Design and evaluation of a gait-adaptive passive ankle exoskeleton for metabolic cost reduction

Conventional active ankle exoskeletons are often bulky and heavily reliant on external power sources. This study presents a lightweight and flexible passive ankle exoskeleton (LFPA-EXO) aimed at reducing metabolic cost of walking. The LFPA-EXO features a gait-adaptive clutch (GA-clutch) and a super-elastic composite booster (SC-booster). By matching the walking gait, it stores gravitational potential energy and converts it into elastic energy through the booster, thereby reducing the metabolic cost of human locomotion. Mechanical and biomechanical evaluations demonstrate that the GA-clutch achieves less than 5% interference and over 85% assistance, indicating that the LFPA-EXO operates within the natural ankle joint range of motion without disrupting normal gait patterns. It delivers a peak assisting moment of 24.56 Nm during normal walking. Notably, it decreases the activation of the soleus muscle while moderately reducing the activation of the gastrocnemius muscle, with minimal impact on the tibialis anterior muscle. The LFPA-EXO achieves a 12.22% reduction in metabolic cost and an 11.17% decrease in average heart rate, underscoring its effectiveness in reducing metabolic cost during walking.