Closed-Loop Control of FES-Assisted Arm-Free Standing in Individuals with Spinal Cord Injury: A Feasibility Study

Objectives: The purpose of the present study was to show that the design of a neuroprosthesis for unsupported (arm-free) standing is feasible. We review findings suggesting that a closed-loop controlled functional electrical stimulation (FES) system should be able to facilitate arm-free quiet standing in individuals with spinal cord injury (SCI). Particularly, this manuscript identifies: 1) a control strategy that accurately mimics the strategy healthy individuals apply to regulate the ankle joint position during quiet standing, and 2) the degrees of freedom (DOF) of the redundant, closed-chain dynamic system of bipedal stance that have to be regulated to facilitate stable standing. Methods and Results: First, we utilized a single DOF model of quiet standing (inverted pendulum) to analytically identify a proportional and derivative (PD) feedback controller that regulates the ankle torque in a physiological manner despite a long sensory-motor time delay. Second, these theoretical results were experimentally validated by implementing the proposed PD controller to stabilize an individual with SCI during quiet standing. Third, a realistic, three-dimensional dynamic model of quiet standing with twelve DOF was used to determine the optimal combination of the minimum number of DOF that need to be regulated with the PD controller to ensure stability during quiet standing. Finally, perturbation simulations confirmed that the kinematics of this system are similar