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



Original Text by Clifford R. Wheeless, III, MD.

Last updated by Data Trace Staff on Monday, December 5, 2011 10:51 am