There are two main types of fibers in your muscles.
Slow Twitch: These are also known as Type I muscle fibers. They are responsible for long-duration, low intensity activity such as walking or any other aerobic activity.
Fast Twitch: These are known as Type II fibers (divided further into A and B). They are responsible for short-duration, high intensity activity. Type IIB fibers are built for explosive, very short-duration activity such as Olympic lifts. Type IIA fibers are designed for regular high-intensity work.
To find the predominant fiber type in a particular muscle in your body, you can try the following test.
- Find your one rep max here for an isolation exercise for that muscle group.
- Take 80% of it and do as many reps as possible.
- If you can do only 4 to 7 reps with it, you have mostly Fast Twitch fibers in that muscle.
- Around ten reps is the typical mix for a muscle.
- Doing 15 to 20 or more reps will be mostly Slow Twitch fibers.
By knowing what type of muscle fibers you have, you can tailor your training towards developing them to their maximum potential.