Optimization of an unpowered energy-stored exoskeleton spring stiffness for spinal cord injuries

The unpowered energy-stored exoskeleton can provide precise walking assistance for spinal cord injury patients for a specified body height, weight and injury level. Since the energy-stored springs are standard components of the exoskeleton, their stiffness should be constant. The aim of this study is to select the optimal stiffness of the exoskeleton springs. The hip joint moment was related to the body height, body weight and hip joint angle through an inverse dynamics model for spinal cord injury patients walking in the single support phase from mid stance to terminal stance. The optimization method minimized the integral of the absolute moment generated by the hip joint and springs. The results show that the stiffness of spring 1 (k₁) has a normal distribution while the stiffness of spring 2 (k₂) does not. The medians of the two spring stiffnesses (the median k₁=3 180 N/m and the median k₂=1 279 N/m) can apply to patients whose heights and weights are in the P1-P99 ranges.