Why Bouncy Legs Work Better

If efficiency were all that matters, animals would hobble around like pirates with two peg legs. That's because, mathematically speaking, running with stiff legs requires less energy. But humans and many other animals have "squishy" legs, and a new simulation suggests why. When attached to real, floppy bodies, so-called compliant legs prove more efficient, absorbing more force and offering more stability in rough terrain. The finding helps explain the long-puzzling paradox of why animals crouch and bounce as they run and may change how researchers model animal locomotion.

From loping camels to scuttling crabs, animals move in a variety of ways. Centuries of research have focused on why certain styles prevail--even Aristotle took a stab at it. In the 1970s, researchers began using mathematics and computers to model locomotion. That's when they ran into a problem: models predicted that stiff legs were most efficient, but real-world animals tended toward crouched, compliant motion. The paradox was especially pronounced among small animals, like rats and chickens, for which motion is costliest. Despite having to expend more energy and take more steps to cover a given distance, they adopt a crouched, Groucho Marx-style run.

Integrative physiologist Monica Daley of the Royal Veterinary College (RVC) in Hatfield, U.K., had observed how adeptly the guinea fowl, an African bird known for its running skill, negotiated sudden drops and other obstacles. She wondered how the shape of an animal's body and the nature of the terrain-details left out of earlier simulations because they're difficult to model-would alter the models' predictions. So she and RVC colleague James Usherwood devised a computer model that didn't sidestep the complexities of animal motion. Instead of attaching legs to an idealized point with a certain mass, the new model linked them to a bouncing body-the seesawing guts and other tissue an animal carries as it moves-and set them on an uneven course.

As Daley expected, the less-idealized runners fared better on compliant legs. The spring in their step offset the bounce of their bodies, resulting in a smaller energy cost. "That's interesting and quite novel," says biomechanist Manoj Srinivasan of OhioStateUniversity in Columbus, who was not involved with the research.

Compliant legs also enabled runners to handle bigger obstacles without falling, an especially useful adaptation for the rough world in which smaller animals live, Daley explains.

"What I want to do now is go out and measure this in real animals," says Daley. She plans to apply the model, published online today in Biology Letters, to a spectrum of running birds ranging from quails to ostriches.

Adding the floppiness of the upper body into motion research is an innovation, experts say. "This is a completely new approach," notes R. McNeill Alexander, a biomechanist at the University of Leeds in the United Kingdom and a pioneer of locomotion modeling. "No one had done anything quite like this before."