The purpose of this study is to investigate the effects of simulated somatosensory deficit and vision on (1) linear measures and (2) rambling-trembling-derived measures of the COP during quiet standing. It was hypothesized that (1) linear COP measures will show greater changes from baseline as deficit severity increases and there will be an interaction between the deficit severity and visual condition, with the effect greater in the eyes-closed condition compared to the eyes-open, and (2) rambling (RM) and trembling (TR) parameters will show similar trends across deficit and vision conditions, but with different magnitudes, and present greater sensitivity to deficit detection compared to the COP measures. The long-term goal of this study is to understand postural sway from a mechanistic perspective and use this information to develop a clinically-relevant measure of balance that is sensitive to changes in somatosensation abilities.

Fifty-two healthy young adults (aged 22.10 ± 1.88 years, 29 male, 23 female) participated in the study. Participants were asked to stand on two force plates (AMTI, Watertown, MA) with a standardized stance and either eyes open (EO) or closed (EC). Five foam thickness conditions (0”, 1/8”, 1/4”, 1/2”, and 1”, corresponding to F0, F1, F2, F3, and F4, respectively) were used to simulate varying degrees of somatosensory deficit. Participants completed three trials with EO and EC for each randomly-ordered foam condition. Foot-floor kinetic data were filtered with a 10 Hz lowpass Butterworth filter and analyzed using MATLAB software (Mathworks, Natick, MA). Force and COP data were used to calculate RM and TR time series, as detailed by Zatsiorsky & Duarte (1999). Velocity, acceleration, and jerk in the mediolateral (ML) and anteroposterior (AP) directions were calculated for every measure type (COP, RM, and TR). Percent changes were calculated using F0 as the baseline. MATLAB software was used to perform three-way analyses of variance with Tukey’s HSD post hoc tests with p<0.01 to determine analyze effects of vision, foam thickness, and measure type. Linear regression of each parameter across foam thickness was performed to estimate measure means across the full spectrum of simulated deficit.

The EO condition produced no statistically significant differences across any foam condition, often plateauing after F2. *+ Therefore, further analysis was performed primarily using EC data. For EC trials, the F4 condition showed greatest percent changes from baseline for all assessed parameters, with an upward trend in mean values from F1 to F4 for COP, RM, and TR measures. In general, standard deviations were very large, likely due to the large sample size and inherent variability in postural sway between subjects. However, some statistically significant differences between COP, RM, and TR acceleration and jerk were still able to be found.

In terms of sensitivity, COP captures the smallest change in foam thickness, but RM provides a robustness across parameters that is not found in COP or TR. Dependence on sway direction is evident, with AP parameters often showing greater percent changes across foam thickness. RM and TR measures showed different behavior in the AP- and ML-direction, with RM greater than COP and TR in the AP-direction. This result is particularly interesting when considering the physiological mechanisms attributed to RM and TR, as these results suggest that movement in AP-direction may be more heavily influenced by the central nervous system. The findings of this study suggest that RM-TR derived measures may:​ (1) provide a greater degree of deficit detection ability than traditional linear COP measures, and (2) reveal previously unknown mechanisms of postural control.