It takes more than a fit body to run an Olympic marathon, at least that's what sports scientists from Houston, Texas are saying. They've calculated an ideal body mass for runners at certain speeds and distances. Sound obvious? It's common knowledge that long-distance runners have slighter frames than their sprinting counterparts, but scientists haven't been able to figure out exactly how weight relates to running distance.

Here's how Houston researchers Peter Weyand and Adam Davis now explain it:

Most of the energy needed for running doesn't actually go into the effort required to move and accelerate our limbs. "During steady-speed running on level ground," says Weyand, "the body has mechanical tricks that allow the energy required for swinging and bouncing the limbs to be recycled from stride to stride." It turns out that our elastic tendons do most of this work, and we end up conserving about 90% of the effort put into moving our limbs. Instead, most of the energy in running goes into pounding the pavement and supporting the weight of the body. This kind of work has no special tricks, since we can't avoid gravity.

Weyand and Davis put a bunch of volunteers on a treadmill, and found that runners exerted less force at lower speeds. If running fast requires a more forceful step, then sprinters need more muscle, tendon, and sinew to support themselves at these speeds. However, too much weight will slow down a sprinter. That's why Weyand and Davis believe there are ideal body weights for different kinds of running. In order to back up their hypothesis, they studied the records of elite athletes over the last fourteen years. They were able to quantify a simple relationship between event distance and a runner's body size.

Weyand and Davis' research contributes to an ongoing debate over the running abilities of the world's largest animals, including dinosaurs. Back in 2002, a group of scientists speculated that, at 6,000 kilograms, the mighty Tyrannosaurus Rex would have been unable to manage more than a brisk walk. Scientists concluded that T. rex would have needed about 85% of its body weight in leg muscles alone in order to run at 50 miles per hour, as previously speculated. This would leave little room for the animal's skeleton or other muscles.