What are Muscle Fibers?
Skeletal muscles are made up of fibers. A fiber is the smallest unit of a muscle that can contract. Muscle fibers are of two types: slow-twitch and fast-twitch.
Slow-Twitch Muscle Fiber
Slow-twitch muscle fibers are also called Type I fibers. They are smaller in diameter than fast-twitch fibers and contract more slowly. These fibers are used for endurance activities, such as marathon running.
Slow-twitch muscle fibers have the following characteristics:
- They use aerobic metabolism to produce energy.
- They have a higher concentration of mitochondria.
- They have a higher concentration of myoglobin.
- They are resistant to fatigue.
- They produce less force than fast-twitch fibers.
Examples of Activities that Make Use of Slow-Twitch Muscle Fibers
- Long-distance running/marathons
- Cycling
- Swimming
- Walking/Power-walking/Jogging
Fast-Twitch Muscle Fiber
Fast-twitch muscle fibers are also called Type II fibers. They are larger in diameter than slow-twitch fibers and can contract more quickly. These fibers are used for explosive movements, such as sprinting.
Fast-twitch muscle fibers have the following characteristics:
- They use anaerobic metabolism to produce energy.
- They have a lower concentration of mitochondria.
- They have a lower concentration of myoglobin.
- They are more susceptible to fatigue.
- They produce more force than slow-twitch fibers.
Examples of Activities that Make Use of Fast-Twitch Muscle Fibers
- Sprinting
- Weightlifting
- Jumping
- Basketball
- Tennis
Training and Muscle Fiber Types
When you know the difference between slow-twitch and fast-twitch muscle fibers, you can better understand how to train for specific activities.
For example, if you mainly want to improve your endurance, make sure to include plenty of aerobic exercises in your training regimen. And if you’re looking to add more explosive power to your game, focus on anaerobic activities that will help increase the size and strength of your fast-twitch muscle fibers.
This knowledge can also be helpful if you’re dealing with an injury. If you have a torn tendon, for example, you might want to focus on exercises that don’t put too much strain on that area. And if you’re recovering from a broken bone, you might want to focus on exercises that don’t require a lot of impact.
Which Type of Muscle Fiber Do I Have?
There’s a misconception that people have one type of muscle fiber or the other. But in reality, everyone has a mix of both slow-twitch and fast-twitch fibers.
The ratio of slow-twitch to fast-twitch fibers varies from person to person and can even differ from one muscle to the next in the same person. The ratio depends on your age—and your activity level.
Testing for Muscle Fiber Type
So how can you find out which type of muscle fiber you have more of? A muscle biopsy is the most accurate way to determine the percentage of slow-twitch to fast-twitch fibers in a particular muscle.
But there’s an easier way to get a general idea of which type of muscle fiber you have more of:
Do a 1.6km/1 mile run test.
If you can complete the 1.6km/1mile run in 8 minutes or less, you most likely have a higher percentage of fast-twitch muscle fibers.
If it takes you more than 8 minutes to complete the 1-mile (1.6-kilometer) run, you most likely have a higher percentage of slow-twitch muscle fibers.
The Bottom Line
Now that you know the difference between slow-twitch and fast-twitch muscle fibers, you can better understand how to train more effectively for specific activities.
And if you’re dealing with an injury, this knowledge can help you focus on exercises that won’t put too much strain on the affected area.
So make sure to keep this information in mind the next time you hit the gym, the court, the rink, the track, or the field—no matter what your specific performance goals are, understanding your muscle fibers will put you that much closer to achieving them.