## 1 Introduction
### Metabolic Demand in Football Players of Different National Leagues  
The corresponding values of blood lactate concentrations in Swedish football players were reported. After the first and second halves, First Division players showed blood lactate concentrations of 9.5 and 7.2 mmol/L, respectively. In comparison, Second Division players recorded values of 8.0 and 6.6 mmol/L, Third Division players had levels of 5.5 and 4.2 mmol/L, while Fourth Division players exhibited the lowest levels at 4.0 and 3.9 mmol/L [24]. The average values of blood lactate concentration in Danish football players of the First and Second Divisions during the first half of a competitive football match were 4.9 mmol/L, while the highest individual value was 10.3 mmol/L [27]. The average values of lactate concentration in the blood of the Italian First League players after a competitive football match were 6.3 \(\pm\) 2.4 mmol/L, while the highest individual value was 11.3 mmol/L [28]. These reports show that the percentage of lactate production in muscles is high during a football match [29]. However, at a work intensity of 60-70% VO2 max, the greatest reduction (16 to 11 mmol/L) of lactate in the blood was observed [24]. It is important to highlight that players with higher VO2 max levels may exhibit lower blood lactate concentrations due to improved recovery from high-intensity intermittent activity. This is attributed to better aerobic responses, more efficient lactate clearance, and enhanced phosphocreatine regeneration [30]. However, they may experience similar blood lactate levels when exercising at a higher absolute intensity compared to their less fit counterparts [30; 31]. Endurance performance is linked to the blood lactate response during sub-maximal continuous exercise, making lactate measurements valuable for assessing long-term exercise capabilities in players undergoing the same tests [32]. In a study of elite Danish football players, treadmill running tests revealed that full-backs and midfielders had higher oxygen uptake at a specific blood lactate concentration compared to central defenders and goalkeepers. Forwards, in line with their VO2 max measurements, showed intermediate values between those of full-backs and midfielders. When oxygen consumption for a given lactate concentration was compared relative to VO2 max, full-backs and midfielders also had slightly higher values. However, like the VO2 max findings, no significant differences were found between regular and non-regular first-team players regarding the relationship between submaximal treadmill speeds and blood lactate concentrations. This suggests that these measurements should be interpreted carefully when distinguishing between top-level players [24]. Despite extensive research across major European leagues, comprehensive physiological profiling of male professional players in the Serbian Super League remains limited. Previous studies have not sufficiently examined intra-league variability in cardiorespiratory and metabolic performance, nor seasonal physiological changes. One recent study by [PERSON] et al. [33] evaluated VO2 max and lactate thresholds in relation to match performance using regression models. However, it did not assess group-based physiological differences. It should also be noted that, to the authors' knowledge, no studies have examined the differences between top-ranked and mid-ranked teams, as most existing research compares top-ranked with lower-ranked teams [34; 35; 36; 37]. Additionally, there is a need for studies that investigate a wider range of cardiorespiratory and metabolic parameters in more detail than previous research [16; 17; 38; 39]. The present study aims to address this gap by conducting a comparative analysis of cardiorespiratory and metabolic parameters between football players from top- and mid-ranked teams in the Serbian Super League.