Variations in foot vortex dynamics with swimming speed during kick swimming

Tsukuba, Japan—When humans kick swim through water, vortices form around their legs, generating the force that propels them forward. However, the mechanisms underlying variations in the structure of these vortices with swimming speed remain unclear. Thus, in this study, researchers analyzed swimmer movement using an optical motion capture system and investigated vortex structure changes with varying speeds. They employed particle image velocimetry to visualize water flow dynamics.

Their results revealed that during underwater undulatory swimming, the vortex structure in the down-kick-to-up-kick transition phase changed as swimming speed increased. Specifically, with rising swimming speed, the direction of the jet flow between the two vortices around the foot shifted to a more vertically downward orientation, a shift hypothesized to enhance forward propulsion during up-kicking.

Notably, this study is the first to observe changes in vortex structure at different swimming speeds during kick swimming. Beyond providing a scientific basis for athletes and instructors to refine underwater undulatory swimming techniques, these findings contribute to a broader understanding of human swimming movements.

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

_{Original Paper}

Title of original paper:
Variations in vortex structure with changes in swimming velocity during human underwater undulatory swimming

Journal:
Experiments in Fluids

DOI:
10.1007/s00348-025-03955-9

_{Correspondence}

Assistant Professor TSUNOKAWA, Takaaki
Institute of Health and Sport Sciences, University of Tsukuba

Associate Professor SHIMOJO, Hirofumi
Department of Health and Sports, Niigata University of Health and Welfare

Professor SAKAKIBARA, Jun
Department of Mechanical Engineering, School of Science and Technology, Meiji University

_{Related Link}

Institute of Health and Sport Sciences