Simulating the soft-tissue interface for a knee exoskeleton for improved interaction dynamics during assisted gait

Exoskeletons assist human movement by transmitting forces and moments through the user-exoskeleton interface. However, the effectiveness of this assistance is strongly affected by the mechanical properties of the interface, as soft-tissues can absorb a substantial portion of the applied power. Accurately modeling the interface, including the softtissue behaviour, is therefore essential for design of efficient and comfortable exoskeletons. In this study, we developed a simulated phantom knee system assisted by an exoskeleton. We modeled the soft-tissue properties and contact mechanics at the interface using MuJoCo's flexcomp feature. Simulated knee moments showed strong agreement with experimental measurements, with correlations ranging from 0.98 to 0.99 across varying interface stiffnesses. Further, increasing interface stiffness resulted in larger moment-deformation loops in the simulations, indicating increased energy dissipation at the interface. Beyond reproducing experimental findings, the model enabled spatially resolved estimation of interface forces, providing pressure maps across the skin surfaces indicating possible pressure hotspots. The proposed approach can be generalized to other joints and exoskeletons, offering a promising tool to design effective and comfortable exoskeletons.