## 3 Results
### Center of Pressure Trajectories  
The overall LMMs demonstrated a statistically significant condition (F = 6.47, \(p\) < 0.001) and condition-by-group interaction (F = 2.11, \(p\) = 0.021) effect in low-frequency power of COP in the x-direction; and condition (F = 69.96, \(p\) < 0.001), trial (F = 6.51, \(p\) = 0.002), condition-by-trial interaction (F = 2.75, \(p\) = 0.002), and condition-by-group interaction (F = 2.14, \(p\) = 0.020) effect in low-frequency power of COP in the y-direction (See Table 2). In addition, there was a statistically significant condition (F = 247.45, \(p\) < 0.001), trial (F = 25.71, \(p\) < 0.001), and age (F = 4.60, \(p\) = 0.036) effect in medium-frequency power of COP in the x-direction; and condition (F = 283.50, \(p\) < 0.001), trial (F = 17.78, \(p\) < 0.001), and condition-by-group interaction (F = 2.06, \(p\) = 0.025) effect in medium-frequency power of COP in the y-direction. Lastly, there was also a statistically significant condition (F = 373.98, \(p\) < 0.001), trial (F = 49.83, \(p\) < 0.001), condition-by-trial interaction (F = 2.40, \(p\) = 0.008), and condition-by-group interaction (F = 2.13, \(p\) = 0.020) effect in high-frequency power of  
\begin{table}
\begin{tabular}{l c c c c c c} \hline  & \multicolumn{2}{c}{**Low Frequency**} & \multicolumn{2}{c}{**Medium Frequency**} & \multicolumn{2}{c}{**High Frequency**} \\
**Source of Variance** & **F-Value** & **_p-Value** & **F-Value** & **_p-Value** & **F-Value** & **_p-Value** \\ \hline
**Trembling in x-direction** & & & & & & \\ Condition & 1.89 & 0.094 & 121.49 & \textless{}0.001 & 214.49 & \textless{}0.001 \\ Trial & 0.20 & 0.817 & 15.27 & \textless{}0.001 & 24.75 & \textless{}0.001 \\ Group & 0.33 & 0.722 & 3.85 & 0.027 & 2.08 & 0.134 \\ Age & 1.14 & 0.290 & 6.14 & 0.016 & 3.82 & 0.056 \\ Condition \(\times\) Trial & 0.37 & 0.961 & 1.24 & 0.261 & 1.65 & 0.087 \\ Condition \(\times\) Group & 2.22 & 0.015 & 1.15 & 0.323 & 1.25 & 0.258 \\ Trial \(\times\) Group & 0.57 & 0.686 & 1.03 & 0.393 & 1.08 & 0.365 \\ Condition \(\times\) Trial \(\times\) Group & 0.19 & 1.000 & 1.00 & 0.456 & 1.07 & 0.380 \\ \hline
**Trembling in y-direction** & & & & & & \\ Condition & 116.23 & \textless{}0.001 & 389.96 & \textless{}0.001 & 546.63 & \textless{}0.001 \\ Trial & 6.37 & 0.002 & 24.40 & \textless{}0.001 & 76.31 & \textless{}0.001 \\ Group & 1.65 & 0.201 & 1.65 & 0.201 & 3.15 & 0.050 \\ Age & 0.03 & 0.873 & 0.14 & 0.711 & 0.18 & 0.670 \\ Condition \(\times\) Trial & 2.40 & 0.008 & 0.90 & 0.536 & 3.56 & \textless{}0.001 \\ Condition \(\times\) Group & 2.09 & 0.023 & 1.11 & 0.350 & 1.78 & 0.060 \\ Trial \(\times\) Group & 1.27 & 0.279 & 1.36 & 0.245 & 0.80 & 0.526 \\ Condition \(\times\) Trial \(\times\) Group & 0.58 & 0.928 & 0.63 & 0.896 & 1.16 & 0.285 \\ \hline \end{tabular}
\end{table}
Table 4: Analysis of variance table for the linear mixed model examining main effects and interactions of condition, trial, and group on the low-, medium-, and high-frequency band of the x and y trembling components, with age as a covariate.  
COP in the x-direction; and condition (F = 541.60, \(p\) < 0.001), trial (F = 75.75, \(p\) < 0.001), condition-by-trial interaction (F = 3.14, \(p\) = 0.001), and condition-by-group interaction (F = 1.99, \(p\) = 0.032) effect in high-frequency power of COP in the y-direction.