Personalized Robotic Achilles Tendon Utilizing a Semi-Passive Spring with Switching Stiffness*

Abstract

Wearable robotic devices have been demonstrated to reduce muscle activation and metabolic cost during walking, but conventional motorized systems often impose significant weight and bulk, leading to user discomfort and limited portability. To address these limitations, the Robotic Achilles Tendon (RAT) was developed as a lightweight, semi-passive spring system that delivers ankle assistance exclusively during the stance phase. The RAT integrates a double-acting pneumatic cylinder and a solenoid valve to emulate spring behavior when the valve is closed and to permit unrestricted ankle motion when the valve is open. Gait-phase detection is achieved via a single inertial measurement unit mounted on the wrist, exploiting the conserved angular momentum that couples arm and leg movements. System architecture was optimized by eliminating motors and minimizing sensor count, resulting in a device weight of 0.45 kg per leg and a total weight of 1.4 kg. Performance evaluation involved surface electromyography and metabolic cost measurements in a cohort of healthy young adults. Compared to unassisted walking, the RAT reduced plantar-flexor muscle activation by 16.9% and decreased metabolic cost by 10.6%. These findings confirm that intent-based actuation of a semi-passive spring can provide effective ankle assistance with minimal hardware complexity. Future work will investigate alternative sensor locations that remain synchronized with lower-limb kinematics, simplify battery and processing modules to further reduce device mass, and extend validation to elderly and pediatric populations.