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The University of Southampton
Southampton Marine and Maritime Institute

Slow and steady vs fast and furious: investigating the energetics of underwater fly kick in swimming

Published: 21 May 2020
Bubble screen
Bubble screen helping capture behaviour of fluid during fly kick

It is an interesting question in performance swimming, especially in longer events, is for how long and with what intensity an underwater fly kick can be used? A more intense, mainly lower leg intense kick may well allow a faster speed but is it worth the energy cost compared to a more whole body wave motion?

Recent papers in Engineering Optimisation and Sports Engineering and Technology by lead author Dr Chris Phillips reported on work from his PhD studies which investigated the energetics of underwater fly kick in swimming.

This work carried out by the performance sport engineering laboratory used a combination of kinematic measurements of a world class swimmer, an applied fluid dynamics code using Lighthill’s method and a state-of-the-art muscoskeletal analysis code to use the measured motion, coupled with estimates of fluid loading to evaluate the likely muscle activity required and hence energy cost of two approaches to underwater fly kick. Optimisation methods were used to generalise the approach from a specific swimmer to a more general result.


Dynamic motion capture
Dynamic motion: Key point locations & corresponding element positions
Same position that allows estimates of muscle group engagement






So how should swimmers trade-off their techniques during a race? Watching the best, especially as they proceed through heats to final clearly shows that they understand that the trade-off is there to be made. But what is clear to all is that having a great underwater fly kick techniques that allows a swimmer to access both methods is a powerful tool to deploy in competition.

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