An Unpowered Exoskeleton Enabling Cross-Limb Energy Transfer from Arm Swing to Assist Contralateral Ankle Plantarflexion

Unpowered exoskeletons commonly assist walking by storing and releasing elastic energy, with existing designs largely limited to energy transfer within the lower limbs. However, arm swing, an integral coupled component of human gait, remains underexplored as a mechanical energy source for locomotor assistance. Motivated by gait biomechanics, we propose a cross-limb energy transfer concept that redirects positive mechanical work from unilateral arm swing to the contralateral ankle. Based on this concept, we developed a novel unpowered exoskeleton consisting of a waist-mounted force-routing module and a shank-ankle assistance module. Treadmill walking experiments (N = 7) were conducted under multiple conditions to quantify exoskeleton torque, spatiotemporal characteristics, gait kinematics, and muscle activity. The results show that the device provided phase-appropriate ankle plantarflexion assistance without significantly altering spatiotemporal parameters, gait stability, or limb coordination. Compared to normal walking, wearing the exoskeleton reduced average soleus muscle activity by 11.1%, indicating effective off-loading of the biological plantarflexor system. Comparisons across assistive conditions further suggest that incorporating arm swing introduced an additional energy contribution beyond lower-limb energy recycling alone. These results demonstrate the feasibility of cross-limb energy transfer in an unpowered exoskeleton and establish a design paradigm that leverages coordinated whole-body motion and active mechanical contributions to expand energy pathways for gait assistance.