Compare athletes in a sprint event to those running a marathon and it's obvious that a runner's body doesn't take one shape -- sprinters tend to have muscular builds, and distance runners are more wiry. The key to the differences, according to a new research study, may lie in a runner's body mass index.
Runners' abilities have long been measured via how much oxygen they can deliver to the muscles, but that doesn't tell the whole story of why their physiques differ so greatly.
"The trick was to figure out what matters from running," says Peter Weyand, a biologist and assistant professor in the kinesiology department at Rice University in Houston. He was the lead author of the study that appears this month in the Journal of Experimental Biology.
He and coauthor Adam Davis, a researcher at the Center for Human Performance at Texas Medical Center in Houston, focused on the force with which the body hits the ground. In running, the arms swing back and forth, and the body bounces up and down.
"Once you're up to speed, those things can be done passively," Weyand says. Once energy is loaded into the tendons, which are elastic, they do the bulk of the work. That way, "the energy in the body can be recycled," he adds. Like a Super Ball, once thrown, it keeps going.
Sprinters, however, have to apply force against the ground to increase speed, "and those forces are really large," Weyand says, about two-and-half times bodyweight, compared with about one-and-a-half times for jogging. "If you're going to go fast,'' he says, "you have to hit the ground faster, and you need more muscles to generate energy."
But bigger doesn't necessarily mean better. Being too large impedes the ability to go fast, Weyand says, "because as you get bigger, there is less force related to body mass." Doing a series of back flips is easy for a gymnast, but might be much tougher for a linebacker.
The study discovered that there may be an ideal body mass index for runners, based on a relationship between BMI and how much force runners use at racing speeds. In addition to studying the height and weight of 45 of the world's fastest male and female runners in various races during the last 14 years, the researchers also had 18 athletic men and women run on a treadmill. While running at a series of constant speeds, the forces their feet exerted on the ground were measured.
Between the two body types there's something of a physiological trade-off, Weyand says. What more muscular runners gain in strength they lose in cardiovascular ability, because they have more body mass to move around. Skinnier runners have more cardio capability but less strength. Applying that to the real world, a firefighter probably needs to be stronger than he is aerobically fit.