BACKGROUND: The swimming start is a key determinant of sprint performance, emerging from complex interactions between kinetic and kinematic variables. However, three-dimensional biomechanical assessment remains largely restricted to laboratory environments. This study aimed to examine the association between markerless motion capture-derived kinematic variables and time to 5 m (T5) during the ventral swim start in national-level competitive swimmers. METHODS: Sixteen swimmers performed five maximal starts under ecological training conditions. Kinematic data were obtained using a smartphone-based markerless system, and T5 was determined from the synchronized start signal to the 5 m reference line using high-speed video analysis. Linear mixed-effects models were applied to account for repeated trials nested within swimmers, and complementary Bayesian analyses were conducted at the individual level. RESULTS: At the trial level, moderate negative associations were observed between T5 and front ankle peak angular velocity (r = *0.39, p = .031), and between T5 and rear hip peak angular velocity (r = *0.33, p = .049). However, mixed-effects modelling indicated that inclusion of these variables did not improve model fit beyond the null model. A high intraclass correlation coefficient (ICC = 0.81) revealed that most T5 variability was explained by between-swimmer differences. At the individual level, Bayesian analysis provided credible evidence for a negative association between the sign-inverted front ankle angle at peak angular velocity and T5 (posterior mean r = *0.42; 95% CrI: *0.79 to *0.02). Because this recoded variable represented plantarflexion magnitude, greater front-ankle plantarflexion at the instant of peak angular velocity was associated with faster 5-m start performance. CONCLUSIONS: Ventral swim start performance over 5 m appears primarily athlete-specific and coordination-driven rather than explained by isolated joint kinematics. Markerless motion capture provides biomechanically coherent data in ecological settings, supporting its use for individualized performance analysis and coaching-oriented feedback
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