Center of mass acceleration feedback control of functional neuromuscular stimulation for standing in presence of internal postural perturbations

Abstract

This study determined the feasibility and performance of center of mass (COM) acceleration feedback control of a neuroprosthesis utilizing functional neuromuscular stimulation (FNS) to restore standing balance to a single subject paralyzed by a motor and sensory complete, thoracic-level spinal cord injury (SCI). An artificial neural network (ANN) was created to map gain-modulated changes in total body COM acceleration estimated from body-mounted sensors to optimal changes in stimulation required to maintain standing. Feedback gains were systematically tuned to minimize the upper extremity (UE) loads applied by the subject to an instrumented support device during internally generated postural perturbations produced by volitional reaching and object manipulation. Total body COM acceleration was accurately estimated (> 90% variance explained) from two three-dimensional (3-D) accelerometers mounted on the pelvis and torso. Compared to constant muscle stimulation employed clinically, COM acceleration feedback control of stimulation improved standing performance by reducing the UE loading required to resist internal postural disturbances by 27%. This case study suggests that COM acceleration feedback could potentially be advantageous in a standing neuroprosthesis since it can be implemented with only a few feedback parameters and requires minimal instrumentation for comprehensive, 3-D control of dynamic standing function.

Posture-dependent control of stimulation in standing neuroprosthesis: Simulation feasibility study

“…The simulation study described in Nataraj et al determined that the typical noise and tracking errors reported for such sensors were within acceptable boundaries to still resolve postural positions with sufficient accuracy for effective feedback control of neuroprosthetic standing [30]. While the required precision of the sensors was not assessed in this study, the previous utilization of such sensors with our customized implantable stimulator for feedback control of neuroprosthetic standing suggests potential feasibility for live-subject implementation for setpoint control as well [2][3]. The model-based study presented here has some intrinsic limitations.…”

“…Our previous studies investigated use of CoM for feedback control based on 3D acceleration of CoM for the express purpose of disturbance rejection about the nominal posture setpoint [2][3]26]. Stimulation levels were modulated to respond to unanticipated, dynamic changes occurring only while standing at that nominal position.…”

“…Advanced FNS systems have been designed and deployed in several settings to aid individuals with SCI to stand erect from their wheelchairs [1] and to modulate stimulation levels according to sensor-based feedback to maintain balance against postural perturbations [2][3]. The use of such systems can have both physiological and functional benefits ranging from increased blood flow and muscle mass and reduced incidence of pressure sores [4][5] to the ability to access and interact with objects in the environment in ways that are difficult or impossible while sitting.…”

Optimizing User Integration for Individualized Rehabilitation

“…Device actuation may be more passive as with AFO assistance from spring-loads being applied proportional to displacements ala Hooke's law (F = -kx) and mediated through well-executed clutching [28]. Device actuation may be more active such as functional neuromuscular stimulation (FNS) to activated paralyzed musculature and restore standing balance capabilities [29] or electromotor actuation to restore walking [30] following spinal cord injury (SCI).…”

Modified Newton‐Raphson Method to Tune Feedback Gains of Control System for Standing by Functional Neuromuscular Stimulation Following Spinal Cord Injury

“…To this end, our group has developed control systems that modulate stimulation to muscles spanning the ankles, knees, hips, and trunk for comprehensive standing balance. We have demonstrated that feedback control can notably reduce the upper extremity (UE) loading required to stabilize against postural perturbations up to 50% in computer simulation [8][9][10] and 27-33% under live-subject conditions [12,13] compared to constant muscle excitation levels. While the procedures implemented to heuristically tune the feedback gains for these controllers were appropriate for initial evaluations, their repeated implementation would be clinically inconvenient by requiring substantial subject time and effort for periodic re-tuning over long-term usage.…”