## 3 Results
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Descriptives**} & \multicolumn{2}{c}{**Point 1**} & \multicolumn{2}{c}{**Point 2**} & \multicolumn{2}{c}{**Point 3**} & \multirow{2}{*}{**F**} & \multirow{2}{*}{\(\mathbf{\eta^{2}p}\)} \\ \cline{3-3} \cline{5-10}  & **Mean** & **SD** & **Mean** & **SD** & **Mean** & **SD** & \\ \hline Interval R-R & 1113.7 & 192.3 & 1070.3 & 235.0 & 852.3 & 117.7 & 4.44 & 0.025 & 0.297 \\ RMSD & 258.6 & 271.7 & 196.2 & 288.3 & 79.8 & 50.1 & 1.24 & 0.310 & 0.106 \\ SDNN & 324.7 & 426.0 & 155.1 & 199.6 & 71.9 & 29.2 & 1.79 & 0.191 & 0.146 \\ PNS Index & 6.7 & 8.0 & 5.3 & 9.9 & 0.8 & 1.4 & 1.42 & 0.264 & 0.119 \\ SNS Index & \(-1.6\) & 0.9 & \(-1.1\) & 1.5 & \(-0.1\) & 0.7 & 4.03 & 0.033 & 0.277 \\ Mean HR & 55.2 & 9.4 & 58.8 & 11.9 & 71.6 & 10.0 & 5.34 & 0.013 & 0.337 \\ VAS pain & 0.50 & 0.75 & 1.25 & 1.03 & 6.87 & 0.83 & 125 & \textless{}0.001 & 0.922 \\ \hline \hline \end{tabular} Note. \(p\)-value < 0.05 statistically significant. Effects size: Small \(\leq\) 0.01, medium \(\geq\) 0.059, and large: \(>\)0.138. Point 1 = one day before without fatigue; point 2 = without fatigue, and point 3 = with fatigue conditions. PNS = Parasymorphic nervous system, SNS = sympathetic nervous system, SDNN = the standard deviation of the R-R intervals, and RMSSD = Root mean square successive differences of R-R intervals.  
\end{table} 
Table 2: Heart rate variability and perceived muscle pain under fatigue and non-fatigue conditions. The data fit the multiple regression model reasonably well for the desaturation response (residual = 0.00 \(\pm\) 6.57; predictive values = 46.44 \(\pm\) 9.91 SmO\({}_{2}\)) with outliers at \(\geq\)30% and \(\leq\)70% SmO\({}_{2}\). For the resaturation response (residual = 0.00 \(\pm\) 7.60; predictive values = 51.90 \(\pm\) 9.29 SmO\({}_{2}\)) with outliers at \(\geq\)30% and \(\leq\)70% SmO\({}_{2}\). For the hyperemic response (residual = 0.00 \(\pm\) 3.91; predictive values = 5.46 \(\pm\) 3.45 SmO\({}_{2}\)) with outliers at \(\geq\)\(-\)0.3% and \(\leq\)15% SmO\({}_{2}\). For the MRT response (residual = 0.00 \(\pm\) 10.07; predictive values = 32.18 \(\pm\) 11.03 s) with outliers at \(\geq\)10 s and \(\leq\)60 s.  
The statistical values from the multiple linear regression showed that HRV influenced the desaturation response (r\({}^{2}\) = 0.695; F = 8.205; \(p\) < 0.000), with the fatigue condition being significant (t = 4.763; \(p\) < 0.000). HRV also influenced the resaturation response (r\({}^{2}\) = 0.599; F = 5.384; \(p\) = 0.003), with the fatigue condition being significant (t = 4.386; \(p\) < 0.000). However, HRV did not show significance for hyperemia (r\({}^{2}\) = 0.408; F = 2.481; \(p\) = 0.070), and the fatigue condition had no influence on hyperemia (t = 0.532; \(p\) = 0.601). Finally, HRV influenced the reoxygenation kinetics (MRT) response (r\({}^{2}\) = 0.545; F = 4.318; \(p\) = 0.009), with the fatigue condition being significant (t = \(-\)3.691; \(p\) = 0.002). Figure 4 illustrates the correlation analysis between VAS pain and the resting muscle oxygen consumption obtained from VOT-NIRS. A significant correlation was observed between VAS pain and desaturation (r = 0.81, 95% CI = 0.60 to 0.91; \(p\)\(\leq\) 0.001), resaturation (r = 0.69, 95% CI = 0.39 to 0.85; \(p\)\(\leq\) 0.001), and MRT (r = \(-\)0.58, 95% CI = \(-\)0.80 to \(-\)0.23;  
Figure 3: Multiple linear regressions between heart rate variability and resting muscle oxygen consumption. **Note. ** The regression model included the following HRV residual analysis as an independent variable and dependent variable: (**a**) desaturation, (**b**) resaturation, (**c**) hyperemia, and (**d**) MRT. The condition (without fatigue or with fatigue) was included as a factor that modulates the multiple regression. \(p\) = 0.003). However, no significant correlation was found between VAS pain and hyperemia (r = \(-\)0.264, 95% CI = \(-\)0.60 to 0.16; \(p\) = 0.212).