Net force analysis during streamlined glide, dolphin kick, and front crawl at high velocities in competitive swimming : implications for optimal propulsion timing.

Abstract

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 $\left(A\right)$ and exponents $\left(n\right)$, following the order: $A_{\mathrm{SL}}>A_{\mathrm{DK}}>A_{\mathrm{FC}}$ and $n_{\mathrm{SL}}<n_{\mathrm{DK}}<n_{\mathrm{FC}}$. The fitted net force curves intersected at two velocities, $U_{\mathrm{SL}-\mathrm{DK}}$ and $U_{\mathrm{DK}-\mathrm{FC}}$, and for most swimmers, these intersection velocities exceeded their respective constant velocities, $V_{\mathrm{DK}\_\mathrm{const}}$ and $V_{\mathrm{FC}\_\mathrm{const}}$. 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.