Fast Twitch and Slow Twitch Muscle Fiber

High intensity, anaerobic, short-burst activity, trigger the fast-twitch fibers, called Type 2 Fiber. Fast twitch fiber are lighter in color. They have low levels of myoglobin (iron-containing protein that stores oxygen) and of mitochondria.

Slow-twitch fiber, called Type 1 Fiber, contain more mitochondria and myoglobin, and are redder in color. They provide for longer activity.

Average people are about equal in both fiber muscles. Dominant athletes tend to predominately possess the muscle fiber composition most utilized by their sport.

Fast-twitch fiber are also divided into two sub-categories:

Type 2b - Fast-Glycolytic (FG) - having few mitochondria and high levels of stored glycogen and the enzymes necessary for producing energy without oxygen; and

Type 2b - Fast-Oxidative-Glycolytic (FOG) - having the stored glycogen and enzymatic properties of Type 2b (FG), but additionally having the benefit of containing high levels of oxidative enzymes which aid aerobic metabolism.

A sprinter achieves his high speed due to the ability to sustain the production of force at a peak effort. Fast-twitch fiber produce no greater force than slow-twitch, but they differ in the rate that they produce force.

Fast-twitch fibers create a relatively high amount of force in a short period, such as the milliseconds that the foot maintains contact with and pushing against the ground during a sprint.

Slow-twitch fibers are utilized during the endurance aspect of longer events. They are fatigue resistant rather than rapid force producers.

The method of muscle contraction also effects the choice of whether fast-twitch or slow twitch fiber engage. Within a motor unit, that is a group of muscle fibers and the motor nerve that activates the group, there is just one type of muscle fiber. With a weak muscle contraction the Slow Twitch Type 1 fibers will take control; if near maximum, Fast Twitch Type 2b will be primarily used; if moderate, Fast Twitch Type 2a fibers will be implemented.

The composition of ones muscles is genetically determined. As a result during the early stages of an athlete's career little change will occur as to which fiber types exist and are utilized. However, over extensive periods of time, that is over several years of training, Fast Twitch Type 2b (FG) fibers apparently can be transformed to Fast Twitch Type 2a (FOG) fibers with their accompanying slow twitch properties.

However, it will take many years of chronic endurance training to achieve this. Typically, far more years than most remain in training. Medical biopsies of elite endurance athletes demonstrate that following years of chronic distance training they have virtually no Type 2b fibers, but a significant percentage of Type 2a.

It also has been shown that Fast Witch Type 2a fibers do not transition to Slow Twitch Type 1.

Intense training over long distances can develop the mitochondria and surrounding capillaries of the muscle leading to an increased ability of the muscle fiber to deal with ongoing distance training.