Assessing Somatosensory Utilization during Unipedal Postural Control

Abstract

Multisensory-visual, vestibular and somatosensory information is integrated for appropriate postural control. The primary goal of this study was to assess somatosensory utilization during a functional motor task of unipedal postural control, in normal healthy adults. Assessing individual bias in the utilization of individual sensory contributions during postural control may help customization of rehabilitation protocols. In this study, a test paradigm of unipedal stance control in supine orientation with and without vision was assessed. Postural control in this test paradigm was hypothesized to utilize predominantly contributions of somatosensory information from the feet and ankle joint, with minimal vestibular input. Fourteen healthy subjects "stood" supine on their dominant leg while strapped to a backpack frame that was freely moving on air-bearings, to remove available otolith tilt cues with respect to gravity that influences postural control when standing upright. The backpack was attached through a cable to a pneumatic cylinder that provided a gravity-like load. Subjects performed three trials each with Eyes-open (EO) and Eyes-closed (EC) while loaded with 60% body weight. There was no difference in unipedal stance time (UST) across the two conditions with EC condition challenging the postural control system greater than the EO condition. Stabilogram-diffusion analysis (SDA) indicated that the critical mean square displacement was significantly different between the two conditions. Vestibular cues, both in terms of magnitude and the duration for which relevant information was available for postural control in this test paradigm, were minimized. These results support our hypothesis that maintaining unipedal stance in supine orientation without vision, minimizes vestibular contribution and thus predominantly utilizes somatosensory information for postural control.

Keywords: balance control; sensory biases; somatosensation; stabilogram-diffusion analysis; unipedal stance.

Figure 1

Subject performing unipedal stance trial on the Gravity-Bed. The coordinate system used is shown marked on the force plate.

Figure 2

Exemplar plots of four parameters for one subject under (A) Eyes-open (EO) and (B) Eyes-closed (EC) conditions. Numerals in the top right of each panel represent RMS values. For this subject, the RMS values for the EC condition were greater than or equal to those for the EO condition for all parameters except Trunk Angular Velocity (Trv).

Figure 3

Exemplar stabilogram-diffusion plots for one subject, under (A) EO and (B) EC conditions. The straight lines in black, fitted to the short- and long-term regions are used to estimate the stabilogram-diffusion parameters. Diffusion coefficients, critical mean square displacements and critical time intervals are also shown on the plots. Hurst exponentials were obtained by corresponding log-transformations of these plots.

Figure 4

Individual values and mean across all subjects for Δ$r_{\text{c}}^2$, for EO and EC conditions. The two conditions were significantly different from each other for this parameter.