Spring in your step helps avert disastrous stumbles

From graceful ballerinas to clumsy-looking birds, everyone occasionally loses their footing. New Harvard University research suggests that it could literally be the spring, or damper, in your step that helps you bounce back from a stumble.

The work, published this week in the journal Proceedings of the National Academy of Sciences, sheds new light on how legged animals maintain a remarkable degree of stability on uneven terrain, highlighting the dynamic elastic and dampening roles of ankles, feet, and other distal extremities in helping us recover after stumbling. It could also help engineers develop better prosthetics and robots robust enough to navigate terrain that would leave today's automatons spinning their wheels.

“Limbs perform wonderfully on uneven terrain,” says author Andrew A. Biewener, the Charles P. Lyman Professor of Biology in Harvard's Faculty of Arts and Sciences. “Legged animals routinely negotiate rough, unpredictable terrain with agility and stability that outmatches any human-built machine. Yet, we know surprisingly little about how animals accomplish this.”

Together with colleague Monica A. Daley of Harvard's Department of Organismic and Evolutionary Biology, Biewener conducted experiments wherein helmeted guinea fowl (Numida meleagris) stepped unexpectedly into a concealed hole while running. Even though the hole's 8.5-centimeter depth equaled some 40 percent of the length of the birds' legs, the fowl remained stable and managed to maintain forward velocity, albeit most often by speeding up.

By monitoring the real-time forces exerted by the limb on the ground, as well as the angles and locations of key joints at the hip, knee, and ankle, Biewener and Daley determined that the stumbling birds' movements were consistent with a mass-spring model that treats the body as a mass balanced atop legs serving as springs. This springiness of the leg was concentrated at its distal end, near the ankle and foot, with only moderate effects seen at the knee and little change occurring at the hip.

“Ordinary walking is a patterned movement of repeating, predictable motions,” Biewener says. “Our work suggests that even falling into a hole while running does not significantly disturb the regularity of hip motion. By contrast, the distal ankle and tarsometatarsophalangeal joints act as dampers, absorbing energy when the limb contacts the ground at an unexpected steep angle and shorter limb length, or as springs, returning energy when the limb contacts the ground at an unexpected shallow angle and more full extension.”

These dynamic processes occur with astonishing speed: In the case of the guinea fowl, the leg modulates itself within 26 milliseconds as it falls into an unexpected void. The lower limbs' spring action helps the birds retain energy and momentum, stabilize their center of mass, and continue forward motion through the hole.

Biewener says this work could help create better prosthetic legs and more stable robots. Most current legged robots engage the proximal “hip” joint to generate limb work but do not incorporate dampening or modulated spring-actuation functions into more distal joints, making the machines more likely to tumble over in an irregular landscape.