The ankle joint is a complex structure of muscles, tendons and bones, that poses a real challenge for rehabilitation when it goes wrong. Now a team of biomechanical experts at MIT has devised a robot they call the “Anklebot” that can measure ankle stiffness in different directions, giving important information for physical therapists.
Neville Hogan, the Sun Jae Professor of Mechanical Engineering at MIT, says the ankle is “nowhere near a simple joint”:
“Imagine you have a collection of pebbles, and you wrap a whole bunch of elastic bands around them. That’s pretty much a description of what the ankle is.”
Prof. Hogan and his colleagues in MIT’s Newman Laboratory for Biomechanics and Human Rehabilitation carried out a study where they used the Anklebot to measure the stiffness of the ankle in various directions.
They write about their findings in a paper published online recently in IEEE Transactions on Neural Systems and Rehabilitation Engineering.
Anklebot is a robot mounted to a knee brace and connected to a custom-designed shoe that the seated patient puts on. Once they are hooked up to Anklebot, the robot moves the patient’s foot according to a preset pattern, in different directions, within the ankle’s normal range of movement.
Electrodes record the angular displacement and torque at the joint (a bit like the amount of flexing, pointing and twisting that the ankle allows the foot to make) to work out how stiff the ankle is.
When they tested the Anklebot on healthy volunteers the team found the ankle is strongest when moving the foot up and down (flexing and pointing, like pressing on a gas pedal). It is less strong when tilting from side to side, and weakest when turning inward.
They were surprised to discover that the ankle’s side to side movement is independent of its up and down movement.
Prof. Hogan says he hopes their findings will help physical therapists and clinicians better understand the kinds of physical limitations that patients with stroke or motor disorders experience.
Prior to this latest study, the team had already been testing the Anklebot as a physical therapy tool to help stroke patients experiencing difficulty walking.
The patients had daily sessions where they sat in a chair and were hooked up to the Anklebot. At first, the robot did all the work, putting the ankle through a range of movements, loosening up the muscles.
But gradually, as the patients started to move their ankles themselves, the robot would help them less and less.
Prof. Hogan explains:
“The key thing is, the machine gets out of the way as much as it needs to so you do not impose motion. We don’t push the limb around. You the patient have to do something.”
The Anklebot is different to other robotic therapies because it allows the patient gradually to do more and more work, instead of doing all the work as a way to train the muscles.
Eric Perreault is a professor of biomedical engineering and physical medicine and rehabilitation at Northwestern University and did not take part in the research. He says the MIT team has given us the first insight into how muscle activation changes the mechanics of the ankle, something that is very relevant to ankle injuries like common sprains. He notes:
“An intriguing extension of this work is that it may be possible to train individuals to activate their ankle musculature in a way that helps reduce the chance of injury. A more immediate benefit of the study is that it presents a method for quantifying the impact of existing rehabilitation therapies on the mechanical properties of the ankle.”
In 2012, researchers in Greece reported that professional soccer players tend to suffer from more ankle sprains when one foot is stronger than the other.