# Wobble Board Performance: A Practical and Useful Quantification in Balance Assessment
## 4 Discussion  
The objective of this study was to assess the validity of WB performance for balance evaluation based on the relationship with force plate CoP variables. For concurrent validity assessment, the linear relationship between a new and an established criterion has frequently been tested via [PERSON]'s or [PERSON]'s correlation analyses previously [17; 24]. The current correlation coefficients remained consistent across three measurement sessions. The inter-session variability in the correlation coefficients showed small standard deviations, ranging from 0.027 to 0.059 for different CoP variables. No systematic deviations were noted over time, indicating the absence of long-term familiarization effects over multiple sessions. The stable correlation between WB and force-plate-derived CoP variables implied that the validity of WB performance was not session-dependent. Clinicians and coaches may use WB performance reliably across at least three repeated sessions in 6-week intervals without the necessity of a prior familiarization session. During a single measurement session, the current coefficients showed a high correlation of WB performance with anterior-posterior (r = \(-\)0.842) and mediolateral (r = \(-\)0.741) sway and sway area (r = \(-\)0.795) of CoP derived from force plates. The correlation with mean sway velocity just missed significance (\(p\) = 0.051) and was only moderate (r = \(-\)0.575). These correlations were higher than the best correlation results (r = 0.52) between WB performance and another typical balance test (i.e., Y-balance test) [17; 26]. Previous authors assumed that WB performance reflected different aspects of balance control than the Y-balance test [17]. Moreover, an intervention study showed that WB training improved WB performance and strategies, but balance performance on firm surfaces was not affected [33]. WB performance was, in the current study, more strongly associated with CoP variables (i.e., valid and reliable for balance assessment) [13; 14] than with Y-balance test results in a previous study [17]. This comparison corroborated that WB performance may serve as a valid measure for dynamic instead of static balance assessment. The adjusted R\({}^{2}\), accounting for the order in polynomial transformation, was higher for all transformed WB data than for raw WB performance. WB performance may be  
\begin{table} \begin{tabular}{c c c c c c} \hline \hline  & **WB [\%]** & **AP [mm]** & **ML [mm]** & **Area [cm\({}^{2}\)]** & **V [mm\(\cdot\)s\({}^{-1}\)]** \\ \hline \multirow{2}{*}{Area} & r & & & 1 & 0.510 \\  & \(p\) & & & & 0.090 \\ \hline \multirow{2}{*}{V} & r & & & & 1 \\  & \(p\) & & & & \\ \hline \end{tabular}  Note: \({}^{\lx@sectionsign}\) CoP values derived from force plate data; \({}^{\lx@paragraph  
better described by a nonlinear than a linear relationship with force plate CoP variables. The coefficients for nonlinear relationships were high to very high. In line with the linear correlation results, the strongest nonlinear relationship was found in anterior-posterior sway (r = \(-\)0.915) and the weakest in velocity (r = \(-\)0.796). Co-linearity was also the weakest between sway velocity and other CoP variables (r = 0.500-0.515, all nonsignificant) and high to very high among fractal sway and area (r \(\geq\) 0.836). This suggested that velocity may reflect a different aspect of balance control than fractal sway and sway area. In recognition of the previously mentioned variety of factors contributing to balance control [3; 5; 6], the different strategies to maintain equilibrium [7; 23], and specific effects of training interventions [33], it seemed reasonable that different biomechanical characteristics reflect different aspects of balance control. Therefore, velocity may be a valuable addition to fractal sway or sway area for holistic balance evaluation in research and practice.