## 4 Discussion  
This cross-sectional study aimed to compare the lactate output profile and aerobic fitness of professional soccer players to those of selected elite youth players. Additionally, the study aimed to determine the LT using four methods, examine their agreement, and conclude if LT detection methods can be used interchangeably in youth, as this was shown not to be possible in professional players [25]. The results demonstrated that professional players exhibited superior cardiovascular fitness compared to youth by performing significantly longer during incremental testing. This was also evident from the fact that 69.23% of professional players had a \(V\)O\({}_{2}\) max running velocity (v/O\({}_{2}\) max) of 18 km/h versus only 56.82% of the youth. Despite the superiority of the professional players in incremental testing, youth players exhibited a significantly higher \(V\)O\({}_{2}\) max. This finding aligns with evidence from other studies, suggesting that other factors might be more sensitive indicators of superior aerobic performance [16; 17], such as shifts in lactate accumulation at higher intensities. According to an extensive review by [PERSON] et al. (2021) [36], the mechanism that most likely regulates the relationship between exercise intensity and increases in muscle and blood lactate is the mitochondrial reticulum volume density. It is also well documented that following endurance training; mitochondrial plasticity is enhanced [37]. Thus, in this study, it is possible that professional players had higher adaptations to endurance training before the preseason period. Perhaps this may be due to their strict adherence to the targeted exercise maintenance programs during the transition period or the more competitive nature of their championship compared to youth leagues. Furthermore, the preseason vLT calculated using the four LT detection methods (Table 2) in this study is in agreement with the literature on both youth [35] and profes \begin{table}
\begin{tabular}{c c c} \hline \hline  & **LT Speed ICC (95\% CI)** \\ \hline  & **Youth (_n = 44_)** & **Pro (_n = 52_)** \\ \hline VI vs. Log–Log & 0.65 * (0.35–0.81) & 0.51 * (0.15–0.72) \\ VI vs. Dmax & \(-\)0.03 (\(-\)0.89–0.439) & 0.44 * (0.02–0.68) \\ Dmax vs. Log–Log & 0.39 (\(-\)0.12–0.67) & 0.57 * (0.26–0.76) \\ \hline \hline \end{tabular} Note.: * = \(p\) < 0.05.  
\end{table} 
Table 4: Analysis agreement between the visual inspection (VI), logarithmic (Log–Log) and maximum distance (Dmax) methods in pro and youth soccer players using Intraclass Correlation Coefficients (ICCs). \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Coefficient of Determination (_R_2)** \\ \hline  & **Youth (_n = 44_)** & **Pro (_n = 52_)** \\ \hline VI & 0.35 ** & 0.48 ** \\ Log–Log & 0.17 * & 0.04 \\ Dmax & 0.01 & 0.08 * \\ FBLA (4 mmol) & 0.27 ** & 0.23 ** \\ \hline \hline \end{tabular} Note.: * \(p\) < 0.05. ** \(p\) < 0.01.  
\end{table} 
Table 5: Predictiveness of running performance on the incremental test based on the various methods of lactate threshold (LT) determination. sional players [17; 38]. However, the LT thresholds were not significantly different between the professional players and the youth (Table 2), despite the lactate accumulation during each stage being slightly lower for professional players (Figure 1). Only the FBLA (4 mmol/L) LT method yielded a running velocity that was significantly higher in professional players than in youth. Interestingly, the FBLA (4 mmol/L) and VI running velocities were also better performance predictors of incremental testing (Table 5). The use of CLT as a predictor of running performance is contradictory in the literature, as studies have shown inconsistent results regarding the superiority of various LT detection methods. Specifically, some studies demonstrated that the Dmax method is more valid and reliable [39; 40] compared to FBLA (4 mmol/L), whereas others demonstrated the opposite [41]. In cyclists, the Dmax intensity threshold is indicative of maximal lactate steady-state (MLSS) intensity [42; 43].