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You may already know about the type Is (slow-twitch) and their impressive endurance capabilities. At the other end of the spectrum, you've got your type IIb fibers. Scientists call these fibers fast-twitch/glycolytic fibers because they contract quickly and use glucose (carbohydrates) as a primary source of energy. (Type I fibers are slow and use fat as a primary source of energy.) Nestled in the middle are your typeIIa fibers. Scientists call these fibers fast-twitch/oxidative-glycolytic fibers. These fibers have an intermediate contraction speed, but what's most interesting is that training can make these type Ha fibers act more like type I fibers, or more like type lib fibers, depending on what kind of training you do. Of course, endurance training will make them act more like type Is, and intense strength training will make them act more like type IIb's.

Now, your type IIb's are the real powerhouses. They're the ones capable of generating the most force the fastest, but they burn out fairly quickly. Think of them as sort of the Mike Tyson of muscle fibers, minus the ear biting and incoherent tirades. They're great in the early rounds but get into trouble if activity lasts longer than a few seconds. The type IIa's, on the other hand, are moderately resistant to fatigue yet can still produce a decent amount of force. These are the fibers you rely on most heavily for activities that take between 15 seconds and 2 minutes such as middle distance running and weight training.

It's worth noting, however, that regardless of the type of activity you do, all of these fibers work on a continuum. So during strength and power activities, some slow twitch fibers are still recruited, just as fast-twitch fibers are active to a lesser extent during endurance work. It's not as if your body flips a switch and selectively recruits a specific type of muscle fiber based on its energy needs at a given time. The other thing that warrants mention here is the fact that each of us is made up of a mix of different fiber types. No one is exclusively fast- or slow-twitch. Fiber compositions vary tremendously from person to person and, in fact, muscle to muscle.


 
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