Fast Twitch and Slow Twitch Muscle Fibers
There are two types of muscle fibers: slow (type I) and fast (IIa and IIb) twitch. Unfortunately, you are born with a certain number of both types, around 50% of each. However, even though you can’t increase the number of fast twitch or slow twitch muscle fibers, you can develop each type of muscle fiber so that you gain the benefits of each type. For training purposes, it is important to train and develop both types of muscle fibers.
Slow Twitch Muscle Fibers
Slow twitch muscle fibers use oxygen efficiently. They are slow to fire and slow to fatigue. This type of muscle fiber is what marathon runners and cyclists have developed most. If you have been blessed with more of this type of muscle fiber, then you will be able to run or cycle farther.
Fast Twitch Muscle Fibers
Fast twitch muscle fibers can deliver more powerful and faster bursts of energy than slow twitch muscle fibers. However, they fatigue faster than do slow twitch fibers. Using this type of muscle fiber, you are more likely to exhaust your glycogen storages. Both muscle fibers have the same force, but fast twitch muscle fibers contract much faster. Sprinters use this type of muscle fiber the most. The greater number of fast twitch muscle fibers you have, the faster you will run.
Fast twitch muscle fibers are divided into two categories. The first type is somewhat of a combination of fast and slow twitch. They have both aerobic and anaerobic metabolism. The second type is the more common. It is purely anaerobic, delivering quick and powerful bursts of energy.
Unfortunately, you can’t change the ratio of fast to slow twitch muscle fibers in your body. However you can perform sprint training in order to run faster or long-distance training in order to run farther. Long-distance runners should train both slow and fast twitch muscles. But be warned. The more often you run slowly, the less ability you will have to sprint fast. If you train for speed, you will develop your fast twitch muscle fibers to the neglect of slow twitch muscle fibers. Be sure to train both in their proper proportion as it relates to your sport.
Glover, Bob, The Competetive Runner’s Handbook