Short-Term Changes, Individual Variability, and Associations with Baseline Physical Performance in Young Male Soccer...

Monitoring physical performance during the pre-season may help coaches and practitioners interpret short-term changes in young soccer players, although group-level analyses may mask individual variability. This observational longitudinal study analyzed short-term changes, individual variability, and associations between baseline performance and change magnitude in 50 under-17 male outfield soccer players during a 47-day pre-season. Physical performance was assessed before and after the pre-season using the squat jump (SJ), countermovement jump (CMJ), 20 m linear sprint (Sprint-20m), 20 m change-of-direction test (COD-20m), and Yo-Yo Intermittent Recovery Test Level 1 (Yo-Yo IRL1). Significant improvements were observed in SJ (+1.43 cm, p < 0.001), CMJ (+1.37 cm, p < 0.001), and COD-20m (+0.31 km·h^-1, p < 0.001), whereas no statistically significant changes were detected in Sprint-20m (-0.14 km·h^-1, p = 0.121) or Yo-Yo IRL1 (-26.20 m, p = 0.721). Individual changes were heterogeneous, with 68.0% improving in SJ and COD-20m, 64.0% in CMJ, 40.0% in Sprint-20m, and 54.0% in Yo-Yo IRL1. Exploratory inverse associations were observed between baseline performance and change scores for SJ (ρ = -0.35, p = 0.014), CMJ (ρ = -0.40, p = 0.004), and COD-20m (ρ = -0.30, p = 0.033), whereas no statistically significant associations were detected for Sprint-20m or Yo-Yo IRL1. These findings support integrating group-level changes, individual variability, and baseline-performance context when interpreting short-term physical performance monitoring in youth soccer.