Assistance Force-line of Exosuit Affects Ankle Multidimensional Motion: A Theoretical and Experimental Study

Abstract

Background: The talocrural joint and the subtalar joint are the two major joints of the ankle-joint complex. The position and direction of the exosuit force line relative to these two joint axes can influence ankle motion. We aimed to understand the effects of different force-lines on ankle multidimensional motion. Methods: In this article, three assistance force line schemes for ankle exosuits were proposed: perpendicular to the talocrural joint axis (PT), intersecting with the subtalar joint axis (IS), and parallel to the triceps surae (PTS). A theoretical model was proposed to calculate the exosuit's assistance moment. Seven participants completed four experimental tests of ankle plantar flexion, including three passive motions assisted by the PT, PTS, and IS, and one active motion without exosuit assistance. Results: The simulation results showed that all three exosuit schemes produced considerable ankle plantarflexion moments. Among the three schemes, the PT scheme generated the highest moments in all three dimensions, followed by the PTS and IS schemes. The experimental findings revealed that there was no significant difference in the angle of plantarflexion between passive and active plantarflexion (P < 0.05). However, there was an increase in the average peak angle of ankle inversion and adduction compared to no assistance, with the increment of approximately 21.07°, 6.57°, and 2.41° for inversion and 8.29°, 2.65°, and 0.33° for adduction, respectively. Our study demonstrated that all three exosuit schemes were beneficial for ankle plantar flexion. The IS scheme had the least effect on ankle inversion/eversion and adduction/abduction and was most similar to active ankle plantarflexion in all three directions compared to the other two schemes. Conclusions: Our research indicates that the position of the exosuit force line to the subtalar joint has a significant impact on ankle inversion and adduction. Among all three schemes, the IS, which has almost no effect on the subtalar joint, has the greatest kinematic similarity to active ankle plantarflexion and might be a better choice for ankle assistance and rehabilitation.