Not all muscle fibers are created equal. Targeting to work different muscle fibers will give you different results, so to understand that, let’s quickly go over what the different types of muscle fibers are:
In the simplest view, there are two general types of muscle fibers. Fast twitch and slow twitch. These are also called Type II and Type I respectively. Fast twitch muscle fibers can be further divided into Type IIa and Type IIb. You can divide these up even further, but that is beyond the scope of this article as further distinctions really do not have very much real-world practicality.
Slow Twitch Muscle Fibers
Type I muscle fibers can be thought of as the “endurance fiber”. They are very good at using oxygen to make more fuel (known as ATP) so that you keep having “energy”. These fibers can go a very long time without fatiguing, so they are great for people who run marathons, ride bikes long distances, or basically any other endurance activity. They do have one drawback however: They contract the muscles slower than fast twitch fibers.
Fast Twitch Muscle Fibers
Type II muscle fibers are virtually the opposite of Type I. They can be thought of at the power and strength fibers. These fibers are not very good at converting oxygen into fuel and can only exert muscle contractions for a short amount of time before they fatigue. Instead of using oxygen, they use anaerobic metabolism to create fuel. These types of fibers can contract more quickly than slow twitch but generate the same amount of muscle force per contraction.
These fibers are sort of in between Type I and Type II fibers, as they use both aerobic and anaerobic metabolism almost equally to create energy (aerobic just means using oxygen to make fuel).
These are normally the type of fibers that everyone means when they talk about fast twitch muscle fibers. These fibers only use anaerobic metabolism for energy and fatigue very quickly. These also have the fastest rate of contraction.
Go on to my second post to see how these different types of fibers affect your performance and how you can use this knowledge to your advantage: http://buildingleanbodymass.webege.com/2010/04/16/how-to-take-advantage-of-different-muscle-fibers/