Toward a hybrid exoskeleton for crouch gait in children with cerebral palsy: neuromuscular electrical stimulation for improved knee extension

Abstract

Background: Neuromuscular Electrical Stimulation (NMES) has been utilized for many years in cerebral palsy (CP) with limited success despite its inherent potential for improving muscle size and/or strength, inhibiting or reducing spasticity, and enhancing motor performance during functional activities such as gait. While surface NMES has been shown to successfully improve foot drop in CP and stroke, correction of more complex gait abnormalities in CP such as flexed knee (crouch) gait remains challenging due to the level of stimulation needed for the quadriceps muscles that must be balanced with patient tolerability and the ability to deliver NMES assistance at precise times within a gait cycle.

Methods: This paper outlines the design and evaluation of a custom, noninvasive NMES system that can trigger and adjust electrical stimulation in real-time. Further, this study demonstrates feasibility of one possible application for this digitally-controlled NMES system as a component of a pediatric robotic exoskeleton to provide on-demand stimulation to leg muscles within specific phases of the gait cycle for those with CP and other neurological disorders who still have lower limb sensation and volitional control. A graphical user interface was developed to digitally set stimulation parameters (amplitude, pulse width, and frequency), timing, and intensity during walking. Benchtop testing characterized system delay and power output. System performance was investigated during a single session that consisted of four overground walking conditions in a 15-year-old male with bilateral spastic CP, GMFCS Level III: (1) his current Ankle-Foot Orthosis (AFO); (2) unassisted Exoskeleton; (3) NMES of the vastus lateralis; and (4) NMES of the vastus lateralis and rectus femoris. We hypothesized in this participant with crouch gait that NMES triggered with low latency to knee extensor muscles during stance would have a modest but positive effect on knee extension during stance.

Results: The system delivers four channels of NMES with average delays of 16.5 ± 13.5 ms. Walking results show NMES to the vastus lateralis and rectus femoris during stance immediately improved mean peak knee extension during mid-stance (p = 0.003*) and total knee excursion (p = 0.009*) in the more affected leg. The electrical design, microcontroller software and graphical user interface developed here are included as open source material to facilitate additional research into digitally-controlled surface stimulation ( github.com/NIHFAB/NMES ).

Conclusions: The custom, digitally-controlled NMES system can reliably trigger electrical stimulation with low latency. Precisely timed delivery of electrical stimulation to the quadriceps is a promising treatment for crouch. Our ultimate goal is to synchronize NMES with robotic knee extension assistance to create a hybrid NMES-exoskeleton device for gait rehabilitation in children with flexed knee gait from CP as well as from other pediatric disorders.

Trial registration: clinicaltrials.gov, ID: NCT01961557 . Registered 11 October 2013; Last Updated 27 January 2020.

Keywords: Crouch gait; Exoskeleton; Functional electrical stimulation (FES); Graphical user interface (GUI).

Fig. 1

The custom-designed digitally-controlled surface NMES system. a custom printed circuit board (PCB) wiring schematic and b printed board that houses a Teensy 3.2 microcontroller, a system of 21 Omron Electronic Components G6L-1P-DC3 SPST-NO electromechanical relays, and inputs for a LiPo battery and external digital input signals, with scale bar for reference. c schematic of the NMES system and d wired system prototype. e a 3D model of the custom protective case and f the enclosed system used for this study

Fig. 2

A graphical user interface (GUI) to communicate with the Teensy 3.2 microcontroller operating the neuromuscular electrical stimulator for digital calibration and control of the neuromuscular electrical stimulation

Fig. 3

a Schematic of the synchronized neuromuscular electrical stimulation (NMES) and exoskeleton communication. The NMES device is calibrated under complete digital control from a Teensy 3.2 microcontroller and system of electromechanical relays housed on a printed circuit board (PCB). Front-end calibration occurs on a Python desktop graphical user interface. The NMES synchronizes with the exoskeleton motor control via digital input from the exoskeleton’s finite state machine. b Schematic of the gait cycle, showing how the timing of stimulation delivery during stance phase as determined by the Finite State Machine (FSM) deviates slightly from the traditional biomechanical definition

Fig. 4

Representation of the experimental setup for clinical testing of walking in the pediatric robotic exoskeleton with synchronized NMES across the Functional & Applied Biomechanics Laboratory in the National Institutes of Health Clinical Center

Fig. 5

Benchtop testing neuromuscular electrical stimulation (NMES) output synchronized with a state change detected by the finite state machine (FSM) of the pediatric robotic exoskeleton; one simulated state change with aligned signals from the NMES and FSM shown for example. Normalized output = 1 indicates stance phase on the FSM and maximum voltage from the NMES

Fig. 6

Knee angle profiles averaged for each gait cycle during all four walking conditions in a left leg and b right leg. Average profile over all gait cycles shown in bold for each condition with transparent spread representing ±1 standard deviation. Black bars along horizontal axes show the average gait cycle duration ±1 standard deviation when electrical stimulation was administered in each limb. The average knee angle profile of a healthy gait pattern from a previous study [20] is shown in grey for reference