Time-dependent tuning of balance control and aftereffects following optical flow perturbation training in older adults

Abstract

Background: Walking balance in older adults is disproportionately susceptible to lateral instability provoked by optical flow perturbations. The prolonged exposure to these perturbations could promote reactive balance control and increased balance confidence in older adults, but this scientific premise has yet to be investigated. This proof of concept study was designed to investigate the propensity for time-dependent tuning of walking balance control and the presence of aftereffects in older adults following a single session of optical flow perturbation training.

Methods: Thirteen older adults participated in a randomized, crossover design performed on different days that included 10 min of treadmill walking with (experimental session) and without (control session) optical flow perturbations. We used electromyographic recordings of leg muscle activity and 3D motion capture to quantify foot placement kinematics, lateral margin of stability, and antagonist coactivation during normal walking (baseline), early (min 1) and late (min 10) responses to perturbations, and aftereffects immediately following perturbation cessation (post).

Results: At their onset, perturbations elicited 17% wider and 7% shorter steps, higher step width and length variability (+171% and +132%, respectively), larger and more variable margins of stability (MoS), and roughly twice the antagonist leg muscle coactivation (p-values<0.05). Despite continued perturbations, most outcomes returned to values observed during normal, unperturbed walking by the end of prolonged exposure. After 10 min of perturbation training and their subsequent cessation, older adults walked with longer and more narrow steps, modest increases in foot placement variability, and roughly half the MoS variability and antagonist lower leg muscle coactivation as they did before training.

Conclusions: Findings suggest that older adults: (i) respond to the onset of perturbations using generalized anticipatory balance control, (ii) deprioritize that strategy following prolonged exposure to perturbations, and (iii) upon removal of perturbations, exhibit short-term aftereffects that indicate a lessening of anticipatory control, an increase in reactive control, and/or increased balance confidence. We consider this an early, proof-of-concept study into the clinical utility of prolonged exposure to optical flow perturbations as a training tool for corrective motor adjustments relevant to walking balance integrity toward reinforcing task-specific, reactive control and/or improving balance confidence in older adults.

Trial registration: clinicaltrials.gov ( NCT03341728 ). Registered 14 November 2017.

Keywords: Coactivation; Elderly; Stability; Virtual reality; Walking.

Conflict of interest statement

The authors declare that they have no competing interests.

Figures

Fig. 1

a Participants walked on a treadmill while watching a speed-matched immersive virtual hallway with and without continuous mediolateral (ML) optical flow perturbations with an amplitude of 0.35 m, applied as detailed in the current methods section. b Participants participated in two sessions performed on separate days separated by at least 1 week and in fully randomized order. Both sessions consisted of a 2 min “baseline” walking period without perturbations. In one session (“Experimental”), participants then walked for 10 min in the presence of perturbations, followed immediately by a 1-min “post” period without perturbations. In the other session (“Control”), participants simply walked for 10 min without perturbations. For analysis, we refer to the first and last minute of each 10-min prolonged walk as “early” and “late”, respectively

Fig. 2

Effects of prolonged exposure to optical flow perturbations on foot placement kinematics. Group average (±standard error) of perturbation-induced effects on a step width (SW), b step length (SL), c step width variability (SWV), and d step length variability (SLV) for older participants. The area between the vertical dashed lines represents the period when visual perturbations were present. Previously published reference data from young participants walking for 8 min with optical flow perturbations is shown for comparison. The repeated measures ANOVA revealed significant (p < 0.001) main effects across the perturbation session (baseline, early, late, post) for all outcome measures. Single asterisks (*) indicate significantly different from baseline values in older adults (p < 0.05). Double asterisks (**) indicate significantly different between early and late time points for prolonged exposure in older adults. We also note differences approaching significance for post and baseline for step width (p = 0.056), between early and late for step width (p = 0.054), and between post and baseline for step length (p = 0.054) in older adult participants (a)

Fig. 3

Effects of prolonged exposure to optical flow perturbations on lateral margin of stability (MoS) and its variability in older participants. Group average (±standard error) (a) lateral MoS and (b) its associated variability calculated at its stance phase minimum and at heel-strike for the perturbation session. The repeated measures ANOVA revealed significant main effects across the perturbation session (baseline, early, late, post) for lateral MoS at heel strike (p = 0.012), the variabilities of lateral MoS at its stance phase minimum (p < 0.001) and at heel strike (p < X). Single asterisks (*) indicate significantly different from baseline values in older adults (p < 0.05). Double asterisks (**) indicate significantly different between early and late time points for prolonged exposure in older adults (p < 0.05). We also note differences approaching significance between baseline and early for lateral MoS at heel-strike (p = 0.051) and between early and late for stance phase minimum lateral MoS variability (p = 0.065) (a)

Fig. 4

Effects of prolonged exposure to optical flow perturbations on antagonist leg muscle coactivations. Group average (±standard error) percent coactivation between (a) vastus lateralis (VL) and medial hamstring (MH), b tibialis anterior (TA) and medial gastrocnemius (MG), and c tibialis anterior and soleus (SOL) during the stance and swing phases of strides taken during the perturbation session in older adults. The repeated measures ANOVA revealed significant main effects across the perturbation session (baseline, early, late, post) for all outcome measures (p-values≤0.020). Single asterisks (*) indicate significantly different from baseline values in older adults (p < 0.05). Double asterisks (**) indicate significantly different between early and late time points for prolonged exposure in older adults (p < 0.05). We also note differences approaching significance between early and late for TA-SOL coactivation during swing (p = 0.066) (a)