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.