# Validation and Application of Two New Core Stability Tests in Professional Football
## 4 Discussion  
### Study I: (validation study)  
The purpose of the validation study was to determine the validity and reliability of two new proposed core strength field tests, using an HHD to evaluate the ability to produce force in challenging positions, representing core stability. The observed ICCs in this study suggest the excellent test-retest reliability of the Prone Plank Test (PPT) and the Closed Kinetic Chain Test (CCT) (see Tables 3 and 4). The ICC is a measure of relative reliability indicating a measure's ability to discriminate between existing differences among two sets of data. It is calculated by dividing the between-patient variance by the total variance [17] (between-patient and within-patient variance). The present findings therefore suggest that the PPT and CCT demonstrated appropriate reliability for their use in the training field setting as a core strength functional evaluation. The SEM provides an absolute measure of reliability with regard to the original test units, and can be used to estimate the error associated with an individual patient's test score [17, 26]. The smaller the SEM value, the more reliable the measurements [17, 20]. In our study, the SEM ranged from 4.01 N to 13.50 N for intratester intrasession and intersession reliability analyses. The results found in our study correlate with a similar study accomplished by [PERSON] et al. [10], in relation to intratester and intrasession reliability, who evaluated maximal isometric trunk flexion and extension strength tests using HHD on a healthy athletic population. They reported excellent intratester and intertester reliability values (ICC = 0.76-0.93) for all tests, except for the intratester reliability of the maximal isometric trunk flexion test starting at 0\({}^{\circ}\), which was good (ICC = 0.67). The ICCs for the intertester and intrasession analysis in our study were lower than what they had found, probably because the  
Figure 4: Pre-/post HHD values for PPT and CCT tests. \(p<0.05\). (*) denotes statistically significance between pre to post evaluations. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**Mean Peak Torque N (\(\pm\)SD)**} & & & \\ \cline{2-7}  & **Pre-Season (T1)** & **Post-Season (T2)** & \(\Delta\) **T2-T1 (\%)** & \(D\) & \(t\)**-Test (\(p\))** & **MDC\({}_{90}\) (N)** \\ \hline \multirow{2}{*}{**Prone plank test**} & 137.87 & 160.73 & \multirow{2}{*}{+22.86} & \multirow{2}{*}{0.9} & \multirow{2}{*}{0.00} & \multirow{2}{*}{47.71} \\  & (\(\pm\)28.13) & & & & & \\ \hline
**Closed kinetic** & 184.64 & 260.92 & \multirow{2}{*}{+76.28} & \multirow{2}{*}{1.0} & \multirow{2}{*}{0.00} & \multirow{2}{*}{93.29} \\  & (\(\pm\)56.10) & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Compared means for pre-season (T1) and post-season (T2) values for the two core strength test (N = 13). N = Newton; SD = standard deviation; Cohen’s d = effect size; MDC\({}_{90}\) = minimal detectable change 90% confidence level. testers from their study had similar physical characteristics, in order to counteract the participant's force exertion. Another study conducted by [PERSON] et al. [30] determined the intertester reliability of six tests of trunk muscle strength evaluation using practical measurement methods which could be applied in most clinical or work settings. For abdominal and back extensor isometric force tests using HHD, they reported poor (0.24 and 0.25, respectively) ICC values. These results contrast with those reported in our study, maybe because the rater's ability to apply force could have been limited, as they argued. In a more recent study conducted by [PERSON] et al. [7], a flexion-extension isokinetic protocol was used to asses trunk muscle strength and endurance in a healthy and physically active population (1-3 h of moderate physical activity in recreational sports, 1-3 days a week). The ICC values obtained for the strength variables in this study ranged from 0.57 to 0.77 in extension, and from 0.62 to 0.84 in flexion, with low SEM values.