A muscle-driven approach to restore stepping with an exoskeleton for individuals with paraplegia

Abstract

Background: Functional neuromuscular stimulation, lower limb orthosis, powered lower limb exoskeleton, and hybrid neuroprosthesis (HNP) technologies can restore stepping in individuals with paraplegia due to spinal cord injury (SCI). However, a self-contained muscle-driven controllable exoskeleton approach based on an implanted neural stimulator to restore walking has not been previously demonstrated, which could potentially result in system use outside the laboratory and viable for long term use or clinical testing. In this work, we designed and evaluated an untethered muscle-driven controllable exoskeleton to restore stepping in three individuals with paralysis from SCI.

Methods: The self-contained HNP combined neural stimulation to activate the paralyzed muscles and generate joint torques for limb movements with a controllable lower limb exoskeleton to stabilize and support the user. An onboard controller processed exoskeleton sensor signals, determined appropriate exoskeletal constraints and stimulation commands for a finite state machine (FSM), and transmitted data over Bluetooth to an off-board computer for real-time monitoring and data recording. The FSM coordinated stimulation and exoskeletal constraints to enable functions, selected with a wireless finger switch user interface, for standing up, standing, stepping, or sitting down. In the stepping function, the FSM used a sensor-based gait event detector to determine transitions between gait phases of double stance, early swing, late swing, and weight acceptance.

Results: The HNP restored stepping in three individuals with motor complete paralysis due to SCI. The controller appropriately coordinated stimulation and exoskeletal constraints using the sensor-based FSM for subjects with different stimulation systems. The average range of motion at hip and knee joints during walking were 8.5°-20.8° and 14.0°-43.6°, respectively. Walking speeds varied from 0.03 to 0.06 m/s, and cadences from 10 to 20 steps/min.

Conclusions: A self-contained muscle-driven exoskeleton was a feasible intervention to restore stepping in individuals with paraplegia due to SCI. The untethered hybrid system was capable of adjusting to different individuals' needs to appropriately coordinate exoskeletal constraints with muscle activation using a sensor-driven FSM for stepping. Further improvements for out-of-the-laboratory use should include implantation of plantar flexor muscles to improve walking speed and power assist as needed at the hips and knees to maintain walking as muscles fatigue.

Keywords: Assistive technology; Biomechanics; Exoskeleton; Finite state machine; Functional neuromuscular stimulation; Gait; Hybrid neuroprosthesis; Spinal cord injury.

Fig. 1

Self-contained exoskeleton architecture overview

Fig. 2

Hydraulic circuit schematics. Top: variable constraint hip mechanism (VCHM). Bottom: Dual state knee mechanisms (DSKMs). Gray arrows in the rod side of the cylinders indicate the direction of rod travel when the corresponding hip or knee joint flexion or extension occurs

Fig. 3

HNP system block diagram. (Meta: metatarsal joint of foot; L: left; R: right; SYS_SHDN: system stimulation shutdown signal; LPF: low pass filter; MCU: microcontroller unit; WSB: wireless sensor board)

Fig. 4

High-level control system that initially began in the sitting function and transitioned user selected functions. Stimulation is indicated in red font color, black arrows indicate transitions that occur based on thresholds, green arrows correlate to pressing the “go” button, and red arrows correlate to pressing the “stop” button

Fig. 5

FSM using GED within the stepping function to transition through phases of gait. Transition between early swing and late swing occurred when the ipsilateral hip joint angle (θ_{hip_i}) exceeded a predetermined hip flexion angle threshold (θ_{hip flex threshold}). Transition between late swing and weight acceptance occurred when the ipsilateral knee joint angle (θ_{knee_i}) reached extension as indicated by being less than the set knee extension joint angle threshold (θ_{knee ext threshold}). Before the user could initiate the next step, the ipsilateral heel FSR signal (FSR_i) had to exceed the set weight acceptance threshold (FSR_{wt accept threhold}). Stimulation is indicated in red font color, black arrows indicate transitions that occur based on thresholds in GED, blue arrows indicate transitions in Timeout phases, green arrow correlates to pressing the “go” button, and red arrows correlate to pressing the “stop” button. (i = ipsilateral)

Fig. 6

Experimental setup for subject testing

Fig. 7

Left step progression for Subject A during gait

Fig. 8

Hip and knee flexion (+) and extension (-) for Subject A. Vertical lines indicate left (blue dotted) and right (red dashed) heel strike