Anchor-Point Modulation for Multiplanar Hip Torque Generation in Cable-Driven Exosuits: A Geometry-Based Empirical Model

Cable-driven hip exosuits are designed to deliver assistive torques during locomotion. However, most studies focus on assistance in a single plane, even though hip mechanics are inherently multiplanar. This study investigates whether modulation of proximal and distal cable anchor-point locations can systematically alter the distribution of hip assistive torque across anatomical planes. To characterize and estimate three-dimensional torque, we propose a geometric model that incorporates empirically identified correction terms. The model was evaluated using a hip–thigh phantom across 12 anchor configurations and 17 joint-angle conditions, resulting in 204 experimental cases. The results show that anchor-point modulation systematically redistributes assistive torque among the sagittal, frontal, and transverse planes, demonstrating that changes in cable routing can generate distinct multiplanar torque profiles. The model captured major trends and directional characteristics of the torque, with strongest agreement observed for the Mz component, for which the mean absolute percentage error ranged from 4.7% to 10.1%. Estimation errors increased under conditions of greater cable wrap, reflecting the influence of torsion and compliance effects not explicitly represented. These findings establish anchor-point modulation as a practical method for achieving multiplanar hip assistance and provide a foundational empirical baseline for extending cable-driven assistance beyond conventional sagittal-plane operation.