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> 2. Muscle: Fiber Type Skeletal muscle can be classified into two different types. Within these types, there are multiple different classifications of fiber with specialized characteristics. Fiber type distribution throughout the body is determined by genetics. These two groups are: < Slow Twitch (Type I) Slow twitch muscle fiber is specialized in producing force for an extended period of time. Some characteristics of type I fiber are a high resistance to fatigue, low force production, and slow contraction speed. Slow twitch cells also have high concentration of mitochondria and capillaries to support energy production through aerobic respiration (metabolism in a later section). < Fast Twitch (Type II) Fast twitch fiber can be further split into two main types, type IIa and type IIb. Type IIa IIa fiber holds characteristics intermediate to type I and type IIb. Type IIa is moderately resistant to fatigue, produces an intermediate amount of force, and has a fast contraction speed. It also has moderate concentrations of mitochondria and capillaries, and increased stores of glycogen-- obtaining fuel through both aerobic and anaerobic means. Type IIb IIb fiber is specialized in creating a large amount of force. Characteristics of IIb fibers are a low resistance to fatigue, high force production, and a very fast contraction speed. Type IIb fibers have low concentrations of mitochondria and capillaries, getting much of their energy through anaerobic metabolism. Latest research further differentiates fast twitch fibers, adding a third type IIx. In reality, characteristics of the fiber types aren't quite as clear cut as is presented. For a more in depth discussion, see Ernest Maglischo-- a nationally acclaimed swim coach and PHD holder in exercise physiology-- discuss fiber types and training: https://youtube.com/watch?v=lUGuc06M39M [Embed]