# Validation and Application of Two New Core Stability Tests in Professional Football
## 2 Methods
### Statistical Analysis
When applicable, the CV was calculated for each subject by dividing the standard deviation of the two testing sessions by the mean of the two testing sessions \(\times 100\) (for intersession CV calculation), and the standard deviation of two testers by the mean of the two testers \(\times 100\) (for intrasession CV calculation). Both the intra-rater (rater 1 and 2, between 2 days) and the inter-rater reliability (between rater 1 and 2, the same testing day) peak isometric force values were assessed using intraclass correlation coefficients (ICC) with 95% confidence intervals (CI) in order to assess relative reliability by utilizing a two-way random effects model with single measure reliability (ICC (2,1)). The mean value from each participant was used for the analysis. Values \(<\) 0.50 represented poor reliability, values \(>\) 0.50 and \(<\) 0.75 indicated fair to good reliability, and values \(>\) 0.75 marked excellent reliability [20]. Given that ICC values may be influenced by the inter-subject variability of scores, the standard error of measurement (SEM) was reported in conjunction with the ICCs. A large ICC may represent poor trial-to-trial consistency if the inter-participant variability is too high [18], in contrast to SEM, which is not affected by inter-subject variability [18]. SEM was calculated using the equation \(\text{SEM}=\text{SD}\times(\sqrt{1}-\text{ICC})\), where SD corresponds to the sample standard deviation [17; 20; 21; 22] for intra- and inter-rater reliability analysis. In the case of the intersession reliability analysis, the SD of the T2 data set class was utilized, assuming that between the two assessment sessions, T2 generated lower SD [21]. The error in an individual's score at one point in time was estimated by multiplying the SEM by the \(z\) value for the 90% confidence level (\(z\) value = 1.65) [23; 24; 25; 26; 27; 28; 29]. This value was then multiplied by the square root of 2 (to account for the measurement error in two test sessions) in order to estimate the minimum detectable change (MDC) at the 90% confidence level. **Study II: (Longitudinal study)**  
A preliminary effect size calculation was performed to determine the adequate sample size required to account for a random error of 0.8. A total of 27 participants would have been necessary, but finally 20 participants were recruited (an entire second Spanish league football club). After that, only 13 players were included in the statistical analysis, because 7 players did not complete the testing session after the core training program due to either injury or national team recruitment for international match play. The compliance of the core training program was 82%, corresponding to 69 core training sessions accomplished out of the 84 total sessions established at the beginning of the intervention. After checking the assumptions of parametric statistics, the mean comparisons before and after the implementation of the core training program were analyzed using a Paired Sample \(t\)-test. The estimation of MDC at a 90% confidence level was also calculated to determine whether the observed change in score between the pre- and post-testing evaluations was truly a change in patient status, or was merely associated with an error (noise) in the measurement process [17].