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 characterised.
“Imagine you have a collection of pebbles, and you wrap a whole bunch of elastic bands around them,” says MIT's Professor Neville Hogan. “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 '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.
Hogan and Krebs, a principal research scientist in MIT’s Department of Mechanical Engineering, developed the Anklebot as an experimental and rehabilitation tool. Much like MIT-Manus, a robot they developed to improve upper-extremity function, Anklebot is designed to train and strengthen lower-extremity muscles in a 'co-operative' fashion, sensing a person’s ankle strength and adjusting its force accordingly.
The team has 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.
In their most recent experiments, the researchers tested the Anklebot on ten healthy volunteers to characterise the normal mechanics of the joint. Understanding the mechanics of the ankle in healthy subjects may help therapists identify abnormalities in patients with motor disorders. It may also be useful in designing safer footwear — a field he is curious to explore.