Table 1: Fiber Type

Our muscles are composed of a wide variety of fibers, which scientists classify according to how they function. Traditionally, three categories are used:

Type IIb: Type IIb fibers are large diameter fibers capable of producing high levels of force at fast contraction speeds. Commonly known as “fast twitch” fibers, Type IIb’s also fatigue very quickly.

Type IIa: These fibers are much like hybrids between IIb’s and Type I fibers. They have moderate force producing capacities and moderate endurance capacity.

Type I: Type I fibers are also known as ‘slow twitch” fibers because they have small diameters, have fairly low force output characteristics, and high endurance capacity.

Despite the above classification format, the important thing to remember is that in reality, there are not three, absolutely distinct types of fibers , but rather a spectrum– on the one end are the biggest, fastest, strongest fibers which require a very high tension to activate (hence, they are often referred to as “high threshold” fibers), and on the other end are the smallest, slowest weakest (but least fatigable) fibers, also called “low threshold” fibers, since it takes only small amounts of muscular tension to activate them.

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