Now I've had a chance to dig through my files and I have found a few pieces of information that might help.
One of the references you might to look up if you are near a library is:
Mookerjee, S., Bibi, K.W., Kenney, G.A., and Cohen, L., Relationship between isokinetic strength, felxibility, and flutter kicking speed in female collegiate swimmers. Journal of Strength and Conditioning Research, 9(2), 71-74 (1995).
A discussion of this topic by Professor Brent Rushall of San Diego State University was available on the web but the link I had is broken and I can't find the new version so I shall summarise some of his points.
(1) Crawl stroke kicks do not propel (See ref above for details)
- Analyses of champion swimmers' strokes show that the only time the lower leg and foot is making a motion that could possible add to forward propulsion, the lower leg and foot are out of the water.
- The surface of water is very turbulent due to the movement of the body of the swimmer through the water. Therefore very little forward force could be generated.
(2) What do crawl stroke kicks actually do?
- the kicking motion counterbalances vertical forces during arm actions
- keep body streamlined
- the movement allows for a better body position in the setup phase of the stroke and therefore a more efficient action
- note it would be possible to use an action that did generate propulsion from the kick however the time taken to perform that action is prohibitive and outweighed by increased drag factors
(3) What is the point of kicking drills?
- they facilitate recovery by providing aerobic activity in a non-fatigued part of the body, especially after hard or exhausting sets
- they provide a respite from face-in-the-water perspectives and so are inherently more entertaining
- when performed with others, they provide an opportunity for social interaction
- kicking drills might assist in kicking effectiveness but that is more likely to be gained through direct whole-stroke swimming
- kicking done with a kickboard has no close relationship to the type of kicking done during the full stroke action!
So now to answer your question about how do you get the propulsion when using a kick board, there a couple of answers that have been alluded to so far.
(1) slower speed and different posture means that the water behind a swimmer's body is less disturbed than during full-stroke swimming. This results in quite different fluid dynamics and can allow some propulsion.
(2) the nature of the kick is very different from that of full-stroke swimming so this different process can create some propulsion.
OK, your second set of questions dealt with (in a starting and superficial way), now on to your first question about swimming with a buoy between the legs.
Improper kicking creates more drag than it saves so stopping kicking can in fact lead to greater speeds, especially if the legs are supported by a buoy. The flotation allows the entire body to roll, reducing drag, and stops the legs moving to a position where they would create more drag. The flotation also provides some compensation for the vertical forces created during the arm stroke, as discussed above.
As to why this seems more effective for older swimmers, I can only guess. I suspect it could be because younger swimmers tend to have the ability to sustain better kicking patterns for longer periods so the legs are not creating so much drag in the first place. Hence, the improvement is not so great.
I hope this helps! As you can see, it's all far more complicated than we at first imagine.
Regards,
David
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David Harris
About.com Physics Guide
physics.guide@about.com
http://physics.about.com
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