Do kinematic metrics of walking balance adapt to perturbed optical flow?

Abstract

Visual (i.e., optical flow) perturbations can be used to study balance control and balance deficits. However, it remains unclear whether walking balance control adapts to such perturbations over time. Our purpose was to investigate the propensity for visuomotor adaptation in walking balance control using prolonged exposure to optical flow perturbations. Ten subjects (age: 25.4±3.8years) walked on a treadmill while watching a speed-matched virtual hallway with and without continuous mediolateral optical flow perturbations of three different amplitudes. Each of three perturbation trials consisted of 8min of prolonged exposure followed by 1min of unperturbed walking. Using 3D motion capture, we analyzed changes in foot placement kinematics and mediolateral sacrum motion. At their onset, perturbations elicited wider and shorter steps, alluding to a more cautious, general anticipatory balance control strategy. As perturbations continued, foot placement tended toward values seen during unperturbed walking while step width variability and mediolateral sacrum motion concurrently increased. Our findings suggest that subjects progressively shifted from a general anticipatory balance control strategy to a reactive, task-specific strategy using step-to-step adjustments. Prolonged exposure to optical flow perturbations may have clinical utility to reinforce reactive, task-specific balance control through training.

Keywords: Gait; Sensorimotor; Virtual reality; Vision; Visual feedback.

Copyright © 2017 Elsevier B.V. All rights reserved.

Figures

Figure 1

(A) Subjects walked on a treadmill while watching a speed-matched, immersive virtual hallway with and without continuous mediolateral optical flow perturbations of different amplitudes. (B) Mediolateral (ML) sacrum motion and lateral step placement during normal walking (“Norm”) and walking with the largest amplitude visual perturbation for a representative subject. Perturbation trials consisted of 8 min of prolonged exposure followed by 1 min of normal, unperturbed walking. L.heel and R.heel refer to the mediolateral locations of markers placed on the left and right heels at the instant of heel-strike, respectively.

Figure 2

Mean (standard error) perturbation-induced effects on step width (SW), step length (SL), step width variability (SWV), and step length variability (SLV) for each perturbation amplitude. Gray shading represents plus and minus one standard error of each measure during normal walking (“Norm”). Vertical dashed lines represent cessation of optical flow perturbations and a return to normal, unperturbed walking. Double asterisks (**) indicate significantly different from normal walking for the onset of visual perturbations and/or following their cessation (p

Figure 3

(A) Group-average (standard error) spectrum of ML sacrum motion reveals distinct peaks of signal intensity at, and thus naturally emerging entrainment to, each of the three perturbation frequencies (0.135, 0.250 and 0.442 Hz). (B) Mean (standard error) peak mediolateral (ML) sacrum motion at each perturbation frequency as a functional of perturbation amplitude and time compared to normal, unperturbed walking (“Norm”). Gray shading represents plus and minus one standard error of each measure during normal walking. Vertical dashed lines represent cessation of optical flow perturbations and a return to normal, unperturbed walking. Double asterisks (**) indicate significantly different from normal walking for the onset of visual perturbations and/or following their cessation (p