A neuroprosthesis for control of seated balance after spinal cord injury

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. © 2015 audu et al.; licensee biomed central.