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 < sub> 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 < sub> 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.