BACKGROUND: Passive drag is a key contributor to resistive forces in swimming. Through adolescence, changes in body size, proportions, and streamlining ability may alter hydrodynamic resistance. This study examined the longitudinal development of passive drag (Dp), drag coefficient (Cd), and body surface area (BSA) in female and male adolescent swimmers. METHODS: Thirty-five competitive swimmers (23 girls, 12 boys; age at baseline: 11.2 ±0.4) participated in annual testing for five years. Passive drag was assessed by towing swimmers in a streamlined position using a 1080 Sprint device, and anthropometric measures were collected. Dp was calculated from the mean force over the central 10 m of each tow. Linear mixed models evaluated year-to-year development and sex differences. RESULTS: BSA increased significantly across Years 1-5, with the greatest growth occurring between Years 1-3. Passive drag also increased over time, though year-to-year changes were modest after the first year. Conversely, Cd decreased overall across adolescence, particularly for girls, indicating improvements in hydrodynamic efficiency. Significant sex differences were observed for all variables, with girls consistently presenting lower Dp, Cd, and BSA. No sex x year interactions emerged, indicating similar developmental trajectories across sexes. CONCLUSIONS: Growth-related increases in body size elevate passive drag, but swimmers concurrently improve streamline control, reducing Cd across adolescence. These findings highlight the interplay between maturation and technical development in shaping hydrodynamic performance.
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