Characterizing the relationship between peak assistance torque and metabolic cost reduction during running with ankle exoskeletons

Abstract

Background Reducing the energy cost of running with exoskeletons could improve enjoyment, reduce fatigue, and encourage participation among novice and ageing runners. Previously, tethered ankle exoskeleton emulators with offboard motors were used to greatly reduce the energy cost of running with powered ankle plantarflexion assistance. Through a process known as “human-in-the-loop optimization”, the timing and magnitude of assistance torque was optimized to maximally reduce metabolic cost. However, to achieve the maximum net benefit in energy cost outside of the laboratory environment, it is also necessary to consider the tradeoff between the magnitude of device assistance and the metabolic penalty of carrying a heavier, more powerful exoskeleton. Methods In this study, tethered ankle exoskeleton emulators were used to characterize the effect of peak assistance torque on metabolic cost during running. Three recreational runners participated in human-in-the-loop optimization at four fixed peak assistance torque levels to obtain their energetically optimal assistance timing parameters at each level. Results We found that the relationship between metabolic rate and peak assistance torque was nearly linear but with diminishing returns at higher torque magnitudes, which is well-approximated by an asymptotic exponential function. At the highest assistance torque magnitude of 0.8 Nm/kg, participants’ net metabolic rate was 24.8 ± 2.3% (p = 4e–6) lower than running in the unpowered devices. Optimized timing of peak assistance torque was as late as allowed during stance (80% of stance) and optimized timing of torque removal was at toe-off (100% of stance); similar assistance timing was preferred across participants and torque magnitudes. Conclusions These results allow exoskeleton designers to predict the energy cost savings for candidate devices with different assistance torque capabilities, thus informing the design of portable ankle exoskeletons that maximize net metabolic benefit.

Evaluation of controllers for augmentative hip exoskeletons and their effects on metabolic cost of walking: explicit versus implicit synchronization

“…The observed diminishing metabolic gain from the provided mechanical power by the exoskeleton could be due to several reasons. Firstly, a generally diminishing trend with increased assistance has been observed in some of the past studies with ankle exoskeletons ( Jackson and Collins, 2015 ; Miller et al, 2022 ). It has also been observed in human-in-the-loop optimization studies that the best assistance strategies from a metabolic gain point of view are not those with the highest exoskeleton mechanical power ( Zhang et al, 2017 ).…”

Optimizing exoskeleton assistance to improve walking speed and energy economy for older adults

“…Variation in exoskeleton power was primarily attributed to variation in optimized torque magnitude (0.54 ± 0.10 N-m/kg), though optimized timing parameters also differed among participants (Additional File 1: Table S1 and Figure S3 ). Rise time was highly varied (27.3 ± 7.0%), suggesting that the parameter is either subject-specific or weakly affects time and energy costs [ 50 ]. Peak time and fall time were more consistent between participants ( t p = 52.3 ± 1.1% and t f = 10.4 ± 2.6%), indicating that exoskeleton torque peaking near the end of stance and quickly falling to zero at toe-off was widely beneficial.…”

Evaluation of Lower Limb Exoskeleton for Improving Balance during Squatting Exercise using Center of Pressure Metrics

“…In this paper, we have performed secondary data analysis of human-in-the-loop optimization of exoskeleton parameters to minimize physical effort during squatting (metabolic cost reduction). The optimal assistance has been provided for various activities such as walking (Ding et al, 2018;Gordon, McGreavy, Christou, & Vijayakumar, 2022;Li et al, 2020;Song & Collins, 2021;Zhang et al, 2017), and running (G. Lee et al, 2017;Miller, Tan, Farina, Sheets-Singer, & Collins, 2022;Uchida et al, 2016;Zhang et al, 2017) to minimize metabolic In this study, in addition to the metabolic cost of squatting, we examined the foot pressure, and it appears that the balance can be influenced for some subjects.…”