The ankle is a complex joint, supported by muscle, tendon, and bones, and maintaining stability and locomotion. Characterizing how it works, however, is not so straightforward says a group of researchers from the Newman Laboratory for Biomechanics and Human Rehabilitation at MIT, Cambridge, MA.

A front view of a volunteer wearing the Anklebot in a seated posture. (Credit: Hyunglae Lee)

They 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. Their measurements also show that the side to side motion is independent of the ankle’s up and down movement.

The Anklebot is designed to train and strengthen lower-extremity muscles by sensing a person’s ankle strength and adjusting its force accordingly. They tested the Anklebot on stroke patients who experience difficulty walking. In daily physical therapy sessions, patients are seated in a chair and outfitted with the robot. Typically during the first few sessions, the robot does most of the work, moving the patient’s ankle back and forth and side to side, loosening up the muscles. The robot senses when patients start to move their ankles on their own, and adapts by offering less assistance.

Their findings may help clinicians and therapists better understand the physical limitations caused by strokes and other motor disorders and aid in rehabilitation therapies.