A muscle consists of various types of muscle fibers. Roughly, one distinguishes between slow twitch, "red" (type I) muscle fibers and fast twitch, "white" (type II) ones. Through their high content in myoglobin as well as mitochondria (in which the oxidative burning of glucose and fatty acids occurs) and oxidative enzymes (for aerobic burning of glucose and fat), type I fibers are specialized in aerobic energy supply.
Fast twitch muscle fibers, on the other hand, are characterized by a high content of energy-rich phosphates and enzymes that decompose these as well as also being able to decompose glycogen without oxygen and are thereby specialized in anaerobic energy supply. Somewhat more precisely:
- Type I fibers
"red" or "slow twitch", small diameter muscle fibers with high resistance to fatigue, higher concentration of ATPase, relatively low glycogen content and lower concentration of SDH (succinatdehydrogenase) as well as - besides the above mentioned high myoglobin content - a large number of mitochondria. They are mainly found in the "red" musculature and possess a good energy supply due to being well capillarized. They are employed in long-lasting movements with limited development of force.
- Type II fibers
"white" or "fast twitch", large diameter muscle fibers
- Type IIA fibers:
"fast" or "fast twitch" fibers with a high fatigue tendency, high content of glycolytic and oxidative enzymes that are needed with longer lasting contractions with relatively higher development of force.
- Type IIB fibers:
fast, easily fatigued fibers with high glycogen and low mitochondria content. Their energy supply occurs very rapidly, mainly via glycolysis, which is important for short or intermittent strain with a high amount of force development.
- Type IIC fibers:
so-called intermediary fibers, which can be ordered between types I and II and, depending on the training, develop more type I or more type II characteristics.