Vertical Vortices Generated by Flutter Kicks Provide Propulsion and Posture Stability

This study visualized the water flow generated by flutter kicks in crawl swimming and analyzed how flutter kicks create propulsive force in the water and stabilize posture. The results confirmed that vertical vortices created by the left and right feet not only assist forward movement but also suppress body sway, scientifically demonstrating the role of flutter kicks.

To swim fast in water, not only arm movements but also leg movements play a significant role. Much research has been conducted on the "dolphin kick" used in butterfly swimming, and it is known that three-dimensional vortices generated by leg movements lead to propulsion. However, for the "flutter kick" used in crawl swimming, the mechanism by which propulsive force is generated in the water has been poorly understood due to the complex water flow created by alternating leg movements. Therefore, in this study, we investigated and compared the water flow generated by flutter kicks and dolphin kicks in detail using a motion capture system and particle image velocimetry.

The results revealed that flutter kicks, similar to dolphin kicks, generate three-dimensional vortices through leg movements, contributing to forward propulsion in the water. On the other hand, unlike dolphin kicks, flutter kicks involve alternating leg movements, causing vertical flows to occur simultaneously around the feet. However, overall, a strong "vertically downward flow" was observed, which was found to lift the body. Furthermore, a significant characteristic of flutter kicks is the formation of asymmetrical vortices, which generate a force that tends to rotate the body sideways, thus playing a role in stabilizing posture during swimming.

These research findings are the first to scientifically demonstrate the propulsion mechanism of flutter kicks through actual measurements of water flow, and are expected to lead to improvements in crawl swimming techniques and instruction methods.

Research Representatives:

Yusaku Nakazono, Project Assistant Professor (Encouragement), Faculty of Health and Sport Sciences, University of Tsukuba

Hirofumi Shimojo, Associate Professor, Department of Health and Sports Science, Faculty of Health Sciences, Niigata University of Health and Welfare

Jun Sakakibara, Professor, Department of Mechanical Engineering, Faculty of Science and Engineering, Meiji University

Research Background:

When a body moves in water, the surrounding water is pushed and flows, and the reaction force propels the body forward. This is a principle of fluid dynamics common to living creatures such as fish and dolphins, and human swimming also operates on the same mechanism. Therefore, in swimming research, it is important to investigate how arm and leg movements create water flow and generate propulsion. Commonly used aquatic propulsion techniques include the "dolphin kick" in butterfly swimming and the "flutter kick" in crawl swimming. Both involve upward and downward leg movements, but the dolphin kick involves moving both legs together simultaneously, while the flutter kick involves alternating left and right leg movements, which is a major difference. Previous research has shown that dolphin kicks allow for faster swimming and generate greater propulsion. Furthermore, for dolphin kicks, it has been thoroughly investigated that three-dimensional vortices form around the feet, providing forward propulsion (Shimojo et al., J. Biomechanics., 2019). However, for flutter kicks, because the left and right legs move in opposite directions, the water flows overlap complexly, and the vortex structure, contribution to propulsion, and involvement in posture stability have not been sufficiently understood. In particular, scientific evidence regarding the role of flutter kicks in crawl swimming has been lacking. Therefore, the purpose of this study was to clarify the propulsion mechanism and role in posture control of flutter kicks by three-dimensionally capturing the water flow generated by flutter kicks and comparing it with dolphin kicks.

Research Content and Results:

In this study, to simultaneously capture the leg movements of swimmers and the surrounding water flow, we combined an optical motion capture system (Note 1) that records leg movements in three dimensions in a circulating water channel with particle image velocimetry (PIV method: Particle Image Velocimetry) (Note 2), which visualizes the movement of fine particles dispersed in water with laser light to measure water flow velocity. This allowed us to simultaneously analyze the swimmer's kicking motion and the complex water flow generated by it. In this study, we measured while gradually changing the swimmer's position and virtually reconstructed the vortex structure formed around the feet in three dimensions (method for constructing a pseudo-three-dimensional flow field).

As a result, it was revealed that flutter kicks, similar to dolphin kicks, generate three-dimensional vortices through leg movements, contributing to forward propulsion. On the other hand, unlike dolphin kicks, flutter kicks involve alternating leg movements, causing vertical flows to occur simultaneously around the feet. However, overall, a strong "vertically downward flow" was observed, which was found to act as a force to lift the body. Furthermore, a significant characteristic of flutter kicks is the formation of asymmetrical vortices.