The contribution of arm motion during the feet-in-place fall-recovery response
Date
2018
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Publisher
University of Delaware
Abstract
Falls are the leading cause of non-fatal, unintentional injury in the U.S. In at-risk groups, such as those with chronic stroke, more than a third of falls are due to externally applied perturbations. In order to maintain stability after such a perturbation, an individual can use different biomechanical mechanisms to regain stability. In this series of studies, we are interested in the mechanism of rotating segments about the center of mass, as commonly done by rapidly moving the arms. We are specifically interested in the ability to recover from a posterior disturbance without taking a step, as this ability has been prospectively related to falls in older adults and can be quantified reliably using single-stepping thresholds. When responding to a posterior disturbance, elevating the arms through shoulder flexion shifts the center of mass anteriorly and applies a reactive moment to the rest of the body that resists rotation of the fall. We do not know, however, if arm motion meaningfully influences the posterior single-stepping threshold measure. For those with chronic stroke, impaired arm function is associated with a greater risk of falls. It is unknown, however, if chronic stroke alters arm rotation during posterior fall recovery. The aims of this project are to specifically 1) quantify how constraints in arm motion alter the response to a posterior postural disturbance, and then 2) determine if individuals with chronic stroke have a diminished, asymmetrical arm response. ☐ To address the Aim 1 hypothesis, ten young, adults with no impairment were recruited for this study. Participants attempted to prevent steps in response to a progressive series of rapid, precise treadmill belt accelerations delivered by a computer-controlled treadmill. The posterior single-stepping thresholds, as represented by the disturbance magnitudes that consistently elicited a step in each direction, were determined for each participant. Kinematics were recorded to quantify the resulting dynamic stability as a means to explain a potential underlying mechanism of hypothesized group differences. When the arms were constrained, lower posterior single-stepping thresholds were observed (p=0.03 Cohen’s d=0.86). When evaluating dynamic stability at the highest disturbance levels, the MoSmin was not different between unconstrained conditions arms–constrained conditions (p = 0.55, Cohen’s d = 0.20). Considering that the unconstrained responses involved larger disturbances, this lack of differences in the minimum margin of stability suggests that the active response to the perturbation were impaired in the arms constrained condition. ☐ To address the Aim 2 hypothesis, ten individuals with chronic completed the same single-stepping threshold assessment, and we compared results to the unconstrained condition of Aim 1. Peak shoulder flexion velocities as well as a symmetry index were compared between groups. Those with chronic stroke displayed more asymmetry than the unimpaired participants (p = 0.005, Cohen’s d=1.34). We did not, however, detect meaningful between-group differences in shoulder motion between unimpaired and impaired individuals (p > 0.20, Glass's Δ < 0.49). ☐ In conclusion, these results demonstrate that arm motion does play a role in posterior fall performance, as characterized by single-stepping thresholds. When applying this concept to those with chronic stroke, a population that has a high fall risk, it was apparent that the arm response was asymmetrical, perhaps altering its contribution to fall recovery. Moving forward, this information justifies consideration of the arm response as a potentially modifiable aspect of the fall recovery response.