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In general, this progression moves from Type I oxidative fibers, to Type IIa fast twitch, fatigue resistant fibers, and finally to Type IIx fast twitch fibers. It should be noted that this recruitment of muscle fibers represents a continuum as opposed to only one fiber type being utilized during exercise that progresses in intensity. The image below illustrates this point nicely and is discussed below:
Since Type I fibers rely on a constant supply of oxygen to produce ATP, any factor that limits this supply would reduce tension development and necessitate the recruitment of Type IIa fibers to generate tension.
Although Type IIa fibers are dependent on oxygen due to a high mitochondrial density; these fibers can also produce ATP via anaerobic glycolysis when the supply of oxygen cannot match the demands of higher intensities of exercise. As with Type I fibers, if oxygen delivery to Type IIa fibers is decreased, or if the ability of the fiber to use oxygen in decreased, tension development will fall requiring type IIx fibers to be recruited to maintain tension.
These fibers are recruited at about 75% VO2 max in order to assist Type I and Type IIa in continuing tension development. Although type IIx fibers can supply energy anaerobically (without oxygen) and generate a greater amount of tension, they are quick to fatigue due to a low mitochondrial density.
This is discussed in the following points below: