A lightweight passive ankle exoskeleton with dual-phase energy harvesting and stiffness adaptation

Passive exoskeletons offer several advantages, including lightweight design, simple structure, and inherent energy efficiency. Most existing passive exoskeletons rely on clutch mechanisms to control spring-based energy storage and release, typically focusing only on recovering biomechanical energy during the stance phase of gait. In this study, we propose and analyze a lightweight passive ankle exoskeleton capable of harvesting and releasing energy during both the stance and swing phases of walking. The device aims to enhance gait assistance while maintaining structural simplicity and minimizing weight. By integrating the optimal stiffness ratio between the stance and swing phases, derived from musculoskeletal model simulations, with previously established optimal stance-phase stiffness parameters, we determined a suitable stiffness coefficient for the swing-phase spring. To validate the design, we conducted comparative experiments on participants walking with exoskeletons configured with different stiffness coefficients. Spatiotemporal parameters, metabolic energy cost, and muscle activation patterns were analyzed to evaluate performance. The results demonstrate that the proposed exoskeleton effectively reduces Soleus muscle activation while increasing tibialis anterior activity, leading to a 6.84% reduction in walking energy cost compared to a nonassistive condition. Furthermore, energy recovery during the swing phase alone contributes an additional 1.67% reduction in energy expenditure, improving overall walking efficiency. The proposed design also eliminates complex clutch components, significantly simplifying manufacturing and reducing costs, thereby enhancing the applicability of passive exoskeletons in daily mobility and rehabilitation scenarios.