For most healthy bipeds, the act of walking is seldom given a second thought: One foot follows the other, and the rest of the body falls in line, supported by a system of muscle, tendon, and bones.
Upon closer inspection, however, locomotion is less straightforward. In particular, the ankle — the crucial juncture between the leg and the foot — is an anatomical jumble, and its role in maintaining stability and motion has not been well characterized.
“Imagine you have a collection of pebbles, and you wrap a whole bunch of elastic bands around them,” says Neville Hogan, the Sun Jae Professor of Mechanical Engineering at MIT. “That’s pretty much a description of what the ankle is. It’s nowhere near a simple joint from a kinematics standpoint.”