Dolphin-kick swimming maximizes water-flow utilization with increasing speed, researchers find

Researchers from University of Tsukuba investigated the propulsion mechanism by visualizing water flow around a swimmer during underwater dolphin-kick swimming in a water channel via particle image velocimetry. Their findings revealed that swimmers can utilize water flow and vortexes more effectively as their speed increases. The research is published in the Journal of Biomechanics.

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. The 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.