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MIT 'Anklebot' gauges joint stiffness to improve rehab

The bot is an experimental rehabilitation tool that uses electrodes to calculate the ankle's stiffness in various directions.

A front and side view of a volunteer wearing the Anklebot, which moves the ankle and records its stiffness.
Hyunglae Lee/MIT

The ankle is something of an anatomical puzzle.

"Imagine you have a collection of pebbles, and you wrap a whole bunch of elastic bands around them," Neville Hogan, a mechanical engineering professor at MIT, said in a school news report. "That's pretty much a description of what the ankle is. It's nowhere near a simple joint from a kinematics standpoint."

So Hogan teamed up with colleagues at MIT's Newman Laboratory for Biomechanics and Human Rehabilitation to test Anklebot, a robot that uses electrodes to record the torque and angular displacement at the joint and calculate stiffness in various directions.

To do this, the bot is mounted to a knee brace that is in turn connected to a custom shoe, and Anklebot moves the person's foot in a programmed trajectory to test the ankle's normal range of motion.

In testing the bot on 10 healthy volunteers, the team learned that the motion of the ankle when moving side to side is actually independent of the motion when moving up and down. They also found that the ankle is weakest when turning inward, stronger when tilting from side to side, and strongest when simply moving up and down.

These findings, published in the journal IEEE Transactions on Neural Systems and Rehabilitation Engineering, could help physical therapists who are working with victims of strokes and other motor disorders, or even inform preventive measures for healthy people who want to stay strong.

Hogan has previously worked with colleagues to develop MIT-Manus, a rehabilitative bot for upper-extremity function. Both are designed to strengthen muscles by detecting strength and adjusting the force applied to the joint accordingly. Think of it as a customized weight press that won't do all the work for you, but won't push you beyond your limit.

"Basically you can fall asleep in [some] machines, and in fact some patients do," Hogan added. "What we're trying to do with machines in therapy is equivalent to helping the patients and weaning them off the dependence on the machine. It's a little bit like coaching."

In this small study, Anklebot was connected to a video display, where muscle activity could be seen in the form of a pixelated bar moving up and down. Each muscle's activity was recorded as the volunteers responded to the bot's opposing force, and the results were plotted on a graph. This is how the researchers were able to see that the ankle was stiffest when toe-up or toe-down and least stiff when turning inward, meaning this movement could be most vulnerable to injury.

Not only does understanding ankle mechanics inform therapies for patients with motor disorders, but it could also help train healthy people to exercise their ankles in specific ways that strengthen them and help reduce future injuries. Hogan suggested it could even lead to better footwear -- i.e., "Could we make aesthetically pleasing high heels that are stiffer in the [side to side] direction? What is that effect, and is it worth doing? It's an interesting question."

We'll have to wait for the answer. At this point, the team is focused on using the bot for rehabilitation purposes.