Balance Adaptation While Standing on a Compliant Base Depends on the Current Sensory Condition in Healthy Young Adults

Abstract

Background: Several investigations have addressed the process of balance adaptation to external perturbations. The adaptation during unperturbed stance has received little attention. Further, whether the current sensory conditions affect the adaptation rate has not been established. We have addressed the role of vision and haptic feedback on adaptation while standing on foam.

Methods: In 22 young subjects, the analysis of geometric (path length and sway area) and spectral variables (median frequency and mean level of both total spectrum and selected frequency windows) of the oscillation of the centre of feet pressure (CoP) identified the effects of vision, light-touch (LT) or both in the anteroposterior (AP) and mediolateral (ML) direction over 8 consecutive 90 s standing trials.

Results: Adaptation was obvious without vision (eyes closed; EC) and tenuous with vision (eyes open; EO). With trial repetition, path length and median frequency diminished with EC (p < 0.001) while sway area and mean level of the spectrum increased (p < 0.001). The low- and high-frequency range of the spectrum increased and decreased in AP and ML directions, respectively. Touch compared to no-touch enhanced the rate of increase of the low-frequency power (p < 0.05). Spectral differences in distinct sensory conditions persisted after adaptation.

Conclusion: Balance adaptation occurs during standing on foam. Adaptation leads to a progressive increase in the amplitude of the lowest frequencies of the spectrum and a concurrent decrease in the high-frequency range. Within this common behaviour, touch adds to its stabilising action a modest effect on the adaptation rate. Stabilisation is improved by favouring slow oscillations at the expense of sway minimisation. These findings are preliminary to investigations of balance problems in persons with sensory deficits, ageing, and peripheral or central nervous lesion.

Keywords: adaptation; balance; compliant surface; repeated trials; sensory conditions; stance; touch; vision.

FIGURE 1

Centre of feet pressure (CoP) excursions and effects of trial repetition on the power spectrum in a representative subject. The recorded CoP displacement in the ML (A,D,G,J) and AP (B,E,H,K) directions are reported for the first and last trials in eyes closed in the absence of touch (EC) (A–E) and eyes open with light-touch (EO-LT) (G–K) conditions. The diagrams of the CoP sway trajectory (grey line) and the 95% ellipse profile (red for EC and blue for EO-LT) are reported in (C,F,I,L). (M–Q) Show the median frequency and the mean level of the spectrum computed for the 8 trials performed in the EC and EO-LT conditions. In (O), a sketch of the experimental condition is shown.

FIGURE 2

Effect of trial repetition on path length and sway area of CoP excursions under different sensory conditions. Path length (A) diminished with trial repetition with EC (red) and EC with light-touch (EC-LT) (yellow), while no adaptation was evident with vision (eyes open (EO), green, and EO-LT, blue). Sway area (B) moderately increased with trial repetition for all conditions (less so for EC). (C,D) Show the mean slope of the lines fitted to path length and sway area values across the trials. The negative slopes with EC and EC-LT indicate a decrease in path length. Slopes were not different from zero with EO and EO-LT. The slopes for the sway area were positive, indicating an increase, except for EC. Asterisks indicate significant differences between sensory conditions (*p < 0.05; **p < 0.01; ***p < 0.001). Symbols indicate slopes different from zero (□p < 0.01; △p < 0.001).

FIGURE 3

Power spectrum differences between the first and last trials. The profiles of the mean power spectrum of the first (black traces) and last (red traces) trials for both ML and AP directions are superimposed for EC (A,B), EC-LT (C,D), EO (I,J), and EO-LT (K,L). In the panels from (E–H,M–P), the differences between the spectrum of the last and the first trial (trial 8 minus trial 1) are reported. The pink area indicates a positive difference, i.e., in the corresponding frequency range, the spectrum of the last trial is greater than that of the first trial. The negative differences are highlighted in light blue, indicating that the spectrum of the last trial is smaller than that of the first trial.

FIGURE 4

Effect of trial repetition on the median frequency and mean level of the spectrum under the different sensory conditions. Median frequency decreased with trial repetition in the no-vision conditions for both ML (A) and AP (B) directions. Median frequency was higher in EC (red) and EC-LT (yellow) than in EO (green) and EO-LT (blue) conditions. (C) Shows the mean slope of the lines fitted to the median frequency data. Except for EO-LT in the ML direction, all the slopes were negative, indicating a reduction in the value of the median frequency. The mean level of the spectrum for the ML (D) and AP (E) direction is also reported. For the ML direction, the mean level of the spectrum showed no clear changes with trial repetition, while in the AP direction the mean level of the spectrum increased with trial repetition. (F) Shows the mean slope of the lines fitted to the mean levels of the spectrum. Except for the EC in ML direction, all the slopes were positive, indicating an increase of the mean level of the spectrum with the trial repetition. Asterisks indicate significant differences between sensory conditions (*p < 0.05; **p < 0.01; ***p < 0.001). Symbols indicate slopes significantly different from zero (◯p < 0.05; □p < 0.01; △p < 0.001).

FIGURE 5

Changes in the mean level of the power spectrum in the six frequency windows, in which the full spectrum (from 0.01 to 2.0 Hz) was separated, for all trials and sensory conditions. Data are expressed as a percent of the spectrum mean level of the first trial (black dots at 100%) in each condition and direction (this prevents a quick comparison of the graphs between conditions). In general, for both ML (A–D) and AP (E–H) directions and for all sensory conditions, the mean level of the spectrum increased over the successive trials in the first two windows (W1 and W2), showed minor changes in W3, and decreased in the windows from W4 to W6.

FIGURE 6

Adaptation rate in the frequency windows (Ws) of ML (A) and AP (B) spectra. The mean slopes of the lines calculated for each frequency window (W) are reported for ML and AP in the different sensory conditions (red, EC; yellow, EC-LT; green, EO; blue, EO-LT). For all sensory conditions, the slopes of W1 and W2 are always positive indicating an increment, during the trial repetition, in the mean level of the spectrum. In W3, the slope is not different from zero in the ML direction for all sensory conditions, while in the AP direction was positive, except for the EC condition, and different from zero only for touch (EC-LT and EO-LT). From W4 to W6, except for the EO-LT condition, the slopes are always negative indicating a reduction in the mean level of the spectrum during the trial repetition. Asterisks indicate significant differences between sensory conditions (*p < 0.05; ^**p < 0.01; ^***p < 0.001). Symbols indicate slopes significantly different from zero (◯p < 0.05; □p < 0.01; △p < 0.001).

FIGURE 7

Comparisons of adapted trials between sensory conditions (coloured). In the upper traces of each panel, the mean power spectra of ML (left column) and AP directions (right column) of the last trial are reported and compared. In the lower traces, the difference between spectra is shown: (A), EC-LT minus EC; (B), EO minus EC; (C), EO-LT minus EO; (D), EO minus EC-LT; (E), EO-LT minus EC-LT. Light blue areas highlight negative differences between the pairs of sensory conditions. The pink area in (D,E) indicates that the spectrum of the most stabilised condition (e.g., EO in D and EO-LT in E) is greater than the contrast condition (EC-LT in D,E).

FIGURE 8

Force applied on the touchpad during trials (means of all subjects). Force was not different between EC-LT (yellow symbols) and EO-LT (blue symbols). In both conditions, the force never exceeded1 N and remained constant across trials.