## 3 Results
### Individual Pairs of EMG-COP Correlations  
Table 2 presents the mean values of the absolute z-transformed EMG-COP correlation coefficients (\(\left|r\right|\)) assessed across four influencing factors: leg dominance, surface stability, sway direction, and foot position. The average \(\left|r\right|\) values range from 0.2 to 0.5, reflecting a spectrum of low-to-high EMG-COP correlations. These variations depend on specific muscles and factors. Notably, the highest \(\left|r\right|\) value is observed in the GM muscle under stable conditions, attributed to the impact of surface stability. The main findings indicate that specific factors--leg dominance, surface stability, and sway direction--affect different muscles under particular conditions. For leg dominance, a higher correlation coefficient is  
Figure 3: An overview of the individual cross-correlations between seven lower limb EMG signals and COPap and COPml displacements during bipedal standing on an unstable surface (US). Data are presented in (**A**) feet-apart and (**B**) feet-together positions. Each participant contributed two data points, one from their non-dominant (ND) leg and one from their dominant (DO) leg, calculated from the middle 60 s of EMG and COP data. The color code is used to differentiate between the two legs: dominant leg (DO) is represented by one color, while non-dominant leg (ND) is represented by a different color. A vertical line denotes the median time delay (\(\tau m\)) for each correlation.  
observed for the TA muscle (\(p\) = 0.004) during the non-dominant leg (ND) stance compared to the dominant leg (DO) stance. Concerning surface stability, stronger correlation coefficients are noted for the RF (\(p\) = 0.001), ST (\(p\) < 0.001), BF (\(p\) < 0.001), GM (\(p\) < 0.001), and SO (\(p\) < 0.001) muscles when balancing on a stable surface (SS) versus an unstable surface (US). Regarding sway direction, higher correlation coefficients are seen for the ST (\(p\) = 0.001) and TA (\(p\) < 0.001) muscles in the anteroposterior (AP) direction compared to the mediolateral (ML) direction.  
Interaction effects also reveal significant relationships between specific factor pairs and targeted muscles. Notably, the interaction between surface stability and foot position  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \multicolumn{5}{c}{**A: Leg Dominance**} \\ \hline
**Muscle** & **ND** & **DO** & \(p\)**-Value** & \(\eta p^{2}\) & **Power** \\ \hline RF & 0.20 \(\pm\) 0.01 & 0.20 \(\pm\) 0.02 & 0.669 & 0.009 & 0.070 \\ ST & 0.23 \(\pm\) 0.02 & 0.23 \(\pm\) 0.02 & 0.933 & 0.000 & 0.051 \\ BF & 0.25 \(\pm\) 0.02 & 0.23 \(\pm\) 0.02 & 0.367 & 0.039 & 0.142 \\ TA & 0.30 \(\pm\) 0.02 & 0.23 \(\pm\) 0.01 & 0.004 * & 0.325 & 0.859 \\ PL & 0.28 \(\pm\) 0.02 & 0.30 \(\pm\) 0.02 & 0.433 & 0.030 & 0.119 \\ GM & 0.46 \(\pm\) 0.02 & 0.43 \(\pm\) 0.02 & 0.190 & 0.080 & 0.253 \\ SO & 0.41 \(\pm\) 0.02 & 0.42 \(\pm\) 0.02 & 0.824 & 0.002 & 0.055 \\ \hline \hline \multicolumn{5}{c}{**B: Surface Stability**} \\ \hline
**Muscle** & **SS** & **US** & \(p\)**-Value** & \(\eta p^{2}\) & **Power** \\ \hline RF & 0.23 \(\pm\) 0.02 & 0.16 \(\pm\) 0.01 & 0.001 * & 0.439 & 0.971 \\ ST & 0.29 \(\pm\) 0.03 & 0.17 \(\pm\) 0.01 & \textless{}0.001 * & 0.485 & 0.988 \\ BF & 0.30 \(\pm\) 0.02 & 0.18 \(\pm\) 0.01 & \textless{}0.001 * & 0.492 & 0.990 \\ TA & 0.25 \(\pm\) 0.02 & 0.28 \(\pm\) 0.01 &significantly affects the RF (\(p\) = 0.001), ST (\(p\) = 0.001), BF (\(p\) < 0.001), TA (\(p\) < 0.001), PL (\(p\) < 0.001), GM (\(p\) < 0.001), and SO (\(p\) < 0.001) muscles. The interaction between leg dominance and foot position is significant for the PL (\(p\) = 0.002) and GM (\(p\) = 0.002) muscles. Additionally, the interaction between leg dominance and sway direction is significant for the GM (\(p\) = 0.003) muscle. Lastly, the interaction of sway direction with itself is significant for the PL muscle (\(p\) < 0.001).