If you are doing it right, except in breaststroke, the legs are used only to keep your lower body up on the surface of the water. The legs don't do much for propulsion (again, with the exception of breaststroke). A pull-bouy can be used to keep the legs isolated, but even if you don't use one, the knees are not stressed at all.
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Cat 2 Track, Cat 3 Road.
"If you’re new enough [to racing] that you would ask such question, then i would hazard a guess that if you just made up a workout that sounded hard to do, and did it, you’d probably get faster." --
the tiniest sprinter