Muscle fibers are designed either for aerobic metabolism or for anaerobic activity. Two muscle fiber types have been identified and classified as slow twitch (Type I) and fast twitch (Type II). Slow twitch (Type I) are more suited for long, low to medium intensity activity and fast twitch (Type II) is more suitable for short duration, high intensity activity. For instance a long distance runner has a greater type I percentage than a sprinter or weightlifter.
Type I and Type II was the standard classification for muscle fiber types. Fast twitch was also futher divided into Type IIa and Type IIb fibers. Type IIa was called fast oxidative glycolytic because of its ability to perform both aerobic and anaerobic metabolism. Type IIb was coined fast glycolytic because it has the greatest anaerobic capability. Eventually more subtypes of each fiber were identified. Type I fibers wer broken down into Type I and Type Ic. Type Ic is believed to have less oxidative ability than Type I. Type II fibers were broken down even more into Type IIc, IIac, IIa, IIab, and IIb. Each has different aerobic and anaerobic characteristics. In a study done where they randomly selected youth athletes and determined the percentage of type I fibers in their Vastus Lateralis. The long distance runners came out on top with roughly 60%-95% Type I fibers while the sprinters had roughly 15%-38% Type I fibers (Åstrand).
| Muscle Fiber (Type I, Type IIa, and Type IIb) Characteristics | |||
| Type I - Slow Twitch | Type IIa - Fast Twitch | Type IIb - Fast Twitch | |
| Force Production | Low | Intermediate | High |
| Contraction Speed | Slow | Fast | Fast |
| Myofibrillar ATPase Activity | Low | High | Hight | Fatigue Resistance | High | Moderate | Low |
| Glycolytic Capacity | Low | High | High |
| Oxidative Capacity | High | Medium | Low |
| Capillary Density | High | Intermediate | Low |
| Mitochondrial Density | High | Intermediate | Low |
| Myoglobin Content | High | Intermediate | Low |
| Endurance Capability | High | Moderate | Low |
| Glycogen Storage Capability | No Difference | No Difference | No Difference |
Is is largely believed that genetic factors influence the percentage of each muscle fiber type in a person. However, in elite athletes depending on which sport they participate in they will have a fiber type that is more favorable for that sport. That simply means endurance athletes had more Type I and a weightlifter or sprinter had more Type II muscle fibers. However, there are conflicting views on exercise changing muscle fiber type. Some studies have shown a change in Type I fibers after endurance training while others have not. Although it seems that high intensity resistance training causes a transformation of Type IIb into Type IIa. Similarly a high intensity and endurance training program increased their Type IIa and Type IIc, IIc is the most oxidative of the Fast twitch.
Subtype transformations seem to occur within two weeks (Åstrand). However, with lack of training the subtype will go to their original state. As more research is done a clearer picture of muscle fiber types should come into view.References:
Borer, Katarina T. Exercise Endocrinology. Human Kinetics, 2003.
Hoffman, Jay. Physiological Aspects of Sport Training and Performance. 1st ed. Human Kinetics, 2002. 7-14.
Åstrand, Per-Olof, Kaare Rodahl, Hans A. Dahl, and Sigmund B. Strømme. Textbook of Work Physiology. 4th ed. Human Kinetics, 2003. 56-59.