Sex Differences in Three-Dimensional Intra-Cycle Velocity Fluctuation and Performance During Freestyle Swimming Among High-Level Swimmers

This study examined sex differences in three-dimensional intra-cycle velocity fluctuations (IVFs) of the body’s center of mass and the relationship between IVFs and swimming velocity (Vswim) in high-level swimmers during freestyle. Maximal-effort freestyle sprints performed by 13 high-level athletes (7 males and 6 females) were captured using a Qualisys three-dimensional motion capture system. Given the limited sample size, robust statistical approaches, including Welch’s t -tests, Bayesian analyses, and permutation tests, were applied. Male swimmers exhibited significantly higher Vswim ( p  < 0.001, ES = 2.94), forward IVF ( p  < 0.001, ES = 2.47), and vertical IVF ( p  = 0.022, ES = 1.57) compared with female swimmers. Bayesian analysis of interaction terms revealed no statistically significant moderating effect of sex on the relationship between IVF and Vswim. These findings indicate that although clear sex differences are evident in specific IVF components and swimming speed among high-level swimmers, the functional relationship between IVF and performance remains statistically consistent across sexes. From a practical perspective, IVF-based interventions should integrate sex-related physiological characteristics with individualized technical profiling to avoid overgeneralization. This study also establishes a high-fidelity methodological reference for three-dimensional kinematic analysis, providing a valuable dataset for future research and technological validation.