## 4 Discussion
### Comparisons by the Age Category  
It was previously indicated that in youth football players, the highest injury rate arises in U-15-16 and U-17-19 players, with notorious advances in chronological age [8], a trend previously associated with greater training loads [35], namely related to the ability to perform at higher intensities as players progress to later adolescence stages [56]. In addition, the U-15 and U-16 age categories have been associated with the highest burden and number of severe incidents [29; 35; 57; 58]; however, this is not consistently reported across studies [52; 59]. These age categories match with the periods of peak height velocity [60] and peak weight velocity [61], in which fast changes in the body of football players may temporarily compromise the structural capacity of body tissues to tolerate the mechanical demand derived from playing football [62].  
In our study, we found age category differences in injury incidence with the older football players, starting in U-14, evidencing a higher total number of injuries within the age category compared to the younger ones and, additionally, a higher individual number of injuries, clearly observed in the U-18 age category. We consider that these results reflect the demand of the training process in elite youth football academies in all age categories and increasing daily demand with the progression of age and competitive commitments, that have greater emphasis and regularity in U-16 and U-18. We should also note that the competitive density should be carefully monitored since most injuries found were most probably associated with fatigue, which may be associated with weeks in which, namely, the U-18 football players in elite youth football academies sometimes play three times in seven days (two times for the Championship on the weekends and in the middle of the week for the Cup or Youth Champions League).  
Some limitations in this study should be addressed. First, the training load outside the academy was not considered and the number of players in each age category was not the same. Secondly, information regarding the biological maturity of the players was not assessed, neither physical fitness nor anthropometric and morphological data, these might provide additional information about the injury risk. Thirdly, we did not compare the results from an elite youth football academy with other lower-level academies or players not engaged in football academy programs, so the results should not be generalized and considered for other level football players. Lastly, we did not consider the playing position and individual competitive involvement throughout the season, which could provide more information about fatigue mechanics, as well as the separation of thigh muscles in the analysis, namely quadriceps and hamstrings.  
Future research should consider the abovementioned limitations. In addition, combining data with match and training load data from a global navigation satellite system could provide very helpful information relating specific training and recovery tasks to fatigue mechanisms in the future, which could support football coaches regarding the possibility of preventing the risk of injury at an early stage. We also suggest, if possible, the severity and burden of injuries to be determined, and perform this evaluation considering more narrow age categories and both sexes regarding the relationship between intervention programs and injury occurrence, with an adequate pre-determined sample power.