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Biomechanical Assessment of Adapting Trajectory and Human-Robot Interaction Stiffness in Impedance-Controlled Ankle Orthosis

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Gait disabilities empowered intensive research on the field of human-robot interaction to promote effective gait rehabilitation. Assist-as-needed strategies are becoming prominent, appealing to the users’ participation in their rehabilitation therapy. This study proposes and assesses the biomechanical effects of an adaptive impedance control strategy that innovatively allows adaptability in interaction-based stiffness and gait trajectory towards a fully assist-as-needed therapy. By modulating the interaction-based stiffness per gait phase, we hypothesize that the strategy appeals to a symbiotic human-orthotic cooperation, augmenting the user’s muscular activity. The interaction stiffness was estimated by modelling the human-orthosis interaction torque vs angle curve with a linear regression model. The strategy also allows for real-time trajectory adaptations at different gait phases to fulfil the users’ needs. The biomechanical assessment of the impedance-controlled ankle orthosis involved eight healthy volunteers walking at 1.0 and 1.6 km/h. The results revealed a stronger muscular activation regarding the non-assisted leg for the gastrocnemius lateralis (increment ratio ≥ 1.0 for both gait speeds) and for the tibialis anterior muscle (increment ratio ≥ 1.0 for 1.6 km/h). The strategy guided users successfully on a healthy gait pattern while allowing deviations (median error < 5.0°) given the users’ intention weighted by interaction stiffness. Findings showed the relevance for adapting gait trajectory as users prefer higher trajectories as the speed increases. No significant temporal variations or neither knee angular compensations were observed (p value ≥0.11). Overall results support that this strategy may be applied for intensity-adapted gait training, allowing different human-robot compliant levels.

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This work has been supported by the FEDER Funds through the Programa Operacional Regional do Norte and national funds from Fundação para a Ciência e Tecnologia with the SmartOs project under Grant NORTE-01-0145-FEDER-030386, and under the national support to R&D units grant through the reference project UIDB/04436/2020 and UIDP/04436/2020.

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Authors and Affiliations



Conceptualization: João M. Lopes, Joana Figueiredo, and Cristina P. Santos; Data curation: João M. Lopes and Cristiana Pinheiro; Formal analysis: João M. Lopes, Joana Figueiredo and Cristiana Pinheiro; Funding acquisition: Cristina P. Santos and Luís P. Reis; Investigation: João M. Lopes, Joana Figueiredo, and Cristiana Pinheiro; Methodology: João M. Lopes, Joana Figueiredo, and Cristina P. Santos; Project administration: Cristina P. Santos; Resources: Cristina P. Santos; Software: João M. Lopes; Supervision: Joana Figueiredo and Cristina P. Santos; Validation: João M. Lopes and Joana Figueiredo; Visualization: João M. Lopes; Writing – original draft: João M. Lopes; Writing – review & editing: Joana Figueiredo, Luís P. Reis, and Cristina P. Santos. All authors read and approved the final manuscript.

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Correspondence to João M. Lopes.

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The study had the ethical approval of the Ethics Committee in Life and Health Sciences with the code reference CEICVS 006/2020.

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Lopes, J.M., Figueiredo, J., Pinheiro, C. et al. Biomechanical Assessment of Adapting Trajectory and Human-Robot Interaction Stiffness in Impedance-Controlled Ankle Orthosis. J Intell Robot Syst 102, 76 (2021).

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