Design and Preliminary Evaluation of a Variable Stiffness Knee Exoskeleton for Walking Assistance

This study presents the design and evaluation of a Variable Stiffness Knee Actuator (VSKA) for walking assistance, featuring a three-spring-group mechanism that provides discrete stiffness levels tailored to the biomechanical demands of the knee joint throughout the gait cycle. Based on gait analysis from seven healthy participants, three target stiffness values were determined for the initial stance, mid stance, and swing phases. A knee exoskeleton incorporating the VSKA was developed and evaluated in two separate treadmill-based experiments. In the first experiment, a healthy adult participant walked with a unilateral VSKA-driven exoskeleton at speeds of 3, 4, and 5 km/h under VSKA-enabled and VSKA-disabled conditions, where dynamic gait tracking performance was assessed using knee joint kinematics, yielding root mean square (RMS) errors below 7° across all speeds. In the second experiment, the same participant performed 10-minute walking trials at 3 and 5 km/h with a bilateral configuration, during which surface electromyography (sEMG) from key lower-limb muscles was recorded. Results showed RMS reductions was up to 9.9% in the vastus medialis and rectus femoris, particularly at higher walking speeds, indicating effective stance-phase assistance. These findings demonstrated that the VSKA-driven exoskeleton can deliver phase-specific assistance, with promising applications in gait rehabilitation and mobility enhancement, particularly for individuals with knee osteoarthritis (KOA).