Unilateral Assistance Using a Lightweight and Back-Drivable Hip Exoskeleton

While lightweight and highly back-drivable exoskeletons enable natural interaction by minimizing resistance to voluntary motion, few studies have investigated the resulting combined physiological and biomechanical effects of unilateral hip torque assistance. This paper presents a hip exoskeleton designed to provide unilateral torque assistance during walking to investigate the effects of unilateral actuation on lower-limb muscle responses. The device employs a quasi-direct-drive actuator and a simplified angle-based control scheme to deliver torque in phase with the hip flexion. Although the mechanical frame was bilaterally symmetrical, only the right hip joint was actuated to evaluate the effects of unilateral assistance while maintaining inertial balance and low mechanical impedance on the unpowered side. Three healthy participants walked on a treadmill under different load (0, 1, and 2 kg) and assistive torque (0, 6, and 9 N·m) conditions. Subsequently, surface electromyography and motion capture data were recorded simultaneously. The activity of the rectus femoris muscle consistently decreased, showing over 30% reduction compared to that observed during unassisted walking under loaded conditions. Furthermore, postural deviation was measurably reduced by up to 46.8% during assisted conditions without hindering the natural swing. These findings demonstrate that a lightweight unilaterally actuated exoskeleton can provide evident physiological benefits even with a minimal control strategy. Notably, moderate torque (6 N·m) proved to be sufficient for yielding significant benefits, in some cases outperforming higher torque levels (9 N·m). This suggests that optimizing assistance levels based on user-specific needs is crucial for maximizing walking comfort and supporting rehabilitation in future designs.