QUESTION: My coach says that I must have a lot of "red" muscle because I am so good at long distance. He didn't know too much more about red muscle but said that the type of muscles you have make a big difference in your ability to do certain sports well. Would you discuss this topic in your column?

ANSWER: Your coach was correct, the type of muscle you have does make a difference in how well you do certain athletic activities. There are two basic muscle fiber types; red and white.The best example of the differences between the two occurs at Thanksgiving when you eat turkey. The breast is "white" muscle, and the legs are "red." The darker color of the red muscle is related to the amount of mitochondria (the energy factories of the muscle) and a chemical called myoglobin (which is a lot like the hemoglobin in the blood that carries oxygen). Having more mitochondria and more myoglobin allows the red muscle to produce energy very efficiently and support long, endurance type of work.

The white fibers, on the other hand, have fewer mitochondrias and low levels of myoglobin, so they fatigue more easily.

Another way to classify these muscles is according to how fast they reach their peak tension. Using this classification, the white fibers are much better because they reach peak tension faster. Because of this, white fibers are related to power as opposed to endurance.

In humans, red fibers are called "type I," and white fibers are divided into "type IIa and "type IIb." Type I fibers are the classic "red" fiber that resists fatigue and produces energy for long, endurance type activities. Type IIa fibers are often called "fast, oxidative-glycolytic" meaning that they resist fatigue fairly well and are quite powerful as well. "Type IIb" fibers have little oxidative capacity and therefore fatigue rapidly, but they have a fast contraction time so are powerful.

So how much of each of these fibers does the average person have? Most of us have about half "type I" and half "type II" fibers in our body. Some outstanding athletes have a much higher percentage of whatever fiber type would be advantageous to their event.

For instance, some elite distance runners have as high as 90 percent "type I" fibers. In contrast, some sprinters have 90 percent "type II." The question can be asked then, do you have to have a high percentage of one or another fiber type to be a great athlete in either power or endurance type activities? A fiber-type study using the best male Swedish sprinters and jumpers (power athletes) showed that they had only 61 percent "type II" fibers, far from the 90 percent reported in some literature. The interesting thing was that the "type II" fibers in these athletes were about 50 percent larger than the "type I"and therefore made up about 75 percent of the total muscle cross-sectional area. The race walkers and the orienteerer (endurance athletes) had 59 percent and 67 percent type I fibers, respectively. It is clear from these data that training can enhance the natural endowment and improve performance dramatically, even if you have only normal values of one or another fiber type. Besides this, there are many sports where fiber type makes little difference.

- Garth Fisher is director of the Human Performance Research Center at Brigham Young University.