Effects of a six-month technical–tactical training program on jump performance and lower-limb muscular symmetry in young football players

Abstract

Explosive lower-limb power and inter-limb muscular symmetry are important factors in athletic performance and injury prevention in youth football. This study investigated the effects of a six-month technical–tactical training program on vertical jump height and lower-limb muscular symmetry in male football players aged 12–13 years. Twenty-three male youth football players (age: 12.71 ± 0.27 years) from the under-13 squad of Olympia Football Club (Sofia, Bulgaria) participated in two assessment sessions: pre-intervention (January 2025) and post-intervention (May 2025). Vertical jump height was assessed using the Squat Jump and Countermovement Jump tests with the OptoJump Next photoelectric timing system (Microgate, Italy). Unilateral lower-limb strength and inter-limb asymmetry were evaluated using the Kineo electromechanical training system (Globus, Italy). Asymmetry was calculated as the absolute percentage difference between the stronger and weaker limb. The Wilcoxon signed-rank test and Cohen's d effect size were used for the statistical analysis. No statistically significant changes were observed in Squat Jump (p = 0.847) or Countermovement Jump height (p = 0.851) during the intervention period, and both showed negligible effect sizes. In contrast, unilateral right-leg strength increased significantly (p = 0.002) with a large effect size, whereas left-leg strength showed a large effect size without reaching statistical significance. Mean inter-limb asymmetry increased substantially from 7.4% before the intervention to 13.2% after the intervention, exceeding the 10%–15% injury-risk threshold reported in the literature. One participant reached an asymmetry value of 54.4%. These findings suggest that a technical–tactical training program of this type is not sufficient to improve bilateral explosive power and may contribute to the development of clinically relevant inter-limb asymmetries. Dedicated unilateral strength training and systematic asymmetry monitoring are recommended in youth football conditioning programs, particularly during the critical developmental stage of early adolescence.