Restoring standing capabilities with feedback control of functional neuromuscular stimulation following spinal cord injury

Abstract

This paper reviews the field of feedback control for neuroprosthesis systems that restore advanced standing function to individuals with spinal cord injury. Investigations into closed-loop control of standing by functional neuromuscular stimulation (FNS) have spanned three decades. The ultimate goal for FNS standing control systems is to facilitate hands free standing and enabling the user to perform manual functions at self-selected leaning positions. However, most clinical systems for home usage currently only provide basic upright standing using preprogrammed stimulation patterns. To date, online modulation of stimulation to produce advanced standing functions such as balance against postural disturbances or the ability to assume leaning postures have been limited to simulation and laboratory investigations. While great technological advances have been made in biomechanical sensing and interfaces for neuromuscular stimulation, further progress is still required for finer motor control by FNS. Another major challenge is the development of sophisticated control schemes that produce the necessary postural adjustments, adapt against accelerating muscle fatigue, and consider volitional actions of the intact upper-body of the user. Model-based development for novel control schemes are proven and sensible approaches to prototype and test the basic operating efficacy of potentially complex and multi-faceted control systems. The major considerations for further innovation of such systems are summarized in this paper prior to describing the evolution of closed-loop FNS control of standing from previous works. Finally, necessary emerging technologies to for implementing FNS feedback control systems for standing are identified. These technological advancements include novel electrodes that more completely and selectively activate paralyzed musculature and implantable sensors and stimulation modules for flexible neuroprosthesis system deployment.

Keywords: Biomechanics; Feedback control; Functional neuromuscular stimulation; Neural prosthesis; Rehabilitation; Standing.

FIGURE 1

Two distinct control systems present during operation of neuroprosthesis to restore standing function: (1) Volitional control of user upper-body governed by human CNS, (2) FNS feedback controller electrically activating lower-extremities and lower trunk according to sensor-based feedback. An important underlying consideration for overall postural control is whether the setpoint and estimated postural state are similar for both control systems.

FIGURE 2

Eight-channel implant recipient (T9 AIS A) releases one hand from walker or surface for manual tasks and overhead reaching activities while standing with the neuroprosthesis.

FIGURE 3

Three-dimensional (3-D) musculoskeletal model of human bipedal stance adjusted to reflect effects of spinal cord injury (SCI) while under feedback control of functional neuromuscular stimulation (FNS) with user upper-extremity support on an assistive device.

FIGURE 4

Experimental set-up for FNS feedback control during bipedal standing against external perturbations has user applying support loads upon stable instrumented handles.

FIGURE 5

(A) The CWRU spiral nerve-cuff stimulation electrode, (B) The C-FINE stimulation electrode developed at the Cleveland APT Center

FIGURE 6

LEFT – Implantable components utilized with networked neuroprosthetic system (NNPS) developed at Cleveland FES Center, RIGHT – Possible architecture for implantable modules used with NNPS for restoring standing function (Nataraj et al., 2015)