Dr. Allan Wrigley shared some thoughts about underwater kicking. He believes that we need to re-think the premise behind underwater kicking off the wall. The underlying premise or reason for doing the kick is to maintain the speed of the push-off. The kick is not used to increase speed. Pushing off the wall is faster than kicking.
Dolphin or fish kicking should be used as it creates less drag than flutter kick. You lose your speed less. As well, under certain circumstances kicking on your side (fish kick) has been shown to be faster than kicking on your stomach. Here, Allan cites a French study (Why have swimmers neglected the "fish kick" technique?Collard, Luc; Auvray, Emmanuel; Bellaunay, Ivan, International Journal of Performance Analysis in Sport Nov2008, Vol. 8 Issue 3, p18) they did with their backstroke and butterfly specialists. Over 15m and at no deeper than 1m below the surface, the swimmers were significantly faster kicking on their side than on their stomach, despite the lack of training in this technique. Make sure that you kick only on your side or on your stomach not while you are transitioning from side to front or back.
Speed of kicking is paramount. First, reduce drag by having a smaller kick. The kick should remain the same size. Kicking speed can be measured through kick rate. Rein Haljund, the Estonian stroke specialist looks for a rate of 75 double kicks per minute. You can measure from the top of one kick through 2 complete kicks. This is an area where we can see a large improvement. Swimmers are often doing dolphin kick off the wall but the rate is not high enough.
Dr. Wrigley also suggests that on the downbeat of the kick don’t bend your knees till later and push the water back with the feet, not down. An analogy would be a soccer kick. He also believes we need to increase the amount of kicking in the streamline position. We should decrease the amount of board kick and use smaller boards when we do.