Exoskeleton suits, or exosuits, have become famous in popular culture , thanks to superheroes like Iron Man. They're also sometimes seen as the poster children for rehabilitation robotics. But this robotics field — which is centered around the design of wearable technologies that assist in movements such as walking — may have a new contender with the potential to be much more helpful than any of Tony Stark's gadgets.
Published in the March 16 issue of Science Robotics, a new study shows proof of concept for an assistive robotic waist tether that pulls from its wearer's center of mass to help its wearer walk and reduce energy consumption from movement, or metabolic rate. The findings show the device can reduce people's metabolic rates by up to 48% as they walk — and it does so by pulling at a counterintuitive time in the walking cycle when both of its user's legs are still on the ground.
"When we walk, our legs alternate between propulsion and braking forces. The robotic pulling mechanism could assist walking in a surprising way by starting a pulling motion while the person is still at the end of the braking phase," said Prokopios Antonellis, a postdoctoral research fellow at Oregon Health & Science University and lead author of the study.
Both the device's design and how it times its pulling force add to research on the development of diverse rehabilitative robots designed to make walking and other movement easier for their wearers. These types of technologies are especially useful for those in physical therapy and rehabilitation, because when it takes less energy to complete a mobility exercise, there is more stamina to do the exercise for longer.
The Many Steps of Walking
Walking is a cyclical, multi-step process. It involves shifting of weight from one leg to another, swinging each leg forward, and balancing throughout it all. On top of that, every person who walks has a unique pattern of walking, or gait, and uses different amounts of energy during this cycle. Those with neurological disorders like cerebral palsy or multiple sclerosis often have gaits with less range of motion and have to use more energy to move.
Exosuits designed to improve motion and lessen metabolic rate typically apply localized forces, such as pushing or pulling, on limbs and joints. This approach can overlook the nuances of gait and energy output. In the last few decades, researchers have also been designing other types of rehabilitative devices, including those that apply force on their wearer's center of mass at the waist.
"These were some of the first studies that used a device to reduce the metabolic cost of walking (and running)," said Antonellis, referencing an early study from the 2000s by researchers at the University of Colorado Boulder that tested the effects of elastic tethers on assisted walking.
For their new research, Antonellis and his colleagues took tethered systems one step further by adding a robotic device to examine how pulling from the waist at different times in the walking cycle could reduce users' metabolic rates.
"We thought it would be interesting to investigate the effect of applying forces at the center of mass but now using a robotic device that could allow us to apply forces at different times in the gait cycle instead of an elastic tether," said Antonellis.
A Change of Pace
To design a device that would act on its wearer's center of mass to support mobility and reduce energy output, the team created a robotic tethered system attached to a treadmill. They tested the device on 10 healthy participants, changing the timing and degree of force with which the device pulled during each user's walking cycle.
What was surprising to the team was that the pulling force was most effective at reducing metabolic cost when the participant was at the end of the braking phase with both feet in contact on the ground — not at the beginning of the propulsion phase when they were starting to step forward. The results also showed that neither the timing nor degree of pulling increased metabolic rate in healthy participants.
"Even the force profiles that occurred entirely during the braking phase of the step cycle reduced the metabolic cost…," said Antonellis. "The finding that none of the force profiles increased the metabolic cost was even more surprising since we definitely evaluated a large range of force profiles, up to the force magnitude where it becomes challenging to stay in place on the treadmill."
He and his colleagues also found the robotic waist tether decreased the metabolic rates of two patients with peripheral artery disease by more than 10%, suggesting the device could be used one day to make rehabilitative exercises easier for patients with conditions that can affect mobility.
"A next step could consist in evaluating the long-term effects of using the device for assisted stepping practice or evaluating whether stepping practice with resistance, instead of assistance, has benefits," said Antonellis.