Matching the Hip Variable Stiffness Feature by an Unpowered Hip Exoskeleton Reduces the Metabolic Cost of Human Walking

Unpowered exoskeletons provide assistance for human walking by recycling negative mechanical energy from lower limb joints. However, current studies fail to further improve the effect of metabolic reduction due to a lack of an optimal match with the variable stiffness of lower limb joints during walking. We analyzed the feature of stiffness variation in the human hip joint and designed an unpowered hip exoskeleton to match that feature. The proposed exoskeleton utilized a mechanism composed of a cam and a linear spring to provide a moment variable in stiffness. This exoskeleton, manually adjustable to fit wearers, reduced the metabolic cost of human walking by $9.14~\pm ~0.85$ % (mean ± s.e.m.) compared to not wearing the exoskeleton. In addition, we found that heavier wearers achieved optimal assistive effects by using exoskeletons with higher stiffness. Our study further enhanced the effectiveness of exoskeletons by optimizing their stiffness feature, which has guiding significance for the subsequent development of exoskeletons.