# Wobble Board Performance: A Practical and Useful Quantification in Balance Assessment
###### Abstract
Another study investigated the resultant horizontal mean velocity of force-plate-derived CoP and documented a stronger correlation with WB tilt angle velocity (r = 0.77) [24]. In the latter study, a wide range of intra- and inter-session reliability was documented for various CoP variables, i.e., sway, horizontal mean velocity, and index of complexity (ICC = 0.17-0.76) [24]. This difference in reliability among CoP variables suggested that various CoP variables may not be equally appropriate for the practical purposes of balance assessment (e.g., longitudinal monitoring of development of balance performance). Moreover, different CoP variables may reflect different aspects of balance control, which makes it difficult to select a single best indicator for balance performance [13]. Alternatively, it was suggested to reflect balance performance as the time spent on the WB at a \(\sim\)0\({}^{\circ}\) tilt angle during a 30 s standing test [17; 21]. This time was then used as a measure of 'WB performance'. The idea is to summarize the variety of balance strategies [7] and motor control systems in the human body [3] involved in postural control into a single representative expression of balance performance. This greatly simplifies data processing and interpretation, which facilitates practical use in the field. Reliability analyses of WB performance achieved ICC results of 0.71 [21] and 0.65-0.89 [17] in standing tests. Validity was only assessed in comparison with a functional balance test (i.e., Y-balance test) with poor results (r = \(-\)0.23-0.34) [17]. It was argued that the two tests may differ in their demands for postural control [17], which is in line with reported differences in strategies to maintain balance on WBs compared with firm surfaces [23]. The Y-balance test evaluates the ability to maintain equilibrium (steady state) on a stable surface, whereas the WB task requires dynamic adjustments to a moving platform, testing the ability to continuously regain balance [23]. As the WB is an unstable platform, the instrumented WB is effective in measuring dynamic, proactive balance performances, providing understandings into an individual's ability to anticipate balance challenges. WB performance was able to accurately detect balance deficits in patients with chronic ankle instability [18; 26]. However, WB performance as time spent at a \(\sim\)0\({}^{\circ}\) tilt angle has never been validated against the gold standard in balance assessment (i.e., CoP variables derived from force plates) [13]. For concurrent validity, investigating the relationship of a tested system with the gold standard is common practice [27]. It is more likely to statistically confirm the relationship between two items in the overall population with a wide range of values (deviations from the mean) than in a specific sub-sample with a narrow range of values [28]. Therefore, an established relationship in the overall population does not imply that the relationship can also be confirmed within a specific sub-sample. Since training interventions are often administered to a specific group of individuals (e.g., athletes of a sports team, patients, elderly), the validation progress should reflect the system's ability to relate with the gold standard within a sub-sample that is relevant in training reality. Otherwise, for example, the tested system may be valid for differentiating between sub-samples within the overall population but not for performance monitoring within a sub-sample (e.g., high-level athletes). High-level athletes are an appropriate sub-sample because inter-individual performance differences are usually smaller (i.e., narrow range of values) at high compared with low competition levels. Regarding a suitable sport for such investigation, volleyball represents a highly relevant choice because of the large number of ankle injuries and the importance of balance training as a prevention measure in volleyball [10].