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

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...

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Bibliographic Details
Published in:	Journal of neuroengineering and rehabilitation Vol. 19; no. 1; p. 46
Main Authors:	Miller, Delaney E, Tan, Guan Rong, Farina, Emily M, Sheets-Singer, Alison L, Collins, Steven H
Format:	Journal Article
Language:	English
Published:	England BioMed Central 12-05-2022 BMC
Subjects:	Aging Ankle Ankle Joint Augmentation Biomechanical Phenomena Cost reduction Design Emulators Energy Energy conservation Energy costs Energy Metabolism Exoskeleton Exoskeleton Device Exoskeletons Exponential functions Gait Human-in-the-loop optimization Humans Metabolic cost Metabolic rate Metabolism Optimization Power Running Torque Walking Weeds Augmentation Running Exoskeleton Metabolic cost Human-in-the-loop optimization
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Summary:	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. 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. 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. 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.
Bibliography:	ObjectType-Article-1 SourceType-Scholarly Journals-1 ObjectType-Feature-2 content type line 23
ISSN:	1743-0003 1743-0003
DOI:	10.1186/s12984-022-01023-5