## 3 Results
### Intra-Rater and Inter-Rater Reliability  
Table 1 presents the intra-rater and inter-rater reliability of the sEMG CoAI1 (ST/VM) for different directions of the YBT-LQ. Considering the lower bound of the 95% CI, the lowest intra-rater reliability was noted for the left lower limb in the posterolateral direction, which ranged from poor to good with ICC = 0.495, 95% CI (0.227, 0.695). Then, the highest level of reliability could be considered for the right lower limb in the anterior direction, which ranged from fair to excellent, ICC = 0.742, 95% CI (0.561, 0.855). Also, based on the lower bound of the 95% CI, the lowest inter-rater reliability was noted for the right lower limb in the anterior direction, which was from poor to fair, with ICC = 0.315, 95% CI (0.002, 0.570). On the other hand, the highest reliability was for the left lower limb for the posterolateral direction and was from good to excellent, ICC = 0.790, 95% CI (0.633, 0.884). Table 2 presents the intra-rater and inter-rater reliability of the sEMG-based CoAI2 (ST/VL) for different directions of the YBT-LQ. Based on the lower bound of the 95% CI, the lowest intra-rater reliability was noted for the left lower limb in the posterolateral direction, which ranged from poor to good with ICC = 0.567, 95% CI (0.317, 0.744). Consecutively, the highest level of reliability was noted for the right lower limb in the anterior direction, which ranged from good to excellent, ICC = 0.765, 95% CI (0.598, 0.869). \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multicolumn{5}{c}{**Surface Electromyography-Based CoAI1 (ST/VM)**} \\ \hline \multicolumn{5}{c}{**Intra-Rater Reliability**} \\ \hline \multicolumn{5}{c}{**Inter-Rater Reliability**} \\ \hline \multicolumn{5}{c}{**YBT-LQ Direction**} & **Right Lower Limb**} & **Left Lower Limb** & **Right Lower Limb** & **Left Lower Limb** \\ \hline Anterior & 0.742 (0.561, 0.855) & 0.613 (0.379, 0.774) & 0.315 (0.002, 0.570) & 0.660 (0.445, 0.804) \\ \hline Posteromedial & 0.636 (0.406, 0.790) & 0.585 (0.339, 0.756) & 0.620 (0.385, 0.780) & 0.667 (0.451, 0.809) \\ \hline Posterolateral & 0.720 (0.528, 0.842) & 0.495 (0.227, 0.695) & 0.756 (0.583, 0.863) & 0.790 (0.633, 0.884) \\ \hline \multicolumn{5}{c}{Values expressed as intraclass correlation coefficient, ICC, and 95\% confidence interval (lower bound, upper bound) of ICC. CoAI, coactivation index; ST, semitendinosus; VM, vastus medialis; YBT-LQ, Lower Quarter Y-Balance Test.} \\ \end{tabular}
\end{table} 
Table 1: The intra-rater and inter-rater reliability of the surface electromyography-based coactivation index, precisely CoAI1 (ST/VM). \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multicolumn{5}{c}{**Surface Electromyography-Based CoAI2 (ST/VL)**} \\ \hline \multicolumn{5}{c}{**Intra-Rater Reliability**} \\ \hline \multicolumn{5}{c}{**YBT-LQ Direction**} & **Right Lower Limb**} & **Left Lower Limb** & **Right Lower Limb** & **Left Lower Limb** \\ \hline Anterior & 0.765 (0.598, 0.869) & 0.707 (0.512, 0.833) & 0.503 (0.227, 0.703) & 0.675 (0.465, 0.813) \\ \hline Posteromedial & 0.761 (0.591, 0.867) & 0.743 (0.565, 0.856) & 0.712 (0.520, 0.836) & 0.743 (0.563, 0.855) \\ \hline Posterolateral & 0.690 (0.487, 0.823) & 0.567 (0.317, 0.744) & 0.779 (0.620, 0.877) & 0.732 (0.544, 0.849) \\ \hline \multicolumn{5}{c}{Values expressed as intraclass correlation coefficient, ICC, and 95\% confidence interval (lower bound, upper bound) of ICC. CoAI, coactivation index; ST, semitendinosus; VL, vastus lateralis; YBT-LQ, Lower Quarter Y-Balance Test.} \\ \end{tabular}
\end{table} 
Table 2: The intra-rater and inter-rater reliability of the surface electromyography-based coactivation index, precisely CoAI2 (ST/VL). Also, considering the lower bound of the 95% CI, the lowest inter-rater reliability was noted for the right lower limb in the anterior direction, which ranged from poor to fair good with ICC = 0.503, 95% CI (0.227, 0.703).