There are two main classifications of muscle fibers: Slow-Twitch muscle fibers (Type I) and fast-twitch muscle fibers (Type II). There is a lot of debate regarding the body’s ability to adapt its muscle fibers from one type to another. Some studies seem to show some ability to affect muscle fiber type with training, while other studies show no correlation.
Type I fibers are able to use oxygen more efficiently. Slow-Twitch muscles, as the name suggests, fire more slowly than fast-twitch muscles. These fibers are most efficient when performing high rep low weight actions. This becomes important in anaerobic exercise such as running a marathon. It is found that marathon runners tend to have higher percentages of Type I fibers, though the cause-effect relationship is not known. It could be that marathon runners have a high percentage of Type I fibers due to the training, or it could be that people with low percentages of Type I fibers simply do not become marathon runners.
Type II muscle fibers are further categorized into Type IIa and Type IIb fibers. Type IIa fibers can be thought of as hybrids between slow twitch and fast twitch muscles. These muscle fibers can alternate efficiently between anaerobic activity and aerobic activity; this essentially gives you the best of both worlds, as far as performance is concerned. Type IIb muscle fibers are the iconic fast-twitch muscles. These fibers allow your body to perform high velocity high power movements, such as jumping high, or bench pressing high weight at low reps. The limitations of Type IIb fibers come from the recovery time. Since these fibers perform best during aerobic activity, these muscle fibers tend to fatigue more easily and tend to take longer to recover than Type I or Type IIa. This effect can be mitigated by making the body more efficient at delivering oxygen to these muscles, so that these fibers do not become oxygen deprived quickly.
Each play an important role in your body, and no person has all of one type or the other. Regardless of your body’s ability to change the muscle fiber types, training in a desired type of performance will always show improvement over time.