The Different Muscle Fiber Types
There are three main types of muscle fibers that make up your muscle tissue, and each fiber take up different characteristics and benefits depending on your training goal.
Type A Fast Twitch Muscle Fibers
Also known as Type 2A fibers, these are the fibers that are needed for very high force outputs. But they will also fatigue the quickest. When performing a small amount of reps with a heavy weight for example, or sprinting, you are mainly stimulating these fibers.
Type A fast twitch muscle fibers are not suited well to activities that call for muscular endurance. They cannot use oxygen very well, and rely mainly on creatine phosphate and stored muscle glucose for fuel.
Type B Fast Twitch Muscle Fibers
Also known as Type 2B fibers, these muscle fibers are mostly called upon in high intensity, short duration activities just like Type A fibers. They are also the largest in size.
The difference is, type B fibers tend to be used for exercise that requires 80-90% of your energy, while on the other hand if you give 100% energy output you are stimulating more type A fibers.
For example, if you sprint all-out for 100 meters you are using more type A fibers, and if you run at roughly 80-90% of your fastest then you are using more type B fibers.
Type B fibers fatigue quite quickly, but not as fast as Type A fibers. They also tend to use oxygen more efficiently.
Slow Twitch Muscle Fibers
Slow twitch muscle fibers (also known as Type 1 fibers) are the smallest fibers in your muscles, and are used when performing low-medium intensity exercise for longer periods of time, such as long distance running.
Of all the muscle fiber types, Type 1 can use oxygen the most and fatigue the slowest. But they are not useful for any high intensity activities.
Type 1 slow twitch fibers primarily use stored fat for fuel.
Which Fibers Should I Work For Muscle Growth?
So, so what does all this mean? Simply put, if your aim is building muscle size then you will want to stimulate as much Type 2B fast twitch muscle fibers as you can, because these are the largest of all the muscle fiber types.
You can achieve this by staying within the range of 5-10 reps on all your weight training exercises. I personally use the rep range of 5-7 as I describe in my article How Many Reps To Build Muscle.