Wheels have served humankind for thousands of years. But they’re overrated, says Robert Full. How many creatures in nature get around by rolling like a wheel? Full counts one, if that: a shrimplike creature called a stomatopod that occasionally curls itself into a hoop and rolls around the beaches of Panama. For the rest of us landlocked animals—except, it seems, for the NASA eggheads who persist in sending wheeled rovers to Mars—it’s legs or bust.
“Everyone thinks the wheel is the most efficient form of locomotion,” Full exclaims. “Wrong!”
Full is a professor of biomechanics at the University of California at Berkeley. His interest in the field goes back to his childhood, when he used to watch ghost crabs scuttling on the beach in Florida during family vacations. “I want to go to graduate school and watch weird animals move around,” he later told his parents.
This he does. As director of the Poly-PEDAL Laboratory, a cross-disciplinary team of researchers—many of them undergraduates—Full is devoted to forming general principles from the study of animal motion. (Polypedal means “many footed,” and PEDAL is an acronym for Performance, Energetics, and Dynamics of Animal Locomotion.) His team’s activities have included counting the hairs on the toes of geckos and building brainless multilegged robots. But their research hit full stride a few years back, when they began using high-speed digital cameras to film cockroaches, centipedes, and other organisms running on small treadmills. A roach moving at top speed, Full found, will tip up on its hind legs and run like a person. (“I still find them disgusting,” he says.)
The Poly-PEDAL team went on to build an insect runway, outfitted with high-speed video, that can measure the exact force, placement, and timing of an insect’s every tiny footstep. The platform consisted of a photoelastic material sandwiched between two polarizing filters and a light that shone through all three layers. When a roach walked across the stage, its legs deformed the material to a tiny yet quantifiable degree. (Initially the team used orange Jell-O as the medium, but the insects were more interested in eating their runway than walking on it.)
Time-lapse studies showed that a roach’s leg—or any kind of leg, in fact—is nothing more than a spring rod, like a pogo stick. If you graph the force it exerts during any one step, the result looks like a sine wave: It increases to a maximum during the push-off, decreases to zero as the foot hovers in midair, and then goes negative during the landing and repeats. A step is just a bounce. Walking or running is simply a matter of bouncing forward from one pogo stick to another, with some side-to-side motion for stability. Dogs bounce on two legs at a time; roaches (and, amazingly, centipedes) bounce on three legs; crabs bounce on four.
This rule is universal and simple to model mathematically. Full’s robot videos helped inspire the insect movements in the animated film A Bug’s Life. But the implications of his work became most clear when he decided to build a walking robot. Most robots with legs are designed to move as if they had wheels, he says. Their motion is preternaturally smooth, with little up-and-down motion, and they keep at least three legs on the ground at all times for stability. But that’s not how real legs work. Animals are “dynamically stable”—unbalanced from one moment to the next but fully stable over a complete cycle of motion. Stability is a state that legged animals are forever falling into.
And that’s the least of it. Although legs come in many shapes and sizes, they’re all equally springy. Every leg in the animal kingdom has exactly the same relative stiffness, Full has found, and all organisms generate the same amount of mechanical energy—one joule to move 2.2 pounds of body weight 3.3 feet—when walking or running on land. “It’s the same for everything,” Full says. “Dogs. Cockroaches. You. It’s crazy!”
