The Twitch You Can’t Switch

“You can train a sprinter to be a long-distance runner, but you can’t turn a long-distance runner into a sprinter,” says Dr. Bengt Saltin, a physiologist in Denmark. He was not talking about athletic strategy, but about the “twitch.”

The “twitch,” or contraction, is the way muscle fibers act to produce the energy needed. Some fibers fire quickly for spurts of energy, but they also tire out quickly. Others act more slowly but last longer. The fast-twitch and slow-twitch fibers are probably better known to you in the form of the light (fast-twitch) and dark (slow-twitch) meat of a chicken or a turkey.

Most people have about equal amounts of each type of muscle fiber. But top-notch long-distance runners average 80 percent slow-acting fibers in their leg muscles, and accomplished sprinters average better than 75 percent of the fast-acting kind. The proportion appears to be fixed in the genes and cannot be changed by exercises. Interestingly, endurance training can make fast-twitch fibers act slower or last longer but seems to have no effect on the slow-twitch fibers. It’s the twitch you can’t switch.