During aerobic activities of progressively greater intensity; changes in the pattern of muscle fiber type recruitment occur. 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:
Type I fibers are recruited to provide tension development up to 40% of an individual’s VO2 max. 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.
As aerobic exercise intensity increases to 40-75% of VO2 max, Type IIa, fast twitch, fatigue resistant fibers are recruited to assist the Type I fibers in tension development. 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.
Type IIx fast twitch fibers are the last fibers to be recruited during aerobic exercise that progresses in intensity. 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.
In contrast to exercise that increases in intensity, exercise that increases in duration rely on a different muscle fiber recruitment pattern. This is discussed in the following points below:
- Events lasting less than ten seconds depend on energy production via the ATP-PC system and anaerobic glycolysis. This indicates a reliance on Type IIa and Type IIx muscle fibers.
- Events lasting anywhere from ten seconds to three minutes primarily relies on anaerobic glycolysis (70% energy supplied at 10 seconds) but also aerobic metabolism (60% energy supplied at 3 minutes). This reliance on anaerobic glycolysis to supply ATP indicates the involvement of type IIa and IIx fibers, with type IIa fibers making the major contribution to performance. After 3 minutes we start to see a shift from anaerobic to aerobic metabolism, which indicates a reliance on Type I muscle fibers.
- As exercise increases from three to twenty minutes, we find that 60-90% of ATP is generated aerobically, which indicates a greater reliance on type I muscle fibers.
- As we progress to events lasting 20-60 minutes we find that ATP production is predominantly aerobic, hence a greater reliance of Type I muscle fibers. It should be noted though that depending on the effort level that Type IIa and IIx muscle could be recruited.
- Events that last one to four hours are clearly aerobic and involve little anaerobic energy production. This indicates Type I muscle fibers are almost being used exclusively with little contribution from Type IIa and Type IIx fibers.
- In sum as the duration of exercise increases, we see a shift from type IIx fibers, to type IIa fibers, to type I fibers.