Associations Between Isokinetic Torque and External Load Metrics During Youth Soccer Matches

Physical fitness, especially strength, is key for football performance and injury prevention, but its role in match running remains unclear. This study examined correlations between knee flexor and extensor isokinetic torque (60°/s, 180°/s, 300°/s) and running performance in 20 U17 players (age 16 ± 0.5 years, height 1.78 ± 0.05 m, weight 71 ± 7.1 kg) across 16 matches, with GPS tracking (Apex, STATSports, Newry, Northern Ireland). Results varied by position. Central defenders showed a negative correlation between non-dominant knee flexor torque at 300°/s and high-speed running distance (r = -0.975, p = 0.025). Side defenders displayed positive correlations between dominant knee flexor torque at 300°/s and both total distance and moderate running (r = 0.885-0.976, p < 0.05), but negative correlations with maximum speed (r < -0.89, p < 0.05). Central midfielders had several negative associations between dominant knee flexor torque at 300°/s and accelerations or decelerations (r < -0.88, p < 0.05). Side midfielders and forwards showed positive correlations between torque at higher speeds (180°/s, 300°/s) and sprint distance (r ≥ 0.85, p < 0.05). Overall, associations differed by position, velocity, and performance variable, reflecting tactical demands and the limits of single-joint testing. However, in SMFs, high positive correlations were observed, highlighting the importance of strength and its necessity in the training process.