Ide, T. (2010). The effects of straight leg kick on race performance in the sprint butterfly. A paper presented at the XIth International Symposium for Biomechanics and Medicine in Swimming, Oslo, June 16-19, 2010.

Butterfly swimmers should keep their body as horizontal as possible during the propulsive phases of the arm stroke. Since 2006, Kohei Kawamoto has been coached to change his butterfly technique, employing the straight knee/leg kick, which should result in a more horizontal stroke. Straight knee/leg swimming is considered when the knee bend is 170-180 degrees. This case study compared Kawamoto's performances in 2009 to those of 2005. All analyses involved swimming 25 m butterfly from a push start with 15 m of underwater kicking. After 15 m, Kawamoto swam all out butterfly stroke until completing a full 25 m. Swimming velocity and stroke angles were analyzed, particularly the bending of the knee and upper body movements.

Velocity improved from 2.5m/sec to 2.7m/sec at the second kick phase. Distance per stroke improved from ~1.90 m to ~2.20 m. Upper body movement increased from 61% to 69%. In 2009, the use of straight leg kicking in strokes was 55% of the time. That was an increase from 39% in 2005.

Implication. While this is a case-study report, it shows the beneficial effect of diminishing knee bend in the kick in butterfly swimming. Smaller (straighter) kicks will force the body to be streamlined, reduce resistance, and orient the swimmer to function with arm-dominance (all good aspects of technique). Although not reported here, similar reductions in kicking size have produced a compensatory hyper-extension of the thoracic region of the spine (similar to what has emerged over the past seven years in breaststroke) which increases the upper body angle while reducing hip and knee angles.

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