The purpose of this study is to investigate (1)the variability of the initial state balance measures within each participant, across different days and within each day, across the participants. (2) the effect of different noise color on the magnitude and the direction of change in the state. (3) if the initial state affects the magnitude and direction in which each treatment color pushes the change in state. We hypothesized that (1) the sway measures before stimulation will differ across subjects and vary within a subject across the sessions. (2). Each vibration color would cause a change in the state different from the placebo. (3)The efficacy of utilizing subthreshold vibration to change postural sway magnitude, predictability, and complexity depends on their initial values and the color used. The long-term goal is to demonstrate that the state of balance can be changed to the desired direction using a specific noise color.
Nine healthy older adults (HO: three male and six females; age: 62.56 ± 1.42 years; height: 169.48 ± 9.32 cm) with no significant history of musculoskeletal, neurologic, vestibular, or mobility deficiencies participated in the study. The participants visited the lab 4 times, and each time their sway before vibration, during vibration and soon after vibration was measured using a force plate. For each visit, different vibration color (white, pink, brown, and placebo) was used, and the order was random for each participant. All sway trials were conducted with people's heels 17 cm apart, eyes closed, hands by their sides, and toes pointing forward. The force plate kinetic data were collected at 2500 Hz using a 16-bit A/D CED Power mkII and Spike2 recorder (Cambridge Electronic Design, UK). All the data analysis was done in MATLAB R2020b (MathWorks, MA, USA). The force and moment data were filtered to remove any motor vibration sway time series with a low pass filter at 20 Hz cut-off frequency. The center of pressure in the anteroposterior(AP) and Mediolateral(ML) directions was calculated from each sway trial . The COP data (COPAP, COPML) were downsampled to 50 Hz and measures of magnitude(RMS), predictability(Sample Entropy), and complexity(DFA) were extracted for each of the sway trials. The statistical analysis was done using Python 3. First, ANOVA was used to determine the statistical significance of the baseline measures (entropy, DFA, and RMS) across the participants. Then a repeated measure of ANOVA was done between baseline measures (entropy, DFA, and RMS) across the four visits. Another repeated measure of ANOVA was performed to determine the statistical significance of the change in measure (measure vibration – measure baseline) in the ML direction for the different treatments (white, pink, brown and placebo). The statistical significance level was set to <0.05. Also, a linear regression model was fitted between the change in measure (measure vibration – measure baseline) and the baseline for each treatment (white, pink, brown, and placebo) in the ML direction
There was a significant difference in the baseline entropy, DFA and RMS value across subjects. Also, within-subjects, baseline measures show different means and variability across subjects. Some had low standard deviations, while others had high standard deviations. However, there was no significant difference in entropy, DFA and RMS baseline across the four visit days.
There was a general increase in predictability (Sample Entropy ) when white, pink, and brown were administered but not the placebo, and this change was found to be statistically significant. The change in RMS and DFA were found not to be significant across the treatments and the placebo. All regression showed negative slopes except the white regression line for the RMS. Most of the regression lines have very low R2 and non-significant slope values. This was expected due to the low sample size and high variability.
The study shows evidence that subthreshold vibration under the feet has the tendency to increase predictability and reduce sway magnitude. The effect of subthreshold vibration on sway could depend on the initial sway and the type of vibration used.
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