## 4 Discussion  
The aim of this study was to analyze the effects of a 7-week combined strength and power training on the body composition, strength, agility, and sprint of elite youth soccer players. The main findings of this work reported that seven weeks of combined strength training produced improvements in body composition, EHS, jump, and pull-up tests of both groups of players. In addition, the U14 team also showed improvements in the agility test. When the U14 and U16 teams were statistically compared, the younger players achieved greater enhancements in the agility and push-up tests. Strength and power are considered requisites for many important soccer actions such as tackling, jumping, change in direction, and kicking [44; 45]. In young footballers, interest in the development of these skills has increased considerably, as a higher ability would increase their chances of reaching the professional level. The combined method seemed to be effective in improving these physical skills in young football players. In this study, one strength session, accompanied by one power session, per week was combined with regular football training. A pre-test and a post-test were performed 7 weeks apart to measure the impact of this protocol on body composition, aerobic capacity, sprint, hamstring eccentric strength, agility, and jumping performance. Our results confirm those of previous studies, showing that there was a decrease in body fat percentage during the pre-season and when normal soccer training was combined with strength training [46; 47]. The decrease in body fat percentage could be associated with improved performance [48], although not all studies confirm this claim [47]. The possible association between a lower body fat percentage and a reduced risk of injury has also not been proven [49; 50]. There were no significant differences between groups in changes in body composition, despite the difference in exposure to official matches during the 7-week period. This agrees with the study by [PERSON] and [PERSON] [51], which showed no relationship between the evolution of body composition during the season and exposure time. Further studies are needed to better understand what impact body composition has on performance and injury risk in young football players. Figure 3: Differences in the total external load parameters (effect size (ES \(\pm\) 95%CI)) measured over the entire intervention period. # (number) Acc+Decaccelerations plus decelerations. During the seven weeks of training, there was an improvement in the aerobic capacity of both groups, as measured by the ISRT submaximal. Although strength training can improve running economy and performance in time trials [52], it was not possible to determine in this study to what extent it contributed to this improvement, given the absence of a control group. What can be stated is that concurrent endurance training (through football training) and strength training did not prevent adaptations of the aerobic system of these youth teams [53], although the presence of an interference effect cannot be excluded [54]. The eccentric strength of the hamstrings increased in both groups, with a higher percentage improvement, although was not statistically significant in the U14 group. This difference may be due to a higher trainability resulting from a lower initial level. This improvement was not associated with an enhancement of sprint test performance, even though this muscle group plays a key role in sprinting [55; 56]. Our results confirm those of [PERSON] et al. [57], who after 4 weeks of eccentric training (Nordic hamstring exercise (NHE) twice a week with between 10 and 18 repetitions) found no improvement in sprinting ability, despite an increase in eccentric hamstring strength. [PERSON] et al. [47] also showed no clear correlations between eccentric hamstring strength and sprint performance, or between changes in both after a strength training intervention. [PERSON] et al.