Pilot evaluation of changes in motor control after wearable robotic resistance training in children with cerebral palsy

Abstract

Cerebral palsy (CP) is characterized by deficits in motor function due to reduced neuromuscular control. We leveraged the guiding principles of motor learning theory to design a wearable robotic intervention intended to improve neuromuscular control of the ankle. The goal of this study was to determine the neuromuscular and biomechanical response to four weeks of exoskeleton ankle resistance therapy (exo-therapy) in children with CP. Five children with CP (12 - 17 years, GMFCS I - II, two diplegic and three hemiplegic, four males and one female) were recruited for ten 20-minute sessions of exo-therapy. Surface electromyography, three-dimensional kinematics, and metabolic data were collected at baseline and after training was complete. After completion of training and with no device on, participants walked with decreased co-contraction between the plantar flexors and dorsiflexors (-29 ± 11%, p = 0.02), a more typical plantar flexor activation profile (33 ± 13% stronger correlation to a typical soleus activation profile, p = 0.01), and increased neural control complexity (7 ± 3%, p < 0.01 measured via muscle synergy analysis). These improvements in neuromuscular control led to a more mechanically efficient gait pattern (58 ± 34%, p < 0.05) with a reduced metabolic cost of transport (-29 ± 15%, p = 0.02). The findings from this study suggest that ankle exoskeleton resistance therapy shows promise for rapidly improving neuromuscular control for children with CP, and may serve as a meaningful rehabilitative complement to common surgical procedures.

Keywords: Cerebral palsy; Exoskeleton; Gait; Muscle synergy; Neurorehabilitation.

Copyright © 2021 Elsevier Ltd. All rights reserved.

Figures

Figure 1.. Wearable adaptive resistance training.

a) Pictures of the exoskeleton used to deliver adaptive ankle resistance b) Schematic depiction of the proposed mechanisms underlying the neuromuscular response to wearable adaptive resistance training, including increased neural complexity (as indicated by an increase in the number of muscle synergies recruited for walking), improved inhibition of antagonist muscles about the ankle, and enhanced muscle activation timing of the plantar flexor muscles for more typical activation timing (experimental data from P1).

Figure 2.. Study visit allocation.

Participants completed 12 total visits: a pre-assessment, ten training visits, and a post-assessment.

Figure 3.. Neuromuscular outcome measures.

Individual (color-coded circles) and group mean (bars) pre- and post-exo-therapy neuromuscular variables, including a) co-contraction index (CCI) between the soleus and tibialis anterior; b) the relationship of the experimental soleus activation profile to a typical activation profile at a non-dimensional speed of 0.25; c) variance in muscle activity of the soleus, tibialis anterior, vastus lateralis, and semitendinosus that could be explained by one muscle synergy (VAF1). ‘Typical’ values were calculated using a publicly available dataset [30] of individuals walking at a non-dimensional speed of 0.25.

Figure 4.. Center of mass mechanical efficiency.

a) Group average pre-exo-therapy (red), group average post-exo-therapy (blue), and typical unimpaired (gray) potential energy (PE, solid lines) and kinetic energy (KE, dashed lines) curves. Pie charts indicate the group average center of mass (COM) energy exchange recovery percentage; For efficient energy exchange and increased recovery, potential and kinetic energies should be 180º out of phase with equal and opposite amplitudes. The typical, unimpaired curve was adopted from Bennett et al [6].

Figure 5.. Metabolic cost.

Individual (color-coded circles) and group mean (bars) pre- and post-exo-therapy metabolic cost of transport, representing the body-mass-normalized metabolic energy required to walk a unit distance. Both pre- and post-measures were collected at the pre-exo-therapy preferred walking speed on the treadmill.