‘Anklebot’ helps determine ankle stiffness

Press release:

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 in specific muscles, which researchers use to calculate the ankle’s stiffness.

From their experiments with healthy volunteers, the researchers found that the ankle lunge 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.

A robotic walking coach

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, the Anklebot is designed to train and strengthen lower-extremity muscles in a “cooperative” 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, “kind of like a massage,” Hogan says. The robot senses when patients start to move their ankles on their own, and adapts by offering less assistance.

“The key thing is, the machine gets out of the way as much as it needs to so you do not impose motion,” Hogan says. “We don’t push the limb around. You the patient have to do something.”

Many other robotic therapies are designed to do most of the work for the patient in an attempt to train the muscles to walk. But Hogan says such designs are often not successful, as they impose motion, leaving little room for patients to move on their own.

“Basically you can fall asleep in these machines, and in fact some patients do,” Hogan says. “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.”

Ankle mechanics

In their most recent experiments, the researchers tested the Anklebot on 10 healthy volunteers to characterize the normal mechanics of the joint.

Volunteers were seated and outfitted with the robot, as well as surface electrodes attached to the ankle’s four major muscles. The robot was connected to a video display with a pixelated bar that moved up and down, depending on muscle activity. Each volunteer was asked to activate a specific muscle — for example, to lift the foot toe-up — and maintain that activity at a target level, indicated by the video bar. In response, the robot pushed back against the ankle movement, as volunteers were told not to resist the robot’s force.

The researchers recorded each muscle’s activity in response to the robot’s opposing force, and plotted the results on a graph. They found that in general, the ankle was stiffest when toe-up or toe-down, while less stiff from side to side. When turning inward, the ankle was least stiff — a finding that suggests this direction of movement is most vulnerable to injury.

Understanding the mechanics of the ankle in healthy subjects may help therapists identify abnormalities in patients with motor disorders. Hogan adds that characterizing ankle stiffness may also be useful in designing safer footwear — a field he is curious to explore.

“For example,” Hogan says, “could we make aesthetically pleasing high heels that are stiffer in the inversion/eversion [side to side] direction? What is that effect, and is it worth doing? It’s an interesting question.”

For now, the team will continue its work in rehabilitation, using the Anklebot to train patients to walk.

Would changing gait pattern decrease your likelihood of running injuries?

ROSEMONT, Ill. (April 6, 2016)–Are runners less injury-prone trekking barefoot than in pricey running shoes? Maybe, according to a new literature review in the March issue of the Journal of the American Academy of Orthopaedic Surgeons (JAAOS). Advances in running shoe technology in the last 40 years have not reduced injuries, but racing “barefoot” in shoes with minimal cushioning could help runners change their strides and landing patterns to prevent repetitive heel pain and stress fractures.

Three of four active runners sustain injuries, mostly in the knee and lower leg. Most distance runners who use cushioned running shoes run heel-to-toe, or in a rearfoot strike (RFS) pattern. This action is associated with longer strides and excessive load force–up three times the runner’s body weight–on the lower leg, knee, and hip. This leads to bone and soft-tissue injuries, tibial stress fractures, and severe heel pain, such as plantar fasciitis.

Minimalist, including barefoot running has become popular in recent years. Minimalist running shoes have thinner soles and less cushioning and are more flexible than conventional runners’ footwear. Advocates believe these shoe changes alter running so the front or middle of the foot strikes the ground first–a forefoot or midfoot strike (FFS and MFS)–which reduces load stress on the knee, lower leg, and heel. Flatter foot placement dissipates load impact on the heel.

“Injury patterns among long-distance runners are unacceptably high, and while some research in minimalist running seems promising regarding injury prevention, there still are a lot of unknowns, and the debate continues,” says lead author and orthopaedic surgeon Jonathan Roth, MD, with Fort Belvoir Community Hospital in Virginia. “Evidence to date shows that changing gait patterns, not shoe selection, is the best intervention to lower the injury prevalence in runners. Minimalist shoes may give better feedback to runners and allow them to focus on changing their gait, but not everyone does, and this could lead to more injury.”

Dr. Roth added that increasing acceptance of minimalist running has outpaced medical evidence of its benefits. Orthopaedic literature, however, has demonstrated that with less-cushioned footwear, runners spontaneously transition from the RFS to the FFS gait pattern. Whether FFS running truly can reduce injuries is unknown, but the most compelling data were published in a 2012 study involving a Division I collegiate cross-country team. The results showed:

  • The athletes had a 75 percent injury rate per year, categorized as either traumatic or repetitive;
  • Strike type was characterized for each athlete and showed that 31 percent ran in the FFS pattern and 69 percent demonstrated RFS; and,
  • There was no difference in the traumatic injury rate between FFS and RFS runners; and,
  • FFS runners were 1.7 times less likely to sustain repetitive injuries than RFS runners.

Other findings in the JAAOS literature review include:

  • Barefoot and minimalist running is not injury-proof and poses risk for metatarsal (toe) stress fractures, plantar fasciitis, and puncture wounds;
  • Runners can transition to the FFS pattern in any shoe with appropriate training; and,
  • Barefoot and minimalist running is an emerging phenomenon that requires further exploration of its orthopaedic implications to identify true long-term benefits and risks.

Runners interested in exploring minimalist running shoes to provide more feel and less of a heel-to-toe offset, and to allow easier landing midfoot to forefoot, “should consider themselves as non-runners and start over by walking and gradually adding running distance week to week,” advises Dr. Roth. “This will help assure proper transitioning to build strength, flexibility, stability, and endurance around the foot and ankle.” Transition from a RFS to FFS gait pattern should be a gradual process–over many months. Runners should expect to run minimal mileage when transitioning and always remember the 10 percent rule when increasing in distance. An abrupt switching of gait patterns can lead to an increase in other repetitive stress injuries if not done correctly.