This study aimed to identify the optimal transition velocities between streamlined glide (SL), underwater dolphin kick (DK), and front crawl (FC) during high-speed swimming. Ten elite female swimmers performed front crawl and dolphin kick trials in a long-course pool, and repeated the same motions under tethered conditions in a water flume to measure net force. Power-law fitting was applied to model net force as a function of flow velocity across the three movement conditions. The results revealed distinct patterns in the scaling coefficients (A) and exponents (n), following the order: ASL>ADK>AFC and nSL<nDK<nFC. The fitted net force curves intersected at two velocities, USK-DK and UDK-FC, and for most swimmers, these intersection velocities exceeded their respective constant velocities, VDK_const and VFC_const. These intersection points indicate the optimal timing for initiating propulsive actions to minimise deceleration during the post-start and post-turn phases. A conceptual model was proposed to explain the phase transition strategy based on velocity-dependent net force profiles. Although individual variation was observed, most swimmers exhibited similar trends, supporting the applicability of the model. These findings provide a fluid mechanical perspective for optimising underwater strategies and may contribute to enhanced start and turn performance in competitive swimming.
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