Isometric knee strength and postural control show no robust baseline associations in young soccer players

To examine baseline relationships between isometric knee-flexion and knee-extension pad force, the protocol-specific flexion-to-extension force ratio, and force-platform-derived postural-control outcomes in academy soccer players aged 9–12 years, and secondarily to characterise exploratory within-player differences between two assessments conducted across a 5-day soccer camp. This single-centre observational study comprised a cross-sectional baseline component and a single-group repeated-session component. Data were prospectively collected, although study registration was retrospective. Seventy players were included (62 male and 8 female; age, 10.53 ± 1.24 years; maturity offset, − 3.78 ± 1.11 years). Knee-flexion and knee-extension pad force were assessed using a ForceFrame isometric dynamometer, whereas jump, landing, stabilisation, centre-of-pressure, and interlimb-asymmetry outcomes were obtained using ForceDecks force platforms. Baseline partial Spearman correlations were adjusted for age group, sex, baseline body mass, and age-group-centred maturity offset. Repeated-session differences were analysed using outcome-specific complete pairs, paired tests, mean differences, Cohen’s d z, bootstrap 95% confidence intervals, and sensitivity analyses. Benjamini–Hochberg false-discovery-rate correction was applied across 18 baseline associations and globally across 24 repeated-session outcomes. Exploratory analyses examined whether maturity offset was associated with T1–T0 differences. Baseline analysis samples comprised 62–64 players, and repeated-session complete-pair samples ranged from 60 to 65. Adjusted baseline coefficients ranged from − 0.194 to 0.327, and none remained statistically robust after multiplicity correction (all q ≥ 0.162). The largest estimate was observed between knee-extension pad force and single-leg-standing centre-of-pressure mean velocity (ρ = 0.327, 95% CI 0.059 to 0.557; p = 0.012; q = 0.162). Knee-extension maximum pad force was 28.86 N higher at T1 (95% CI 16.62 to 41.09), corresponding to 16.3% of the paired-sample T0 mean and d z = 0.60 (95% CI 0.33 to 0.81; p < 0.001; q < 0.001). This was the only repeated-session outcome that remained robust after correction across all 24 outcomes. Knee-flexion pad force, the flexion-to-extension force ratio, stabilisation time, and centre-of-pressure mean velocity showed unadjusted differences, but none remained robust after multiplicity correction. In exploratory analyses, age-group-centred maturity offset was associated with T1–T0 differences in bilateral land-and-hold time to stabilisation (ρ = 0.484, 95% CI 0.268 to 0.660; q = 0.002) and single-leg land-and-hold time to stabilisation (ρ = 0.380, 95% CI 0.124 to 0.585; q = 0.043). No baseline association between the selected knee-force and postural-control measures was statistically robust after adjustment for developmental and anthropometric covariates and correction for multiple testing. Knee-extension pad force was higher at the second assessment, but the uncontrolled

Design: prevents attribution of this difference to the soccer camp and does not exclude familiarisation, measurement variability, recovery status, or other time-dependent influences. Associations with anthropometrically estimated maturity offset indicated heterogeneity in repeated-session stabilisation-time differences but should not be interpreted as causal effects of maturation. Trial registration : Retrospective study registration ClinicalTrials.gov ID NCT06719752 registered 30 November 2024.