PURPOSE This study employed a markerless motion analysis system to categorize the block start of elite sprinters into a four-phase sequence and examined the relationship between the kinematic characteristics of each phase and first-step performance. METHODS Seven elite male sprinters (100 m personal best: 10.52 ± 0.14 s) participated in this study. Block start movements, from block clearance to first-step touchdown, were captured at 120 Hz. The collected video data were processed using Theia3D Markerless software. The block start motion was divided into four phases (P1–P4). For each phase, the following were analyzed: knee joint extension angular velocity in the sagittal plane, pelvic rotation angular velocity in the transverse plane, a coordination index, and a timing index. RESULTS Knee joint extension angular velocity differed significantly across the phases (F=41.11, p <.001), increasing rapidly after block clearance. Notably, the ability to maintain a high angular velocity through the initial flight phase (P3) was strongly positively correlated with normalized firststep length (r =0.872, p <.05). In contrast, the coordination index did not differ significantly across the phases. CONCLUSIONS During the block start, maximizing and maintaining knee extension angular velocity through P3 is a determinant of first-step performance. This phase-based analysis offers a scientific basis for training strategies focused on movement improvement in specific phases.
