Strength and jumping performance in youth athletes: do sport specialization and age categories affect strength asymmetry?

Methods: A total of 181 young male athletes were recruited and categorized according to sport specialization (soccer or athletics) and age categories (U15, U17 or U19). Isokinetic strength was measured as peak muscle torque (PT), normalized to body mass, for the knee extensors (PT_KE) and knee flexors (PT_KF) during concentric muscle contraction at three angular velocities (60^°s^-1. 180^°s^-1. 300^°s^-1). Vertical jump performance was measured during a countermovement jump with arms fixed (CMJ) and a squat jump (SJ).

Results: Significantly higher values of bilateral asymmetry (BA) of PT_KF for angular velocity 60°s^-1 and 180°s^-1 were found in the athletics group compared to the soccer group in the U17 category (14.40% to 16.02% vs 9.07% to 10.45%). Significantly higher values of BA for angular velocity 300°s^-1 for both PT_KE and PT_KF were found in the U15 compared to U19 category. Significantly higher values of H:Q ratio at all angular velocity except for the non-dominant leg in the highest angular velocity in soccer compared to the athletes in the U17 category were found. Soccer players exhibited significantly higher values of PT_KF compared to those in athletics and jump height in the U17 category.

Conclusion: Soccer players displayed increased isokinetic strength and more balanced BA compared to the athletics group. Physiotherapists and strength coaches should focus on younger age groups, especially U15, due to the higher incidence of BA and lower relative strength regardless of specialization.