A neuroprosthesis for control of seated balance after spinal cord injury

Abstract

Background: A major desire of individuals with spinal cord injury (SCI) is the ability to maintain a stable trunk while in a seated position. Such stability is invaluable during many activities of daily living (ADL) such as regular work in the home and office environments, wheelchair propulsion and driving a vehicle. Functional neuromuscular stimulation (FNS) has the ability to restore function to paralyzed muscles by application of measured low-level currents to the nerves serving those muscles.

Methods: A feedback control system for maintaining seated balance under external perturbations was designed and tested in individuals with thoracic and cervical level spinal cord injuries. The control system relied on a signal related to the tilt of the trunk from the vertical position (which varied between 1.0 ≡ erect posture and 0.0 ≡ most forward flexed posture) derived from a sensor fixed to the sternum to activate the user's own hip and trunk extensor muscles via an implanted neuroprosthesis. A proportional-derivative controller modulated stimulation between trunk tilt values indicating deviation from the erect posture and maximum desired forward flexion. Tests were carried out with external perturbation forces set at 35%, 40% and 45% body-weight (BW) and maximal forward trunk tilt flexion thresholds set at 0.85, 0.75 and 0.70.

Results: Preliminary tests in a case series of five subjects show that the controller could maintain trunk stability in the sagittal plane for perturbations up to 45% of body weight and for flexion thresholds as low as 0.7. The mean settling time varied across subjects from 0.5(±0.4) and 2.0 (±1.1) seconds. Mean response time of the feedback control system varied from 393(±38) ms and 536(±84) ms across the cohort.

Conclusions: The results show the high potential for robust control of seated balance against nominal perturbations in individuals with spinal cord injury and indicates that trunk control with FNS is a promising intervention for individuals with SCI.

Figure 1

Schematic of trunk feedback control system showing subject seated in work volume of motion capture cameras. Three computers – actuator computer, target and host manage the real-time environment for the tests. A linear actuator applied measured pull pulses to the chest of the subject. Settings for the tilt sensor are defined as OE for the upright threshold and OF for the flexion threshold.

Figure 2

Flow diagram for the feedback control of erect seated posture. The error in trunk tilt measured a body-mounted sensor provides input to a PD controller which produces a normalized control signal that is converted to PW values to be applied to all the extensor muscles of the hip and trunk. s is the Laplace transform parameter.

Figure 3

Typical complete trial consisting of 3 cycles of perturbation force application from a trial with Subject S1. Top plot is the normalized disturbance, second plot is the accelerometer sensor output (in g’s) which was used as a measure of trunk forward flexion, third plot is the trunk angle computed from marker data and last plot is the muscle pulse-width for the erector spinae muscle activated as a consequence of change in trunk tilt.

Figure 4

Typical result of feedback perturbation rejection control experiment with Subject S4. (a) Pull magnitude = 35%BW; flexion threshold = 0.70 where saturation pulse-width was not attained and (b) Pull magnitude = 45%BW; flexion threshold = 0.75 where saturation pulse-width was attained. The thick lines in the plots represent response means over 6 pull cycles ±1 standard deviation (thin lines). The trunk angles in the middle subplots were calculated using the marker positions captured with the Vicon cameras.

Figure 5

Typical response showing sensor output, trunk angle and muscle stimulus for different pull amplitudes for Subject S4. In all cases, the flexion threshold was set at 0.75. With lower pull magnitudes the trunk did not reach the set flexion threshold before muscle action restored it to erect. With the larger pull magnitude of 45%BW, the trunk flexed up to the set flexion threshold before muscle action was strong enough to restore it to erect.

Figure 6

Typical response trunk tilt, angle and muscle stimulus for different flexion threshold settings at the same pull magnitude of 45% BW for Subject S1.

Figure 7

Mean settling times for trunk angle for each of the 5 subjects shown by the grey blocks. The leftward axis was the flexion threshold, the rightward the perturbation amplitude, while the vertical axis was the settling time in seconds. The thick black lines are the error bars that represent the standard deviations from the means.

Figure 8

Means of the response times for trunk angle for each of the 5 subjects shown by the grey blocks. The leftward axis was the flexion threshold, the rightward the perturbation amplitude, while the vertical axis was the response time in milliseconds. The thick black lines are the error bars that represent the standard deviations from the means.