Researchers at University of Tsukuba used advanced techniques to visualize the water flow generated by flutter kicking during front-crawl swimming. They analyzed how this kicking motion generates propulsive force and contributes to body stabilization, demonstrating that the vertical vortices resulting from the alternating left and right leg movements not only impart forward propulsion but also suppress body sway. These results provide a fluid-dynamical explanation of the functional value of the flutter kick.

In competitive swimming, both upper-and lower-limb motions play important roles in propulsion. Extensive research has focused on the dolphin kick used in the butterfly stroke, revealing that this kicking technique generates three-dimensional vortex structures that contribute directly to propulsion. In contrast, the propulsion mechanism of the flutter kick used in the front crawl has remained poorly understood, largely because the alternating motion of the left and right legs induces complex flow patterns.

Therefore, in this study, published in Physics of Fluids, the researchers investigated the flow fields generated by the flutter kick by combining a motion-capture system with particle image velocimetry—an optical method for visualizing and measuring flow.