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Muscle Fiber Types


- See: Muscle Menu

- Two Major Fiber Types: type I and type II fibers;
    - in lower mammals and other animals, muscles are generally composed entirely type I or type II fibers;
    - human muscle is made up of a mixture of type I and type II fibers;
    - muscle fiber type depends not on any intrinsic feature of the fiber itself but on the motor neuron supplying that particular fiber;
    - all muscle fibers supplied by one particular axon will be of either type I or type II;

- Type I:
    - there are significant differences in the fiber types;
    - type I muscle fibers are rich in the enzymes necessary for oxidative metabolism and are darker in appearance;
    - when stimulated, they have a slow contraction or "twitch" time;
    - these fibers have increased resistance to fatigue;
    - type I fibers are well suited for activities related to physical effort requiring strength and endurance that depend for energy metabolism on oxidative processes;

- Type II Fibers:
    - type II fibers obtain their energy through much faster glycolytic process;
    - as glycogen stores are more rapidly depleted than oxygen supplies, type II fibers are less suited to continuous types of activity and are more suited
          to rapid alternating effort;
          - those muscles most accustomed to slow, continuous work have a lower percentage of type II muscle fibers;
    - type II fibers may be more prone to anatomic changes following altered energy demands than are type I;
    - type II fibers tend to be smaller than type I in children and in adults who do not carry out strenuous physical exercise, although they increase in size with
          repeated physical demands on the muscle;
    - type II fibers are subdivided into type IIa & type IIb;
    - type IIa fibers have an admixture of glycolytic and oxidative enzymes and show an intermediate twitch time;
    - type IIb fibers (fast glycolytic fibers) have the largest motor unit size, have the fastest rate of contraction, and are most susecptible to fatigue;
    - muscle injury:
          - type II muscle fibers are more suseptible to injury than type I fibers.
          - other risk factors for injury include muscle that cross two joints and eccentric loading