## 4 Discussion
However, the reliability of the Dmax method is challenged [44; 45], as the accuracy of this method depends on the first and last lactate measurements. Interestingly, our results demonstrated that the final lactate level was significantly higher in the youth group than in the professional group (Figure 1). This is because the youth challenged their anaerobic energy system contribution towards the end of the incremental testing before volitional fatigue. Presumably, youth consider incremental testing as a criterion for their professional development, aiming to achieve longer running times. This speculation is not supported by [PERSON] et al. (2018) [46], who demonstrated that elite youth soccer players have higher autonomous motivation, most likely resulting from their engagement in diverse developmental activities rather than being driven by external rewards related to pro status. It could also be due to their lack of experience with the pacing required for incremental testing, unlike the professional players who routinely perform the test throughout their careers. Alternatively, it could simply indicate the superiority of professional players in aerobic fitness, since [PERSON] et al. (2015) [47] demonstrated that peak lactate concentrations decrease after endurance training interventions. Thus, the lower value of the last stage in professional players could cause an underestimation of the vLT using the Dmax method [47]. Consecutively, in addition to other disadvantages associated with LT determination, such as the inability of the literature to establish a gold standard for laboratory LT determination [48], Dmax's low reliability [41], the LT underestimations of the log-to-log transformation method, and the experience dependence and subjectivity of the VI method [49], the youth's last value might have influenced the results as outliers could affect these estimation methods that rely on regression analyses [33]. There is limited research on the interchangeability of the various LT detection methods. Specifically, only four other studies have examined the agreement between the various LT detection methods. [PERSON] et al. (2007) [49] examined the agreement between three LT detection methods for cyclists of both sexes. Despite the strong correlations detected, the Bland-Altman analysis indicated that the wattages had a large LOA and thus lacked agreement. Similarly, [PERSON] et al. (2014) [50], in a larger study using 57 cyclists, indicated poor agreement between the LT concepts used in their study and the MLSS. Furthermore, [PERSON] et al. (2019) [44] examined the agreement of six different LT detection methods in elite cross-country skiers. Their results revealed that these six methods should not be used interchangeably. To our knowledge, only one similar study has examined LT agreement among professional soccer players [25]. Their running velocities at LT, detected with four different methods, were higher than our professional group, but this was expected as they collected the data towards the end of the season, during which the values [35] shifted at higher intensities. Similarly to the literature mentioned above, our results show a failure to meet the criteria for agreement as set for players before [25] and, thus, poor agreement among the LT detection methods investigated in this study (Figure 2A-F and Table 5). Furthermore, there is a paucity of evidence on which method is more accurate in determining vLT in both professional and youth soccer players. Thus, further research is required to determine the most appropriate LT concept, and follow-up validation studies are required to examine its accuracy. A major limitation of LT determination and its practicality is that soccer entails a large interplay of running velocities, many of which are above maximal aerobic speed. Additionally, high-intensity interval training formats are tailored based on the different locomotor profiles (speed, endurance, or hybrid profiles) that characterize soccer players [51]. On the same note, [PERSON] et al.