Soleus Bioinspired VSM-Driven Soft Ankle Exosuit for Walking Assistance

Abstract

Soft ankle exosuits typically employ cable-driven configurations to emulate muscle-tendon units for gait assistance. However, existing actuation designs remain limited in reproducing the phase-specific roles of individual ankle muscles throughout the gait cycle. Based on the biological contraction characteristics of the soleus muscle, this letter proposes a novel cable-driven exoskeleton architecture incorporating a variable stiffness mechanism (VSM), alongside a time-independent heuristic phase switching strategy and a torque-stiffness tracking strategy. This system aims to simulate the dynamic contraction characteristics of the soleus muscle, thereby emulating its functional roles during the isometric, concentric, and transparent phases. The proposed strategy enables coordinated VSM deformation and effective torque transmission for phase-dependent walking assistance. Experimental results involving six subjects across level, uphill, and downhill terrains demonstrate that the system significantly reduced average soleus activity while emulating targeted muscle behaviors. Notably, a 14.21% reduction was observed during level walking compared to unassisted walking. Furthermore, terrain-specific analysis suggests the exoskeleton’s capability to facilitate uphill propulsion and provide downhill braking, thereby validating the efficacy of mimicking concentric and isometric contractions, respectively. These results demonstrate that the series integration of a VSM into cable-driven exoskeletons for soleus-inspired functional emulation is an effective pathway to enhance gait assistance.