Age-Related Characteristics of Lower-Limb Muscular Balance in Bulgarian Male Football Players Aged 13 to over 19 Years

Muscle strength and bilateral asymmetry of the lower limbs are relevant to injury risk and performance in football, yet their age-related development in adolescent players outside elite Western European cohorts is insufficiently described. This study determined age-related differences in lower-limb strength and muscular balance in Bulgarian male football players. A total of 122 players aged 13 to over 19 years from three clubs were assessed at the end of the competitive season. Maximal concentric knee extension and flexion peak force of both legs was measured at 60°/s using a system with electronically controlled resistance (Kineo); relative strength (force per body mass) was used for the main between-age comparisons to control for growth. Age differences were analysed with ANOVA and Tukey tests (extension) and Kruskal–Wallis with DSCF comparisons (flexion) across seven age groups. Relative strength differed significantly with age (extension η2p = 0.28–0.36; flexion ε2 = 0.43–0.49), rising most up to 16 years and then plateauing. The concentric H/Q balance index ranged from 0.48 to 0.62. The findings provide a descriptive developmental profile relevant to individualised strength programming in youth football.