Neuromuscular strength, not sagittal-plane joint or modeled muscle-tendon kinematics, is associated with 50-m sprint performance in youth male sprinters.

INTRODUCTION: Maximum-velocity sprinting imposes extreme mechanical demands on the hamstring muscle-tendon unit, yet how neuromuscular function, sprint kinematics, and modeled muscle-tendon behavior jointly differentiate youth sprinters of varying performance levels remains poorly characterized. We investigated the associations among these factors and 50-m sprint performance in youth male sprinters. METHODS: Fifty-six athletes (14-20 years) were recruited; 46 with valid 50-m times weretertile-split into faster (n = 16, 5.97 ± 0.09 s), intermediate (n = 15, 6.30 ± 0.09 s), and slower (n = 15, 6.77 ± 0.27 s) groups. NordBord hamstring eccentric strength, ForceFrame isometric strength, isometric mid-thigh pull, standing long jump, and nine flexibility measures were assessed. In 43 athletes, two-dimensional high-speed videography with deep-learning pose estimation yielded 129 kinematic variables, and OpenSim/Pose2Sim modeling quantified 2,480 muscle-tendon kinematic parameters across 20 lower-limb muscle groups. Between-group comparisons used ANOVA or Kruskal-Wallis tests with Benjamini-Hochberg false discovery rate (FDR) correction; cross-validated penalized regression (LASSO/ridge) identified independent predictors, with age as a covariate in sensitivity analyses. RESULTS: All four 50-m segments showed complete three-group separation (all p < 0.001, η2 = 0.70-0.84). Hamstring bilateral peak eccentric force (η2 = 0.369), hip flexor isometric peak force (η2 = 0.316), and standing long jump (η2 = 0.417) showed large between-group differences. Because the faster group was significantly older, these associations were re-examined with chronological age (and an estimated maturity offset) as covariates and remained robust. In exploratory analysis, a larger popliteal angle (poorer hamstring extensibility) was associated with slower times but did not survive age adjustment. Penalized regression retained standing long jump, hip flexor isometric force, and hamstring eccentric force among the leading predictors of 50-m time (LASSO leave-one-out R2 = 0.44). Of 129 kinematic variables, only 10 (all spatiotemporal) reached FDR significance; none of the 44 joint angles, 16 angular velocities, 53 inter-segmental coordination measures, or 2,480 muscle-tendon parameters survived correction. DISCUSSION: These findings indicate that hip flexor and hamstring eccentric strength, rather than joint or muscle-tendon kinematics, are the key neuromuscular discriminators of youth sprint performance, with differences at maximum velocity expressed through temporal force application during ground contact rather than joint- or muscle-level motion.