Relationships between quadriceps and hamstring strength and sprinting, change-of-direction, and single-leg jump performance in youth soccer players

Background There is limited knowledge about the relationships between concentric and eccentric lower-limb strength and sprint, change-of-direction (CoD), and forward or lateral jump performance in elite youth soccer players. Objective This study investigated the associations between knee strength and sprinting, CoD, and single-leg jump performance in youth soccer players (mean age = 16 years; n = 41). Methods Maximal isokinetic knee extension and flexion strength (60°/s and 180°/s) and Nordic hamstring strength were assessed and correlated with single-leg forward hop and side-bounce distance, 10 m and 30 m sprint times, and 90° and 180° CoD test times. Results Pearson correlations revealed task- and leg-specific relationships. Stronger knee extensors at 60°/s were associated with faster 90° CoD performance, whereas stronger knee flexors and higher flexion-to-extension ratios at 180°/s were linked to faster 10 m and 30 m sprints. When the right leg acted as the inside turning leg, greater knee flexor strength (180°/s) was related to slower 180° CoD performance but faster 10 m and 30 m sprint times. When the right leg acted as the outside leg: (i) higher flexion-to-extension ratios (60°/s) corresponded to slower 90° CoD performance; (ii) stronger knee flexors (180°/s) with slower 180° CoD; and (iii) higher strength ratios at 180°/s with slower 90° and 180° CoD performance. No significant associations were found for the left leg or single-leg jump tests. Conclusions These findings highlight the movement- and limb-specific nature of lower-limb strength contributions in youth soccer players. Overall, hamstring strength supported sprint performance, whereas quadriceps strength underpinned CoD ability.