Background: In elite youth soccer, the interplay between the development of key physical qualities, change-of-direction (COD) performance, explosive strength, and flexibility, remains poorly understood. Conventional training often assumes transfer effects, yet empirical evidence on how these qualities co-occur within elite youth cohorts across a competitive season remains limited. We tested the hypothesis that flexibility and explosive strength would be positively associated with COD performance at end-season (T2). Methods: Forty-five elite male youth players (Serie A academy; U12-U14) were monitored across one competitive season. COD performance (T-Test completion time), flexibility (sit-and-reach), and explosive strength (standing broad jump, SBJ) were assessed at the season's start (T0; September 1, 2024) and end (T2; May 1, 2025). Primary analyses used paired t-tests with Cohen's d_z and 95% confidence intervals (CI). Pearson correlations (with 95% CI) were computed cross-sectionally at T2. Results: Significant season-long improvements were observed in flexibility [deltasit-and-reach = + 1.83 ± 1.42 cm; 95% CI (1.40, 2.26); p = 4.87 × 10-11; d_z = 1.29], explosive strength [SBJ delta = + 12.8 ± 3.8 cm; 95% CI (11.66, 13.94); p < 1 × 10-20; d_z = 3.37], and COD performance [deltaT-Test = -1.16 ± 0.87 s; 95% CI (-1.42, -0.90); p = 1.86 × 10-11; d_z = -1.33]. At T2, SBJ was moderately associated with COD performance [SBJ vs. T-Test: r = -0.41, p = 0.005; 95% CI (-0.63, -0.13)] and with flexibility [SBJ vs. sit-and-reach: r = 0.32, p = 0.032; 95% CI (0.03, 0.56)], whereas sit-and-reach was not significantly related to T-Test time (r = -0.15, p = 0.31). Conclusions: Elite youth players improved COD performance, flexibility, and horizontal power across the season. Cross-sectionally, higher SBJ performance co-occurred with faster COD performance at end-season, while sit-and-reach flexibility did not. These findings support multi-domain monitoring and targeted training prescriptions, while highlighting the need for maturation-informed designs to test longitudinal coupling of individual adaptations.
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