The breast stroke kick uses lots of knee extension and gets power from that whipping extension, vs the power in the frog kick being from adducting/clapping the feet together. Yes, many of you said that, but there was a short circuit between my brain and body mechanics. I was doing it totally backwards, trying to get power during what should have been the loading phase and resting during what should have been the power phase. Not only that, but I was wrongly trying to generate circular force from the knees instead of a more efficient power from the hips. Combine that with totally wrong foot and ankle position, and there's the recipe for knee injury.
I'm athletically declined, and still try too hard to muscle through skills instead of finesse. Just got home from almost 24 total hours' of GUE Primer with Garth McMurdie in UT, and finally understand the mechanics of the frog kick as it relates to my body. That feeling of water being squeezed between the fins when they are clapped together was a real "a-ha" moment. I still have a long way to go with that and the other skills, but something amazing happened.
After countless pool laps with a very patient instructor, I have zero pain in the knees. None. My back muscles, deltoids, and glutes are smoked from trying to learn a stable platform, but that's all a "good hurt." Even with several steel 72s sucked down over the pool hours, my knees are completely unbothered.
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Now, if I could just stop pointing my fin tips away from horizontal in a hover, it would keep me from rolling & corkscrewing out of plane. It was almost comical seeing myself on video - I got to the point of being able to predict how long it would take me to destabilize when a foot drifted. My mantra while watching the videos was: "There goes the fin tip. There goes the other leg to try to counterbalance, unhooking that fin. And roll starts...now!" There was slow but definite progress. My helicopter turns and back kicking still are way inefficient, and somehow were better when concentrating on other things like the Basic 5 or holding a 5' stop during a practice ascent. That fits with the whole "over-think and try to muscle it" issue.
I'm going to grab some JetFins, too, after being more stable with them. It was a surprising difference both in feel and appearance on video. At my very newbie level, their flatter shape helped with learning foot position. The gentle curve built into my Hollis F1s probably wouldn't bother anyone with better innate proprioception, but it was harder for me to learn how to hold a curved fin in a neutral position.