A Biologically Inspired Cross-Type Ankle-Foot Exotendon: Assisting Plantarflexion Moment and Movement Stability

Cable-driven ankle-foot exoskeletons have attracted numerous researchers over the previous decade. The assistive forces of most exoskeletons pulled the back bottom of the shoes, across talocrural and subtalar joints. The talocrural joint is inherently mediolateral unstable at the plantarflexion position due to its sliding mortise structure, while the subtalar joint allows inversion/eversion. In this paper, a biologically inspired cross-type double-cable-driven ankle-foot exotendon was proposed to assist not only the plantarflexion moment but also the movement stability. The novel structure was bio-inspired by the behind-calf anatomically symmetric layout and under-foot cross-configuration of the ankle-foot muscles. To examine the combined functions, we conducted a forward pelvis perturbed standing experiment on five subjects without and with exotendon assistance and recorded the biomechanical data. Compared to the unpowered condition, the biological ankle plantarflexion moment was reduced by 39% with 0.1 Nm/kg exotendon assistance for one leg. Besides, the forward margin of stability was increased by 17% during the late perturbation period, which indicated the improvement of balance in the sagittal plane. In addition, the standard deviation of the lateral CoP and three-dimensional marker trajectories for the ankle condylar and heel all descended, which provided evidence for ankle-foot stability improvement. The results suggested that the proposed biological exotendon can provide the compound ankle-foot assistance, reducing plantarflexion moment and improving movement stability.