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.”
Now, Hogan and his colleagues in the Newman Laboratory for Biomechanics and Human Rehabilitation have measured the stiffness of the ankle in various directions using a robot called the “Anklebot.”
The robot is mounted to a knee brace and connected to a custom-designed shoe. As a person moves his ankle, the robot moves the foot along a programmed trajectory, in different directions within the ankle’s normal range of motion. Electrodes record the angular displacement and torque at the joint, which researchers use to calculate the ankle’s stiffness.
From their experiments with healthy volunteers, the researchers found that the ankle is strongest when moving up and down, as if pressing on a gas pedal. The joint is weaker when tilting from side to side, and weakest when turning inward.
Interestingly, their measurements indicate that the motion of the ankle from side to side is independent of the ankle’s up and down movement. The findings, Hogan notes, may help clinicians and therapists better understand the physical limitations caused by strokes and other motor disorders.
The researchers report their findings in the journal IEEE Transactions on Neural Systems and Rehabilitation Engineering. The paper’s co-authors are Hyunglae Lee, Patrick Ho, and Hermano Krebs from MIT and Mohammad Rastgaar Aagaah from Michigan Technological University.