your exact proportion of muscle fibers isn’t necessary.

“Instead, focus on doing activities that build both types,” Lawton advises.

How to build fast-twitch muscle fibers

Cardio exercises like walking, swimming and biking have many physical and mental health benefits. But they’re not exercises geared toward building fast-twitch muscle fibers.

“Activities that you can do for five minutes or longer are slow-twitch muscle fiber activities,” clarifies Lawton. “To engage your fast-twitch fibers, you have to work your muscles to the point of fatigue or failure.”

Basically, you have to do the activity until you physically can’t do any more without a break. You can build your fast-twitch muscle fibers by:

Strength training

Lifting weights is one of the best ways to build and maintain fast-twitch muscle fibers. But your training must be at least moderate in intensity. In other words, lift until your muscles reach failure or fatigue with a moderate amount of weight.

“Doing many reps with a lower amount of weight is using more of your endurance muscle fibers,” Lawton explains. “You want to lift a weight that’s heavy enough that you can only do about 10 to 15 reps at a time.”

High-intensity interval training (HIIT)

High-intensity interval training calls your fast-twitch muscle fibers up from the bench because it requires sudden, full-body movements like jumping and burpees. Many HIIT workouts also include strength training.

“HIIT workouts can work your fast-twitch fibers if they’re shorter and more intense than a typical aerobics class,” says Lawton.

Start with an expert

The best way to fire up your fast-twitch fibers is to see an exercise physiologist, sports medicine specialist or personal trainer first.

“Weightlifting and HIIT exercises should be different for everyone,” notes Lawton. “A professional can help you determine what’s difficult enough to work your fast-twitch fibers, but still doable for you.”

Most importantly, a professional can help you exercise safely and efficiently. “If you don’t train properly, you won’t get all the benefits for your fast-twitch fibers and could get injured,” she continues.

Be consistent

No matter which fast-twitch exercises you choose, try to do them at least twice a week. (You start to lose your muscle gains after four days of inactivity, shares Lawton.) But don’t push yourself to do extra-hard workouts every day.

“Even highly trained athletes need a day of rest between intense exercise sessions,” she says. “Overtraining can lead to injuries.”