Automatic application of neural stimulation during wheelchair propulsion after SCI enhances recovery of upright sitting from destabilizing events

Abstract

Background: The leading cause of injury for manual wheelchair users are tips and falls caused by unexpected destabilizing events encountered during everyday activities. The purpose of this study was to determine the feasibility of automatically restoring seated stability to manual wheelchair users with spinal cord injury (SCI) via a threshold-based system to activate the hip and trunk muscles with electrical stimulation during potentially destabilizing events.

Methods: We detected and classified potentially destabilizing sudden stops and turns with a wheelchair-mounted wireless inertial measurement unit (IMU), and then applied neural stimulation to activate the appropriate muscles to resist trunk movement and restore seated stability. After modeling and preliminary testing to determine the appropriate inertial signatures to discriminate between events and reliably trigger stimulation, the system was implemented and evaluated in real-time on manual wheelchair users with SCI. Three participants completed simulated collision events and four participants completed simulated rapid turns. Data were analyzed as a series of individual case studies with subjects acting as their own controls with and without the system active.

Results: The controller achieved 93% accuracy in detecting collisions and right turns, and 100% accuracy in left turn detection. Two of the three subjects who participated in collision testing with stimulation experienced significantly decreased maximum anterior-posterior trunk angles (p < 0.05). Similar results were obtained with implanted and surface stimulation systems.

Conclusions: This study demonstrates the feasibility of a neural stimulation control system based on simple inertial measurements to improve trunk stability and overall safety of people with spinal cord injuries during manual wheelchair propulsion. Further studies are required to determine clinical utility in real world situations and generalizability to the broader SCI or other population of manual or powered wheelchair users.

Trial registration: ClinicalTrials.gov Identifier NCT01474148 . Registered 11/08/2011 retrospectively registered.

Keywords: Event detection; Functional neuromuscular stimulation; Spinal cord injury.

Fig. 1

Experimental Setup. Experimental set-up of the ramp with a guidance ramp for (a) collisions and (b) 90-degree turns

Fig. 2

Detection Algorithms. a Collision detection algorithm b Turn detection algorithm

Fig. 3

Stim Timings for S2. a AP acceleration of a collision event for S2, with stimulation activated during the shaded portion (b) SI angular velocity of a left turn event for S2, with stimulation activated during the shaded portion. Right turns are symmetrical to left turns over the x-axis

Fig. 4

SCI AP Trunk Angles during Collision. The AP trunk angle during a collision trial with FNS (red lines) and without FNS (blue lines) for S1, S2, and S3. Zero seconds marks the start of the collision event

Fig. 5

Comparison of Maximum Trunk Angles and Return Time to Erect Posture with and without FNS. a Average maximum AP trunk angles during collision trials with FNS versus without FNS (b) Average return time to an erect seated posture during collision trial with and without FNS (c) Average maximum ML trunk angles during right turns with FNS and without FNS (d) Average maximum ML trunk angles during left turns with FNS and without FNS (* denotes statistical significance with p < 0.05)

Fig. 6

Collision Event. Collision trial without stimulation (top row) and with stimulation (bottom row). The images include: (1) Initial impact: wheelchair contacts obstacle and rear wheels lift off the ground (2) Maximum AP trunk angle (3) Return to erect posture, which requires manual re-adjustment in the trial without stimulation

Fig. 7

Turn Event. Left turn trial without stimulation (top row) and with stimulation (bottom row). The images include: (1) Initial turn: wheelchair enters turn and detection is triggered (2) Maximum ML trunk angle: without stim, the trunk leans out of the turn due to inertia, whereas with stim, the trunk leans into the turn (3) Return to erect posture, which requires manual re-adjustment in the trial without stimulation