Figure 3. The seven types of skeletal fibresMost skeletal muscles are either fusiform or pennate fibers.
Muscle Fiber Types
As mentioned there are three types of muscle tissue; white fast twitch, red fast twitch and red
slow twitch muscle fibers. These different fibers have exactly the same internal components,
but vary in number and size. They have differing levels of a substance called myosin-ATPase,
which breaks down ATP for energy. The more of this enzyme, the faster the contraction, thus
more power as well. These fiber types have varying cardiovascular “support systems” and fuel-
utilization characteristics, and have different types of nerves that stimulate them. Therefore, they
are coordinated, or organized functionally, as well as structurally. The individual fiber is stimulated
to contract by a branch of the neuron called an axon, which varies in thickness and the degree of
myelination, or insulation it possesses. The thicker or more myelinated it is, the faster the signal
can travel down it. For highly efficient aerobic metabolism to occur, oxygen must be present.
This is delivered to the muscle cell by a very small tube called a capillary. Microscopic capillaries
extend throughout intramuscular areas surrounding the muscle fibers.
White, Fast Twitch Muscle Fibers - Type II b
White, fast twitch muscle fibers have fewer capillaries and, therefore, must rely on glycolysis for
energy – thus, the name fast-glycolytic fibers. In the absence of this oxygen-rich blood supply,
they cannot function for very long periods. However, as a lot of fibers are contained within the
fast twitch motor unit, and the girth of the individual fiber is large, possessing more contractile
proteins, they are extremely strong.