## 4 Discussion  
The aims of this study were to analyze the total muscle and hamstring injury incidences and burdens in different chronological age (i.e., U14, U16, U19 and senior) groups in an elite Spanish soccer academy during three consecutive seasons. Previous studies have analyzed injury incidences in male professional soccer players [11; 25]; however, very few have included elite male youth players, with a specific focus on muscle and hamstring injuries. Our results indicate that the incidence rate was greater in match play than training. Overall, the incidences of muscle injuries in elite young male soccer players increases with age, with the greatest injury burdens evident in the U16 age group. In addition, muscle tears accounted for the greatest percentage of injuries, and the most frequent anatomical site of injury is the hamstring. The biceps femoris was the most commonly injured muscle of the hamstring complex. FB, WM and FW suffered the greatest number of hamstring injuries, and seasonal variation indicated two peaks of incidence, specifically in August and October. Finally, soccer players were more susceptible to suffer an injury in the final period of each half of match play. Previous literature has analyzed muscle injury incidences in elite adult soccer [11; 14; 26], reporting that muscle injury incidence rates are high and relatively unchanged over an 11-year period [27]. However, the number of prospective studies exploring muscle injury incidences in young soccer players is limited [5]. Our results indicate that the injury incidence rate was significantly higher during  
Figure 3: Hamstring injuries distribution, attending to the time periods during matches. Figure 2: Hamstring injuries distribution, attending to the month of the year. matches (6.78 muscle injuries/1000 h of exposure) in comparison to training sessions (3.20 muscle injuries/1000 h of exposure). This finding agrees with those results reported by several authors [5; 10; 28] also in elite young soccer players. Despite this, the overall muscle injury incidences in our study (1.74 injuries/1000 h of exposure) was lower than the study conducted by [PERSON] [28] with elite young male Turkish players (6.71 muscle injuries/1000 h of exposure). These differences could be due to variations in the play style among geographic areas, which might influence the injury risk [29]. On the other hand, in our study, muscle injury incidences presented a linear increase with age. The greater frequency of injuries in senior players may also be due to heightened intensities of play, as well as their status as full-time academy players [5]. This increases their total weekly training load due to a greater number of training sessions per week performed by these players during the three seasons. Additionally, previous studies have observed that adolescent soccer players who are approaching professional-league levels of play are more susceptible to sustaining muscle injuries [9; 30]. This finding suggests that senior and U19 age groups should be targeted for screening and preventive strategies. Injury incidences have been frequently used as a quantitative parameter to analyze the impact of muscle injuries [31]. However, the consequences of an injury must be also considered in relation to its severity and the burden [21], to help understand the meaningfulness of the injury episodes. Moderate injuries were the most common injuries in all studied age groups, and this is similar to previous data in adult professional soccer players [11]. In addition, the U19 age group presented the highest incidence of major injuries (>28 days). A possible explanation could be that U19 players are able to continue with soccer activity despite the presence of pain and reduced function, which gives rise to increasing muscle damage [32]. Even though the highest injury incidence was observed in the senior age group, the U16s experienced the greatest burden of all the age groups studied. This is in general agreement with the recent study of [PERSON] et al.