Slow-Twitch Muscle Fibers: What They Are and How To Work Them

You might not think you’re working your muscles when walking to your car, pushing a grocery cart or sweeping the floor. But every time you do one of these low-intensity activities, you engage your slow-twitch muscle fibers.

These muscle cells, also called type I muscle fibers, keep you going during everyday movements and longer workout routines. Exercise physiologist Katie Lawton, MEd, explains slow-twitch muscle fibers and how to keep them healthy and strong.

Do you need slow-twitch muscle fibers?

Absolutely! “Your slow-twitch fibers are necessary for everyday life and endurance,” says Lawton. “Every time you sit, stand, walk or do moderate-intensity exercise, you use your slow-twitch fibers.”

But if you follow fitness experts and trends, you’ve probably heard more about fast-twitch muscle fibers, also known as type II muscle fibers. Your fast-twitch muscle fibers are muscle cells that produce a lot of power very quickly, allowing you to do explosive movements and intense weight training.

As the name suggests, slow-twitch muscle fibers utilize energy more slowly than fast-twitch muscle fibers. But that doesn’t mean they’re second best. In fact, slow-twitch muscles are actually used first and more often in everyday activities.

Plus, they last much longer. “Fast-twitch muscles fatigue after a few seconds or a couple of minutes,” says Lawton. “Slow-twitch muscle fibers, on the other hand, keep going for extended periods.”

Why do slow-twitch fibers punch the clock for more hours than their fast-twitch counterparts? The main reason is fuel.

“Your slow-twitch muscle fibers are aerobic, meaning they use oxygen for fuel,” she continues. “You replenish this oxygen as you breathe so your slow-twitch muscles can keep going. Fast-twitch fibers use glucose and lactic acid, which take longer to regenerate as fuel.”

How many slow-twitch muscle fibers do you have?

You’re born with a mix of fast-twitch and slow-twitch muscle fibers, says Lawton. But everyone has a different ratio of these fibers. The numbers depend on:

• Age: In general, older people have more slow-twitch fibers. “As we age, many people lose muscle mass through sarcopenia,” says Lawton. “Sarcopenia affects your fast-twitch muscle fibers more, so your slow-to-fast-twitch ratio increases. However, you can avoid or slow this process by doing high-intensity exercises and strength training.”
• Exercise habits: A long-distance runner will have more slow-twitch fibers than a sprinter. Doing a mix of longer, moderate-intensity and short, high-intensity exercise will help you keep a healthy amount of both types of muscle fibers.
• Genetics: Like eye color, hair color and many other traits, your genes play a role in your muscle composition. “But how you train and the stimulus you place on muscles can also have an impact on muscle fibers,” notes Lawton.

How to build type I muscle fibers

Training and building slow-twitch muscle fibers requires moderate-intensity exercise. If your intensity gets too high, like with high-intensity interval training or weightlifting to your maximum, your fast-twitch fibers will take over.

“Slow-twitch muscle fiber exercises are aerobic exercises,” says Lawton. “These activities are great for your cardiovascular health. Slow-twitch fiber exercises also burn fat, which can help with weight loss.”

Slow-twitch muscle exercise examples include:

Walking

This tried-and-true exercise is widely recommended for a reason. “Walking has mental health benefits and improves cardiovascular and metabolic health,” says Lawton. “Every time you walk, you work your slow-twitch muscle fibers.”

Plus, you don’t need special equipment and it’s great exercise for beginners and seasoned athletes alike.

Swimming

Got sore joints? Swimming will work your slow-twitch muscle fibers and has practically zero impact, thanks to the buoyancy of water.

“Water exercises offer moderate resistance and engage your slow-twitch fibers,” says Lawton. “Swimming is often a good choice for people with reduced mobility, arthritis, fibromyalgia or obesity. But even hard-core athletes can get a great full-body workout with swimming.”

Jogging

Like to run longer distances? You’re using your slow-twitch muscle fibers when you do. “Fast-twitch fibers are necessary for sprinting, but your slow-twitch fibers carry you through a longer, slower run,” explains Lawton.

Primal movement workouts

A primal movement workout involves movements we use every day, like bending, twisting and lunging. It’s a moderate-intensity exercise and targets muscle groups you need to stay mobile and active, which makes it a good way to target slow-twitch muscle fibers.

Don’t ignore your fast-twitch muscle fibers

It’s easy to assume that you don’t need fast-twitch muscle fibers unless you’re a powerlifter, but that’s not the case.

“Everyone needs fast-twitch muscle fibers because they improve reaction time,” clarifies Lawton. “If you’ve ever tripped and caught yourself before you fell, you can thank your fast-twitch fibers. Many older adults are prone to falls due to declining amounts of fast-twitch fibers.”

Finding the right routine

Creating a workout routine that works all of your muscle fibers can be overwhelming. If you’re not sure where to start, consider getting help from a pro.

“Many people are surprised to find out they’re only working one muscle fiber type,” says Lawton. “That’s why it’s helpful to consult a personal trainer or exercise specialist to evaluate your routine. They can help you find activities that are best for your fitness goals and personal preferences.”