Characterization of eccentric hamstring strength and inter-limb asymmetry profiles in national-level youth male soccer players

Eccentric hamstring strength and inter-limb asymmetry are commonly monitored in youth football; however, evidence comparing their magnitude and directional consistency across age categories remains limited. This study compared eccentric hamstring strength and inter-limb asymmetry magnitude between national-level U12 and U18 male footballers and assessed within-cohort agreement in stronger-limb classification. In this cross-sectional study, 63 national-level male youth footballers were included: 35 U12 players aged approximately 12.0–12.9 years and 28 U18 players aged approximately 17.3–18.9 years. Participants completed repeated maximal bilateral Nordic Hamstring Exercise trials. Absolute peak force (PF), body mass-normalized peak force (RPF), inter-limb asymmetry magnitude, and stronger-limb classification were assessed. Trial-to-trial reliability was evaluated using coefficients of variation and intraclass correlation coefficients. Between-cohort differences in PF and RPF were examined using independent-samples t-tests, whereas the Mann–Whitney U test was used for inter-limb asymmetry. Hedges’ g quantified between-cohort effect sizes, and Cohen’s kappa assessed agreement in stronger-limb classification across trials. The U18 cohort demonstrated greater absolute PF (383.1 ± 37.8 vs. 230.7 ± 46.6 N; g = 3.51, p < 0.001) and RPF (5.1 ± 0.5 vs. 4.5 ± 0.6 N·kg⁻¹; g = 1.01, p < 0.001) than the U12 cohort. Inter-limb asymmetry was greater in U12 players (4.5 ± 2.3%) than in U18 players (3.3 ± 2.8%; g = − 0.48, p = 0.036). The U12 cohort demonstrated substantial within-cohort agreement in stronger-limb classification (κ = 0.860, p < 0.001), whereas no evidence of agreement was observed in the U18 cohort (κ = −0.143, p = 0.450). National-level U18 male footballers exhibited greater absolute and body mass-normalized eccentric hamstring strength and lower mean inter-limb asymmetry than U12 players. The U12 cohort showed substantial within-cohort agreement in stronger-limb classification, whereas no evidence of agreement was observed in the U18 cohort. Strength magnitude, asymmetry magnitude, and asymmetry direction should therefore be interpreted as complementary monitoring indicators, preferably alongside repeated assessments rather than as interchangeable measures.