## 4 Discussion
The average values of VO2 max are 61.15 \(\pm\) 3.89 mL/kg/min, which are slightly lower than the values achieved by footballers from Spain (65.5 mL/kg/min) [53] and approximately align with professional soccer players from other countries such as Norway (63.7 mL/kg/min), England (61.6 mL/kg/min), [54], the United Kingdom (59.4 mL/kg/min) [55], the Czech Republic (59.2 mL/kg/min) [56], and slightly higher values compared to footballers from Greece (58.8 mL/kg/min) [47], Belgium (58.0 mL/kg/min) [48], Croatia (57.63 mL/kg/min) [49], and Cyprus (57.35 mL/kg/min) [57]. It is noteworthy that these values are consistent with the observation that professional soccer players' VO2 max can vary from 55 to 65 mL/kg/min [58; 59]. However, it should be noted that these values can vary based on playing position, the playing style of individuals, and the team. Additionally, some authors [60; 61] suggest that the VO2 max of elite players should be above 60 mL/kg/min to cope with the demands of modern football, which corresponds to the values achieved by Serbian football players in this study. Other cardiorespiratory parameters were not analyzed in other studies, making them difficult to compare. One significant finding of this research is that heart parameters, such as HRmax, HR 2\({}^{\prime}\), and HR AT, are associated with the total distance covered during a match. This relationship between heart parameters and total distance has a high degree of correlation (R2 = 54.3%), indicating that cardiac function is crucial for the endurance of football players. Unlike HRmax and HR AT, where a positive correlation was achieved, HR 2\({}^{\prime}\) showed a negative correlation, meaning that a lower heart rate during recovery is associated with a greater overall distance covered. This result may suggest that the speed of recovery after intensive effort is important for the efficiency of football players during matches. In other words, players with better results in these parameters possess a higher level of physical preparedness, enabling them to efficiently execute a greater number of high-intensity movements during the game and eliminate fatigue in shorter time intervals, preparing them for subsequent exertions. Furthermore, the research indicates that respiratory parameters, such as VO2 max/v, influence the overall distance covered. This emphasizes the importance of optimal respiratory efficiency for achieving better sports results. Concerning high-intensity running, the relationship between heart parameters and fast running has also been confirmed. HR 2\({}^{\prime}\) and HRmax made the most significant contribution to the model, highlighting the role of these two variables in achieving high speeds during matches. Recent studies have revealed a connection between cardiorespiratory parameters and MRP in football. Studies have shown that average VO2 max values are moderately correlated with the total distance covered [29] and high-intensity running in footballers [30; 31]. However, conflicting results were found in a study by [PERSON] et al. [32], who did not find a connection between VO2 max and MRP. In our study, although there was no direct link established between VO\({}_{2}\)max and MRP, a moderate connection was achieved between the VO\({}_{2}\)max model and MRP, with the greatest contribution coming from the VO\({}_{2}\)max/v parameter. This suggests that VO\({}_{2}\)max and HR should be further examined as they are linearly interconnected [62]. These results highlight the importance of the development of the cardiovascular and respiratory systems in professional footballers. The efficiency of the aerobic energy system, which provides about 90% of the energy during a football match, is crucial for achieving better cardiorespiratory endurance [7]. Adaptive changes, such as an increase in the dimensions of the left ventricle and improved circulation, contribute to an increase in the capacity of oxidative metabolism and overall cardiorespiratory endurance [63].