An investigation of the neuroplasticity underlying an improved reactive balance response
Date
2020
Authors
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Publisher
University of Delaware
Abstract
Perturbation-based protocols, in which participants repeatedly attempt to recover from rapid surface translations, seek to improve the reactive response needed to maintain upright balance following an external disturbance. While these protocols have reduced fall incidents in some at-risk populations, the neural adaptations underlying their effects are unknown. To address this gap in knowledge, we examined the neuroplasticity associated with changes in the ability of healthy adults (aged 25-55yrs) to respond to brief (i.e., 600-700ms) rapid surface translations, without needing to step. Our objective was to determine the impact of this training on lower leg spinal reflexes and to determine whether this training affects motor behavior during other postural tasks. The central hypothesis was that training would improve performance on the reactive balance task, and would change specific spinal reflexes (e.g., up- or down-regulation of the soleus Hoffman reflex (H- reflex)). These changes might, in turn, alter motor behavior during tasks such as postural steadiness during quiet stance. ☐ Fifteen participants completed six sessions of reactive balance training; during each session s/he stood on a computer-controlled treadmill and responded to ~70-85 perturbations; all perturbations were directed posteriorly (i.e., induced anterior sway); participants were instructed to try not to step when responding to perturbations. The size (i.e., velocity and displacement) of each disturbance was adjusted trial by trial, based on the participant’s performance. Stepping Threshold, defined as the perturbation level (size) that elicited a step on three consecutive trials, increased significantly, pre- to post-training, for the group. ☐ To determine the effect of our training protocol on spinal reflex behavior, soleus H-reflex control trials (i.e., H-reflex responses elicited while background muscle activity and M-wave size were kept constant) were obtained from both legs during standing. For six participants H-reflex amplitude increased bilaterally, pre-to post-training; for four participants H-reflex amplitude decreased bilaterally; five participants showed a mixed response. Additionally, there was a significant positive correlation, of moderate effect size, between change in Stepping Threshold and change in the soleus H-reflex amplitude of the Stepping leg (i.e., the leg used most often for stepping during unsuccessful perturbation trials). ☐ To determine if our protocol led to changes in postural steadiness, under-the-feet center-of-pressure data were collected during quiet bipedal stance, before and after training. We found a positive trend in the relationship between change in the H-reflex of the Stepping leg and changes in postural steadiness. Specifically, an increase in the amplitude of this reflex tended to correlate with a reduction in postural sway in the anterior-posterior direction. ☐ In sum, our data suggest that changes in reactive balance and postural steadiness during standing are mediated, in part, by changes in the reflex behavior of the soleus muscle. However, future analyses of associated kinematic and electromyographic data are needed to investigate why the direction of H-reflex change differed among participants. Additionally, the positive relationship between changes in postural steadiness and changes in soleus H-reflex amplitude warrants further investigation given its potential impact on fall prevention. That is, training that specifically targets up-regulation of this reflex (e.g., operant conditioning protocols) may improve upright postural control in some populations at-risk for falls.
Description
Keywords
Motor learning, Perturbation-based training, Postural control, Reactive balance, Soleus H-reflex, Spinal plasticity