These visible physical characteristics carry through down to the individual fiber of athletes' muscles. Those in short track have muscles primarily composed of fat muscle fiber called fast-twitch. Those in biathlon have muscle of primarily small fiber called slow-twitch. These fibers function differently, too, the fast performing explosive movements and the slow best at endurance feats.
You, too, have a higher ratio of either fast- or slow-twitch muscle fiber. Are you at the more Nordic end of the range? Or the short track? When you know which, you can leverage it to achieve greater success at your sport. You can probably guess. If you excel at endurance sports, your muscles are likely composed of more slow-twitch fiber. If you have a high vertical jump or do better at sports that require sprinting, you probably have more fast-twitch fiber. (For a quick at-home test to find out, go to www.fasterbetterstronger.com.)
The exact ratio of your muscle composition is determined by genetics and it's largely unchangeable. This is true for everyone. It is difficult for many people to understand, but even Carl Lewis - the great 100-meter runner, blessed with mostly fast-twitch fibers - could never have won a marathon. He could never change his muscles' ratio of fast- to slow-twitch fiber enough to do so, no matter how long and hard he trained. Which is to say, neither can you.
Your fast-twitch fibers do become more slow-twitch as you age, however; people in their 40s often run their personal best in marathons but never in sprints. You can also influence your muscles' composition via the training you do. About 15 percent to 20 percent of your muscle fiber is available to transfer from the work of slow-twitch to fast-twitch fiber and vice versa, depending on what you ask of it, moment by moment or over a lifetime.
Elite athletes such as those in the Winter Olympics use this to their advantage. They leverage the natural composition of their muscles by competing in the arena in which they have a natural advantage. They then use training to recruit the changeable fibers to do even better. If their sport demands explosive movement, their exercise is almost exclusively focused on strengthening exercise. If it requires endurance, they do aerobics.
According to exercise performance physician Max Testa, M.D., an excellent exercise for developing fast-twitch muscle fibers - say, if you want to do well in a sport such as basketball, volleyball or soccer - is lifting heavy weights with a low number of repetitions. To maintain overall fitness, it's important to do some exercise in your areas of weakness, but Testa cautions against doing a lot of aerobic work, which may cost you explosive strength because you will transform some of the fast-twitch fibers to slow-twitch.
Likewise, if you are a marathoner, a cyclist or an endurance athlete of any kind, Testa cautions against doing a lot of bodybuilding, because the bulk may work against you. Instead, he recommends doing resistance training to make your muscles stronger and more resistant to fatigue.
( Eric Heiden, M.D., a five-time Olympic gold medalist speed skater, is now an orthopedic surgeon in Utah. He co-authored "Faster, Better, Stronger: Your Fitness Bible" (HarperCollins) with exercise performance physician Max Testa, M.D., and DeAnne Musolf. www.heidenortho.com)