Adaptive ankle exoskeleton gait training demonstrates acute neuromuscular and spatiotemporal benefits for individuals with cerebral palsy: A pilot study

Abstract

Background: Gait abnormalities from neuromuscular conditions like cerebral palsy (CP) limit mobility and negatively affect quality of life. Increasing walking speed and stride length are essential clinical goals in the treatment of gait disorders from CP.

Research question: How does over-ground gait training with an untethered ankle exoskeleton providing adaptive assistance affect mobility-related spatiotemporal outcomes and lower-extremity muscle activity in people with CP?

Methods: A diverse cohort of individuals with CP (n = 6, age 9-31, Gross Motor Function Classification System Level I - III) completed four over-ground training sessions (98 ± 17 min of assisted walking) and received pre- and post-training assessments. On both assessments, participants walked over-ground with and without the exoskeleton while we recorded spatiotemporal outcomes and muscle activity. We used two-tailed paired t-tests to compare all parameters pre- and post-training, and between assisted and unassisted conditions.

Results: Following training, walking speed increased 0.24 m/s (p = 0.006) and stride length increased 0.17 m (p = 0.013) during unassisted walking, while walking speed increased 0.28 m/s (p = 0.023) and stride length increased 0.15 m (p = 0.002) during exoskeleton-assisted walking. Exoskeleton training improved stride-to-stride repeatability of soleus and vastus lateralis muscle activation by up to 51 % (p ≤ 0.046), while the amount of integrated stance-phase muscle activity was similar across visits and conditions. Relative to baseline, post-training walking with the exoskeleton resulted in a soleus activity pattern that was 39 % more similar to the typical pattern from unimpaired individuals (p < 0.001).

Significance: This study demonstrates acute spatiotemporal and neuromuscular benefits from over-ground training with adaptive ankle exoskeleton assistance, and provides rationale for completion of a longer randomized controlled training protocol.

Keywords: Exoskeleton; Gait training; Mobility; Neuromuscular; Rehabilitation.

Figure 1.

Experimental setup and design. A) Exoskeleton ankle assembly and control module. B) Electromyography (EMG) of the soleus (Sol), vastus lateralis (VL), and semitendinosus (ST) were monitored bilaterally when participants walked with the ankle exoskeleton. C) Plantarflexion and dorsiflexion torque set-points for each participant. D) Desired and actual torque profiles of ten continuous steps from one participant. E) Experimental protocol for training and assessment and total training time (ranged from 65 to 120 minutes) with the exoskeleton across the cohort.

Figure 2.

Walking speed, stride length and cadence pre- and post-training for each condition. Over-ground exoskeleton-assisted gait training improved walking speed and stride length. Speed and stride length were greater during the post-training Exo vs. post-training Shod walking. Neither training nor assistance statistically affected walking cadence. * indicates a significant difference between conditions within the same visit or between visits of the same condition. Ψ indicates a significant difference between pre-training Shod and post-training Exo conditions. Error bars indicate standard error of the mean. Grey shading depicts the typical range (mean ± standard deviation) for unimpaired individuals walking in a straight line [37,38].

Figure 3.

A) Variance ratio (VR) for soleus, vastus lateralis, and semitendinosus pre- and post-training for the Shod and Exo conditions. Training improved the uniformity of muscle activity for all muscles except the semitendinosus. * indicates a significant difference between the first and last visits for Shod (red) or Exo (green). Ψ indicates a significant difference between pre-training Shod and post-training Exo conditions. Error bars indicate standard error of the mean. B) Normalized soleus EMG during the Shod and Exo walking conditions pre- and post-training for a representative participant (P2). Each color indicates one gait cycle.

Figure 4.

A) Stance phase integrated EMG (iEMG) of the soleus, vastus lateralis, and semitendinosus pre- and post-training for the Exo and Shod conditions. B) Coefficient of cross-correlation between the soleus activity pattern of our cohort and that of unimpaired individuals walking over-ground at 1.25 m/s [36]. Post-training, soleus activation pattern during walking with the device increased in similarity to that of unimpaired individuals. C) Normalized soleus EMG from a representative participant (P2) for pre-training Shod (red line) and post-training Exo (teal line) conditions; normative data from the average unimpaired pattern are also shown (dashed black line). * indicates a significant difference between visits within the same condition. Ψ indicates a significant difference between pre-training Shod and post-training Exo conditions. Error bars indicate standard error of the mean.

Figure 5.

Relationship between A) walking speed and stride length and B) walking speed and cadence of our cohort (black solid line) and, for reference, the relationships reported for unimpaired young adults (gray dotted line) [39]. C) Relationship between height-normalized walking speed and muscle variance ratio (combined average of the soleus, vastus lateralis, and semitendinosus) across both assessments (pre- and post-training) and both conditions (Shod and Exo). Stride length and cadence explained 92% and 57% of the variance in walking speed, respectively. Average variance ratio explained 81% of the variance in normalized walking speed, and lower muscle variance ratio corresponded to faster walking speed. Each color represents a participant, and each shape indicates a condition (Shod/Exo) within each visit (pre-/post-training).