Findings describe a novel way to reduce the energy people spend to walk, as much as by half, which could have applications for therapy received by patients with impaired walking abilities.
The research, conducted at the University of Nebraska at Omaha and published in the journal Science Robotics, demonstrates that the optimal way to assist with a wearable device does not always align with intuition.
Based on previous literature, the researchers believed they would see the highest energy savings by pulling with a waist tether when the individual is trying to propel forward against the ground. That hypothesis was based on a bioinspired assistance strategy, meaning it is inspired by how our biological muscles work during walking.
“Although bioinspired actuation can have certain benefits, our study demonstrates that this is not necessarily the best strategy for providing the greatest reduction in metabolic cost or energy expended,” said Prokopios Antonellis, Ph.D., first author of the study and now a postdoctoral fellow at Oregon Health & Science University.
“This finding supports a greater emphasis on biomechanical testing rather than trying to predict optimal bioinspired strategies,” said Antonellis, who performed the research during his doctoral program at UNO.
The approach of using biomechanical testing to optimize a robotic waist tether is highlighted as one of different unique approaches for designing personalized assistance in an editorial published March 30 by Amos Matsiko, Ph.D., senior editor of Science Robotics.
This research shows that a strategically-timed pull from a waist belt connected to a pulley can help an individual use less energy for each step while walking. However, the optimal timing of that forward pull was what came as a surprise.
“When we walk, there is a short period between steps where one foot is stopping its forward motion while the other is preparing to accelerate to take the next step forward. Our research shows that this brief window where both feet are on the ground is the best time to apply force to assist walking most efficiently,” said Philippe Malcolm, Ph.D., assistant professor in biomechanics at UNO
The device works by providing timed pulls from a motorized pulley while an individual walks on a treadmill. Since it only requires wearing a waist belt, it is relatively easy to make individualized adjustments compared to more complicated devices.
The findings about optimal timing could have applications for exercise therapists in clinical settings providing care for patients with conditions such as peripheral artery disease. Iraklis Pipinos, M.D., a vascular surgeon at the University of Nebraska Medical Center and the Omaha VA Medical Center, who collaborated with the study team, sees the benefits of this research.
“My patients have hardening of their arteries causing problems in the circulation to their legs, resulting in leg pain and reduced mobility,” said Pipinos. “I was touched to hear that certain patients felt relief in their legs for the first time when they tried the device. We are now thinking of ways these methods can be used in everyday practice, for example, by using systems for assisted walking exercise therapy at physical therapy clinics.”
More information: Prokopios Antonellis et al, Metabolically efficient walking assistance using optimized timed forces at the waist, Science Robotics (2022). DOI: 10.1126/scirobotics.abh1925
Amos Matsiko, Robotic assistive technologies get more personal, Science Robotics (2022). DOI: 10.1126/scirobotics.abo5528
The field of wearable robotics has evolved from sophisticated full-body exoskeletons (2) toward simpler single-joint exoskeletons that first achieved metabolic rate reduction (9, 21, 22). Our study suggests that a simpler strategy of accelerating the COM with linear forces can provide further gains in stationary applications such as treadmill exercise therapy. Although robotic tethers cannot assist with overground mobility similar to exoskeletons, the greater reductions could enable treadmill exercise therapy applications.
Clinical practice guidelines recommend that combining robotic assistance with higher intensity stepping is an area that remains to be investigated (23). The large effects of robotic waist tether assistance could allow for higher intensity stepping training. However, it remains to be seen whether the effects observed in healthy participants transfer to patients.
Although the effect of timing on the horizontal GRF was small, we found large effects on the COM velocity. Using their pulley mechanism that allowed for applying nonconstant force profiles, Penke et al. (6) found a 12% reduction in metabolic cost in individuals poststroke but no reduction using constant forces.
Thus, timing could potentially have a greater influence in populations with nonconstant walking velocity, such as individuals poststroke, but this remains to be tested. Evaluating the effect of timed assistance at the COM in clinical populations requires an investigation of the targeted clinical population because there are different instances where the effects of assistive devices in healthy participants did not translate to patients (24–27).
In clinical populations or older adults, it is possible that the effects of nonconstant forces on maintaining balance eliminate any metabolic cost reduction benefit. We evaluated the applicability of sinusoidal waist tether assistance in impaired gait in a healthy participant and two older adults with PAD and found that it could reduce the metabolic rate of walking. However, it is not yet possible to draw general conclusions from these small sample size experiments. Further experiments are needed to evaluate whether these results are reproducible in larger samples of patients.
Our work shows that it is possible to reduce the metabolic rate of walking by half by accelerating the COM. Although the results from our main study in healthy participants show benefits of optimally timed forces, the effects of timing were relatively small compared with the effect of aiding work rate. This could be considered as a limitation given the fact that applying different timings is more challenging than applying different constant magnitudes, which can be done with a passive tether system. Another limitation is that we only tested sinusoidal force profiles and did not evaluate whether other force profiles could provide greater assistance.
The observation that all force profiles with suboptimal timing still reduced the metabolic rate suggests that further reductions can be obtained with profiles that do not stay at a force level of 0% BW during a part of the step cycle. Using a simple pendulum model, we predict that one could approach an 80% reduction in metabolic rate by accelerating the COM during the first half of single support, followed by decelerating the COM during the second half (with a backward force) (fig. S10) (15).
Such strategies could be implemented in cable robots for treadmill exercise therapy (28) or mobile devices that assist via the trunk, such as motorized rollators (29).
A number of wearable robots for clinical populations are designed to assist specifically during propulsion (30) or mimic biological kinematics or kinetics (31). We found that assisting during propulsion can reduce the metabolic rate but does not optimally reduce the metabolic rate, and even net aiding force profiles that occur entirely during braking reduce the metabolic rate. This finding indicates that bioinspired controls that mimic biological kinetics are not necessarily optimal for reducing the metabolic rate.