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

Abstract

BackgroundSeveral 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.MethodsIn 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.ResultsAdaptation 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.ConclusionBalance 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.

Incongruity of Geometric and Spectral Markers in the Assessment of Body Sway

“…We have confirmed the stabilising effects of the vision and solid base of support on the CoP excursions during standing without additional constraints beyond keeping equilibrium in a spontaneous attitude (e.g., there was no instruction to “stand as still as possible”). When vision was occluded, or a compliant foam support was used, or both, all geometric and spectral measures underwent major and significant changes, but with notable departures from a simple scheme of concurrent increase as a function of the hypothesised unsafe stance condition (e.g., EC or Foam support) ( 1 , 68 ) or of concurrent decrease with adaptation.…”

“…Furthermore, we aimed to contrast the results of the spectral analysis of the CoP excursions (mean Spectrum Level and Median Frequency) with Sway Area and Path Length. This issue has been addressed here by reanalysing from an unconventional perspective the body sway data obtained during a recent investigation on the adaptation of stance to repeated trials in a population of young, healthy volunteers ( 1 ). The necessary data variability has been offered by two support surface conditions (Foam or Solid), two visual conditions (eyes open, EO; or eyes closed, EC) and the adapted conditions in the same population of healthy young subjects.…”

“…Adaptation to repeated stance trials (not to be confused with accommodation to some imposed visual or physical perturbations of posture) produces selective changes in sway variables ( 1 ). The mean Spectrum Level shows minor changes between the non-adapted (the first trial) and the adapted trial on either Foam or Solid surface in the ML direction.…”

“…The participants were resident physicians or physiotherapists at the Istituti Clinici Scientifici Maugeri SB, Pavia, Italy. Data had been recorded in a previously published study ( 1 ). Two subjects were removed because stance on Solid support was not completed.…”

“…We have become recently interested in this, perhaps, vacuous issue by the observation that these two markers of body sway diverge in their values during the process of adaptation to repeated consecutive standing trials ( 1 ). When standing on foam without vision, Path Length decreased while Sway Area remained approximately constant or moderately increased over time.…”