## 1 Introduction  
L-lactate, contrary to some old beliefs, is considered an important energy source for many tissues of the human body, such as the heart [1] and the brain [2]. This is due to the Monocardoxylate Transporters (MCTs) [3] that allow L-lactate to be absorbed by the cells of various tissues. Additionally, the metabolic process of L-lactate formation from the breakdown of glucose is an essential step in anaerobic metabolism and is mandatory for glycolysis sustainability [4]. L-lactate formation also occurs under aerobic conditions, but not due to mitochondrial hypoxia. In fact, mitochondrial activity is very high during maximal aerobic exercise, and L-lactate accumulation and, presumably, formation are high [5, 6, 7]. Under conditions in which the muscle metabolic rate increases, the glycolytic rate also increases, resulting in enhanced L-lactate release and extraction by the active muscles. While L-lactate release by the active muscles increases throughout the exercise phase [8], plasma L-lactate concentrations rise only during high-intensity exercise [2, 5]. During continued exercise with gradual increases in intensity, L-lactate will start building up in a curvilinear manner [9]. Soccer demands superior levels of physical fitness [10], and the aforementioned metabolic processes are favored during high-level professional matches [11]. In addition, player physicality is constantly evolving [12], and varying playing positions impose distinct metabolic demands on players [11]. Thus, particular interest is given to determining the inflection point at which L-lactate starts to build up more rapidly, referred to by many as the lactate threshold (LT) [13]. The particular breakpoint for the commencement of lactate accumulation has been the focus of interest for many researchers, and several methods have been used to determine and define it. Furthermore, exercise intensity at the LT inflection point has become the most important determinant of aerobic fitness [14,15] and is often used by coaches to evaluate athletes' training status or to establish training intensities. Additionally, running velocity at LT (vLT) and running economy (RE) are more sensitive than maximal oxygen consumption (\(\dot{V}\)O\({}_{2}\) max) in predicting aerobic performance [16] and are also indicative of soccer players' endurance characteristics and their playing standards [17]. The most commonly used methods for detecting LT are visual inspection (VI) [18] and log-to-log transformation [19]. The VI method defines the LT as the workload needed to cause lactate to increase exponentially, whereas the log-to-log transformation technique involves fitting two regression lines of best fit on the curvilinear graph, with the point of intersection denoted as the LT. Another method proposed was Dmax, where the lowest and highest lactate points on the curvilinear graph are connected with a linear line [20]. The longitudinal tangent on the curvilinear path placed at the furthest point between the straight line and the curvilinear curve is considered the LT. Other approaches have proposed a fixed value of lactate, defined as the fixed blood lactate accumulation (FBLA), with the most commonly accepted value being 4 mmol/L [21]. While lactate profiling in the laboratory is a common practice for professional soccer teams, youth players are mainly assessed using field-based tests. Field testing for aerobic power is associated with game performance [22] and has gained acceptance in evaluating aerobic fitness in youth soccer [23]. However, these tests have not been optimized [24] and lack sensitivity for predicting aerobic performance [16]. Moreover, comparisons and assessments of readiness to meet the playing standards of professional soccer players are difficult because lactate profiling is scarce in youth athletes.