Pounding the pavement with quick steps takes plenty of muscle. Credit: © Punchstock

When it comes to athletics, going the distance is not just a matter of fitness. Researchers have found that there is an ideal body mass for running at a certain speed, and for a certain distance.

It's common sense that long-distance runners are more lithe than sprinters, but sports scientists have never before managed to work out how or why weight relates to event distance.

Now Peter Weyand from Rice University, Houston, and Adam Davis from Houston's Center for Human Performance, claim to have done so.

Weyand and Davis argue against the idea that most of the energy needed for running goes into the effort required to move and accelerate your limbs.

"This is an important and somewhat controversial issue," says Weyand. "During steady-speed running on level ground, the body has mechanical tricks that allow the energy required for swinging and bouncing the limbs to be recycled from stride to stride," he says.

That means our elastic tendons do most of the work, letting us bounce along once we are up to speed without requiring much extra energy. Perhaps 90% of work put into our limbs is conserved this way, Weyand estimates.

Instead, he says, most of our energy is spent pounding the pavement and supporting the weight of the body. "There are no tricks for avoiding gravity," he says. The faster you go, the more energy is spent hitting the ground.

Pounds and ounces

To find the relationship between running speed and the force needed to support the body, the researchers tested volunteers on a treadmill. At their personal top speeds both men and women were hitting the ground with a force of around 2.5 times their own body weight. At lower speeds, they exerted less force, the team reports in The Journal of Experimental Biology1.

If running fast requires a more forceful step, the researchers thought, then you need more muscle, tendon and sinew to support those speeds. But being too heavy would slow you down, because it takes energy to carry weight around. This led them to believe that there might be an ideal body mass for a given speed.

The researchers looked at records of élite athletes from a variety of running events, each of which suits itself to a different speed. By collecting the heights and weights of the world's 45 fastest men and women over eight different track distances for the past 14 years, they uncovered a surprisingly simple relationship between event distance and a runner's body size. If you are of a similarly athletic frame, you can find where you lie on this curve, and what your ideal running distance is (click here).

Lumbering dinosaurs

The team says that its investigation into the force runners exert on the ground might explain this curve, with more muscular runners pounding the ground hard for short sprints and skinny runners having a more gentle step over the long haul.

The research also contributes to ongoing debate about the running ability of large animals, including dinosaurs. Based on observations of skinny long-distance runners and the lumbering elephant, scientists have previously concluded that heaviness of any kind is bad for track times.

In 2002, scientists hit the headlines by suggesting that, at 6,000 kilograms, Tyrannosaurus rex would have been unable to manage more than a brisk walk2.

Weyand says that it may be time for a rethink about the running abilities of larger animals. "Without sufficient musculoskeletal mass, faster running speeds cannot be attained. Having to carry extra mass is not always a performance detriment."