Closed-Loop Control of FES-Assisted Arm-Free Paraplegic Standing: A Feasibility Study

Using a simplified model of quiet standing, we demonstrated analytically that a proportional-derivative (PD) controller can mimic the physiologic control strategy applied by healthy individuals during quiet standing. In a second step, utilizing transcutaneous functional electrical stimulation (FES) of the ankle extensors in experiments with a spinal cord injured subject, it was verified that the PD controller can, in fact, compensate for long neurological delays and muscle nonlinearities. The third phase of this study was focused on better understanding the nonlinear dynamics of quiet standing. In particular, a realistic model of quiet standing was developed that could be stabilized by actuating six out of twelve degrees of freedom (DOF). A PD plus gravity compensator was used to control these six DOF, and asymptotic stability was demonstrated using Lyapunov stability analysis. These results suggest that quiet standing can be achieved in paraplegic individuals by controlling only six DOF in the lower limbs using FES, and that a PD plus gravity compensator should be able to stabilize the system despite long neurological delays and muscle nonlinearities