A prototype ankle exoskeleton developed at Stanford increases the self-selected walking speed, to the potential benefit of older adults and others with a reduced pace.
Being unable to walk quickly can be frustrating and problematic, but it is a common issue, especially as people age. Noting the pervasiveness of slower-than-desired walking, engineers at Stanford University have tested how well a prototype exoskeleton system they have developed – which attaches around the shin and into a running shoe – increased the self-selected walking speed of people in an experimental setting.
The exoskeleton is externally powered by motors and controlled by an algorithm. When the researchers optimised it for speed, participants walked, on average, 42% faster than when they were wearing normal shoes and no exoskeleton.
"We were hoping that we could increase walking speed with exoskeleton assistance, but we were really surprised to find such a large improvement," said Steve Collins, associate professor of mechanical engineering at Stanford and senior author of the paper. "Forty percent is huge."
For this initial set of experiments, the participants were young, healthy adults. Given their impressive results, the researchers plan to run future tests with older adults and to look at other ways the exoskeleton design can be improved. They also hope to eventually create an exoskeleton that can work outside the lab, though that goal is still a ways off.
"My research mission is to understand the science of biomechanics and motor control behind human locomotion and apply that to enhance the physical performance of humans in daily life," said Seungmoon Song, a post-doctoral fellow in mechanical engineering and lead author of the paper. "I think exoskeletons are very promising tools that could achieve that enhancement in physical quality of life."
The ankle exoskeleton system tested in this research is an experimental emulator that serves as a testbed for trying out different designs. It has a frame that fastens around the upper shin and into an integrated running shoe that the participant wears. It is attached to large motors that sit beside the walking surface and pull a tether that runs up the length of the back of the exoskeleton. Controlled by an algorithm, the tether tugs the wearer's heel upward, helping them point their toe down as they push off the ground.
For this study, the researchers had 10 participants walk with five different modes of operation. They walked in normal shoes without the exoskeleton, with the exoskeleton turned off and with the exoskeleton turned on with three different modes: optimised for speed, optimised for energy use, and a placebo mode adjusted to make them walk more slowly. In all of the tests, participants walked on a treadmill that adapts to their speed.
The mode that was optimised for speed – which resulted in the 42% increase in walking pace – was created through a human-in-the-loop process. An algorithm repeatedly adjusted the exoskeleton settings while the user walked, with the goal of improving the user's speed with each adjustment. Finding the speed-optimised mode of operation took about 150 rounds of adjustment and two hours per person.
In addition to greatly increasing walking speed, the speed-optimised mode also reduced energy use, by about 2% per meter travelled. However, that result varied widely from person to person, which is somewhat expected, given that it was not an intentional feature of that exoskeleton mode.
"The study was designed to specifically answer the scientific question about increasing walking speed," Song said. "We didn't care too much about the other performance measures, like comfort or energy. However, seven out of 10 participants not only walked faster but consumed less energy, which really shows how much potential exoskeletons have for helping people in an efficient way."
The settings that were optimised specifically for energy use were borrowed from a previous experiment. In the current study, this mode decreased energy use more than the speed-optimised settings but did not increase speed as much. As intended, the placebo mode both slowed down participants and boosted their energy use.
Now that the researchers have attained such significant speed assistance, they plan to focus future versions of the ankle exoskeleton emulator on reducing energy use consistently across users, while also being more comfortable.
In considering older adults specifically, Collins and his lab wonder whether future designs could reduce pain caused by weight on joints or improve balance. They plan to conduct similar walking tests with older adults and hope those provide encouraging results as well.
"A 40% increase in speed is more than the difference between younger adults and older adults," said Collins. "So, it's possible that devices like this could not only restore but enhance self-selected walking speed for older individuals and that's something that we're excited to test next."
Optimizing exoskeleton assistance for faster self-selected walking
Seungmoon Song, Steven H Collins
Published in IEEE on 20 April 2021
Self-selected walking speed is an important aspect of mobility. Exoskeletons can increase walking speed, but the mechanisms behind these changes and the upper limits on performance are unknown. Human-in-the-loop optimization is a technique for identifying exoskeleton characteristics that maximize the benefits of assistance, which has been critical to achieving large improvements in energy economy. In this study, we used human-in-the-loop optimization to test whether large improvements in self-selected walking speed are possible through ankle exoskeleton assistance. Healthy participants (N = 10) were instructed to walk at a comfortable speed on a self-paced treadmill while wearing tethered ankle exoskeletons. An algorithm sequentially applied different patterns of exoskeleton torque and estimated the speed-optimal pattern, which was then evaluated in separate trials. With torque optimized for speed, participants walked 42% faster than in normal shoes (1.83 m s-1 vs. 1.31 m s-1; Tukey HSD, p = 4 × 10-8), with speed increases ranging from 6% to 91%. Participants walked faster with speed-optimized torque than with torque optimized for energy consumption (1.55 m s-1) or torque chosen to induce slow walking (1.18 m s-1). Gait characteristics with speed-optimized torque were highly variable across participants, and changes in metabolic cost of transport ranged from a 31% decrease to a 78% increase, with a decrease of 2% on average. These results demonstrate that ankle exoskeletons can facilitate large increases in self-selected walking speed, which could benefit older adults and others with reduced walking speed.
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