This Is MS Multiple Sclerosis Knowledge & Support Community

Slow Twitch (Type I)
The slow muscles are more efficient at using oxygen to generate more fuel (known as ATP) for continuous, extended muscle contractions over a long time. They fire more slowly than fast twitch fibers and can go for a long time before they fatigue. Therefore, slow twitch fibers are great at helping athletes run marathons and bicycle for hours.

Fast Twitch (Type II)

Because fast twitch fibers use anaerobic metabolism to create fuel, they are much better at generating short bursts of strength or speed than slow muscles. However, they fatigue more quickly. Fast twitch fibers generally produce the same amount of force per contraction as slow muscles, but they get their name because they are able to fire more rapidly. Having more fast twitch fibers can be an asset to a sprinter since she needs to quickly generate a lot of force.

It is now generally recognized that skeletal muscle fibers do not exist in three discrete forms at the subcellular level, but rather in a continuum based on the multitude of combinations of myosin heavy and light chain isoforms, polymorphic expression of protein isoforms, metabolic potential, and Ca2+ handling characteristics. Moreover, it is clear that all of these cellular characteristics exhibit some degree of plasticity in response to exercise training.