Dolphin-kick swimming maximizes water-flow utilization with increasing speed

The swimming motion imparts momentum to water, a fluid, thereby generating a propulsive force. Thus, we can understand the propulsion mechanism by examining the water flow generated by a swimmer's motion. However, observing colorless, transparent water with the naked eye or a camera is challenging. To address this issue, researchers employed particle image velocimetry, a technique utilized in fluid dynamics, to visualize water-flow patterns. They investigated how water flow changes as swimmers change their speed while executing the dolphin-kick swimming technique. This investigation was conducted in an experimental circulating-water channel (a pool with flowing water).

The results revealed that the water-flow velocity increased with increasing swimming speed during the underwater dolphin-kick lower-limb action, generating a strong vortex during the kicking action. This phenomenon possibly contributes to the increased propulsive force. Additionally, recycling of the flow generated during the downward-kick phase was observed during the transition to the upward-kick phase, with the effect becoming more pronounced as the swimming speed increased. This study marks the first observation of water-flow changes during dolphin-kick swimming at varying speeds.

This study is expected to advance research on water flow, a critical topic in swimming research. It offers scientific evidence for instructors to adopt kick-swimming techniques.

This work was supported by Japan Science and Technology Agency, Grant Number 22 K19725.