Ankle Exoskeletons in Walking and Load-Carrying Tasks: Insights into Biomechanics and Human-Robot Interaction

Abstract

Background: Lower limb exoskeletons are in the focus of the scientific community due to their potential to enhance human quality of life across diverse scenarios. However, their widespread adoption remains limited by the lack of comprehensive frameworks to understand their biomechanical and human-robot interaction (HRI) impacts, which are essential for developing adaptive and personalized control strategies. To address this, understanding the exoskeleton's effects on kinematic, kinetic, and electromyographic signals, as well as HRI dynamics, is paramount to achieve improved usability of wearable robots. Objectives: This study aims to provide a systematic methodology to evaluate the impact of an ankle exoskeleton on human movement during walking and load-carrying (10 kg front pack) tasks, focusing on joint kinematics, muscle activity, and HRI torque signals. The methodology is designed to account for individual and device-specific factors, ensuring adaptability across users and exoskeletons. Materials and Methods: The study employed an inertial data acquisition system (Xsens MVN), electromyography (Delsys), and a unilateral ankle exoskeleton. Three complementary experiments were performed. The first examined basic dorsiflexion and plantarflexion movements. The second analysed the gait of two subjects without and with the device under passive and active assistance modes. The third investigated load-carrying tasks under the same assistance modes. Results and Conclusions: The first experiment confirmed that the HRI sensor captured both voluntary and involuntary torques, providing directional torque insights. The second experiment showed that the device slightly restricted ankle range of motion (RoM) but supported normal gait patterns across all assistance modes. The exoskeleton reduced muscle activity, particularly in active mode. HRI torque varied according to gait phases and highlighted reduced synchronisation, suggesting a need for improved support. The third experiment revealed that load-carrying increased GM and TA muscle activity, but the device partially mitigated user effort by reducing muscle activity compared to unassisted walking. HRI increased during load-carrying, providing insights into user-device dynamics. These results demonstrate the importance of tailoring exoskeleton evaluation methods to specific devices and users, while offering a framework for future studies on exoskeleton biomechanics and HRI.