## 4 Discussion  
The aim of this study was to examine the association between cardiorespiratory and metabolic parameters and match running performance (MRP) in national-level football players. Based on multiple regression analysis, the key findings were as follows: (i) cardiac parameters HRmax, HR 2\({}^{\prime}\), and HR AT are associated with total running distance, with HR 2\({}^{\prime}\) and HRmax also being associated with high-speed running, while HR 2\({}^{\prime}\) is associated \begin{table}
\begin{tabular}{c c c c c c c} \hline  & **\textless{}8 km/h** & **8.1–15 km/h** & **15.1–19 km/h** & **19.1–23 km/h** & **\textgreater{}23 km/h** & **Total** \\ \hline HR max & 0.258 & 0.429 & 0.478 & 0.526 * & \(-\)0.114 & 0.573 * \\ HR AT & 0.011 & \(-\)0.311 & \(-\)0.383 & \(-\)0.250 & \(-\)0.168 & \(-\)0.360 * \\ HR 1\({}^{\prime}\) & 0.111 & 0.087 & \(-\)0.112 & 0.059 & 0.298 & 0.128 \\ HR 2\({}^{\prime}\) & \(-\)274 & \(-\)0.420 * & \(-\)0.146 & \(-\)0.551 * & \(-\)0.064 & \(-\)0.528 * \\ R & 0.379 & 0.580 & 0.390 & 0.681 & 0.251 & 0.737 \\ R2 & 0.144 & 0.336 & 0.152 & 0.464 & 0.063 & 0.543 \\ \(p\) & 0.219 & 0.004 & 0.191 & 0.000 & 0.661 & 0.000 \\ \hline VO2 max & 0.472 & 0.162 & 0.005 & 0.197 & 0.085 & 0.342 \\ VO2 max/v & \(-\)0.272 & \(-\)0.313 & \(-\)0.005 & \(-\)0.262 & \(-\)0.248 & \(-\)0.403 * \\ VO2 max/HR & \(-\)0.374 & \(-\)0.217 & \(-\)0.204 & \(-\)0.391 & 0.034 & \(-\)0.398 \\ R & 0.344 & 0.371 & 0.203 & 0.447 & 0.196 & 0.498 \\ R2 & 0.118 & 0.138 & 0.041 & 0.200 & 0.038 & 0.248 \\ \(p\) & 0.194 & 0.135 & 0.664 & 0.039 & 0.690 & 0.014 \\ \hline LA 4\({}^{\prime}\) & \(-\)0.500 & 0.793 & 0.367 & \(-\)0.050 & \(-\)0.869 & 0.342 \\ LA 10\({}^{\prime}\) & 1.216 & \(-\)0.149 & \(-\)0.178 & 0.361 & 1.210 & 0.534 \\ Index LA & 1.072 & \(-\)0.329 & \(-\)0.279 & 0.278 & 1.300 & 0.325 \\ Index ME & 0.684 & 0.545 & \(-\)0.055 & 0.186 & 0.360 & 0.725 \\ R & 0.468 & 0.272 & 0.240 & 0.156 & 0.415 & 0.368 \\ R2 & 0.219 & 0.074 & 0.058 & 0.024 & 0.173 & 0.135 \\ \(p\) & 0.058 & 0.583 & 0.699 & 0.922 & 0.136 & 0.250 \\ \hline \end{tabular} Legend: R—multiple correlation; R2—coefficient of determination; \(p\)—statistical significance; * denotes statistical significance of \(p\) < 0.05.  
\end{table} 
Table 6: The result of multiple regression for cardiorespiratory and metabolic parameter effects on the match running performance parameters. with jogging; (ii) respiratory parameter VO2 max/v is associated with total running distance; and (iii) lactate parameters are not associated with movement performance. Upon examining cardiac parameters, it was noted that the average HRmax values among footballers in the top level of competition in Serbia were 192.90 \(\pm\) 7.95 bpm, which is consistent with findings from other studies, such as 195 bpm in Greece [46], 193 bpm in Greece [47], 192.9 bpm in Belgium [48], 191.3 bpm in Croatia [49], and an HRmax of 188 bpm in Greece [50]. The average values of heart rate at the anaerobic threshold (HR AT) were 167.20 \(\pm\) 8.02 bpm, which closely corresponds to the values of 169 bpm for Greek footballers [46], but they are slightly lower than the values estimated for Belgium (178.2 bpm) [48] and, especially, for Croatian footballers (182.96 bpm) [49]. However, these differences may be due to different assessment methodologies. Our resting heart rate (RHR) values, specifically HR 1\({}^{\prime}\) and HR 2\({}^{\prime}\), are 170.05 and 129.44, respectively, with HR 1 representing 88% of HRmax, which aligns with the results for Champions League footballers (91% of HRmax) [51] and Scottish players (90% of HRmax) [52]. The other RHR parameter (HR 2\({}^{\prime}\)) is 67% of HRmax; however, it is not possible to compare this data with recent studies as there are no studies that have examined this parameter.