Pre-Season Changes in Physical Performance and Early Competitive Squad Inclusion in Under-15 Male Soccer Players

Background: Pre-season physical monitoring may help coaches understand short-term changes in youth soccer players and examine whether physical indicators are associated with early squad inclusion. This study aimed to analyze changes in physical performance during the pre-season period and to examine whether pre- and post-assessment physical performance differed between squad-included and non-included under-15 male soccer players in the first five official matches of the main competition. Methods: Thirty-seven male outfield players were assessed using the Squat Jump (SJ), Countermovement Jump (CMJ), 20 m linear sprint (Sprint-20 m), 20 m change-in-direction test (COD-20m), and Yo-Yo Intermittent Recovery Test Level 1 (Yo-Yo IRL1). Squad-included players were those listed in at least one of the first five official match reports, whereas non-included players were not listed in these reports but remained part of the broader under-15 program. Results: Significant increases were observed in SJ (+1.44 cm, p = 0.007), CMJ (+1.36 cm, p = 0.006), and COD-20m (+0.38 km·h^-1, p = 0.002), whereas Sprint-20 m and Yo-Yo IRL1 did not change significantly. Fourteen players were squad-included and 23 were non-included. Squad-included players presented higher pre-assessment values for SJ, CMJ, COD-20m, and the exploratory composite physical score, and higher post-assessment values for SJ, CMJ, Sprint-20 m, COD-20m, and the exploratory composite physical score. The largest between-group difference was observed for post-assessment COD-20m (14.19 ± 0.49 vs. 13.40 ± 0.74 km·h^-1, p < 0.001, d = 1.25). Conclusions: Early squad inclusion was associated with higher absolute physical performance, particularly post-assessment COD-20m, whereas the magnitude of pre-to-post changes was not clearly associated with squad inclusion.