## 4 Discussion
In addition, the complexity and close coherence in the synergistic muscle recruitment of the biceps femoris and the semitendinosus probably increase the injury risk [41]. Conversely, [PERSON] et al. [36] observed higher incidences in the semitendinosus and in the semimembranosus (0.04 injuries/1000 h) muscles together in comparison to the biceps femoris (0.06 injuries/1000 h) in adolescent elite soccer players. However, this information is biased, since these authors analyzed the semitendinosus and the semimembranosus injuries together and only included players between 13 and 16 years old. Playing positions that demand greater distances running at high intensity or sprinting (i.e., fullbacks, wide midfielders and forwards) present higher hamstring injury rates than other playing positions [1; 42]. For example, previous research has indicated that FB, WM and FW perform a total of 29-35 sprints in comparison with other positions (approx. 17-23 sprints) [43]. Thus, appropriate management of training loads according to the playing positions' physical demands could be an effective strategy to reduce the injury risk for this muscle complex [44]. Additionally, players must also be conditioned appropriately and be exposed to enough load to be able to tolerate the demands of their position [44]. Previous studies have shown different peaks of hamstring injuries across the season [25; 36]. [PERSON] et al. [5] reported that overall injuries also had two peaks in youth players across a single season in September and January. These results partially agree with our study, in which seasonal variation of hamstring injury incidences showed two peaks: (1) the pre-season (August), which may be due to the accumulation of fatigue following intensive periods of soccer-specific training sessions [45] and (2) after the start of the in-season period (October), which is likely influenced by the physical stress derived from official matches, increasing the injury risk in early sport-specialized young players [46]. In this sense, the application of shorter but more frequent restitution periods could help players to rest and recuperate while avoiding a rapid decline in their conditioning levels, reducing injury risks due to a more gradual accumulation of the required training volume [47]. Similar to previous studies [9; 25], hamstring injuries occurred more frequently toward the end of each half of match play (i.e., 30-45 min and 75-90 min), and this might be due to fatigue-related effects. In this sense, [PERSON] et al. [48] observed that professional soccer players experienced a reduction in the eccentric strength of the hamstrings as a soccer match progresses, increasing the injury risk in this muscle complex [49]. In addition, [PERSON] et al. [41] postulated that, under fatigue conditions, the biceps femoris have to partly compensate for the lack of endurance capacity of the semitendinosus, increasing the hamstring injury risk. This finding supports the importance of building fatigue tolerance in young soccer players through targeted training and build-up of high chronic training loads, as well as to periodize preventive programs in fatigue situations, mainly based on strength training [50]. One of the limitations of the current study was that it was not performed by maturation stage but by chronological age group only. Thus, an assumption was made that players were no longer circa peak height velocity, which may have influenced the results obtained in this study. Secondly, availability in matches and training sessions were not recorded during the experimental period. Despite this, our study presents several strengths. First, it is a study with a large sample (227 unique elite male young soccer players belonging to seven teams and grouped in four age groups) and conducted over a longitudinal time period (three consecutive seasons). Finally, future studies should consider quantifying internal and external workloads to examine their effects on injury incidence and more accurately identify and contextualize the injury risks in these age groups.