Im figthing against rotulae chondromalacia / pathelar thenidinitis for the last couple of months. though I too use speedplay zero and I have also changed my fit to lower stack height after swapping to sidi ergo 3 speedplay specific in April, I believe the roots for my issues aren't caused by speedplay's float, but gotta admit this thread has made me wonder..
I started cycling last november, was training hard from Jan-March (on speedplay since january) and did some nice 60-70 mile rides with no pain after the new fit. one thing I remember we spotted on the retul fit was how my knees didnt rock at all during the pedal stroke (0% change in axis, which apparently was a good thing, as atested for the good shape of my stabilizers). then one day I joined some friends that have been cycling forever on an 80 mile ride with some very steep ascents, some (20-25% inclination) to the point I couldnt pedal forward anymore even putting all my power to the pedals. Clearly my muscles werent prepared for such efforts and getting back home that day was painful. the worse ascents I've ridden previously were around 13%, so half that...
to make it worse, I trained regularly that following week and on the next weekend went on a 90 mile ride which I couldnt even finish. from that day, my knees never recovered. I also tried ice, stretching and 1cm extra of saddle height, 10 days without cycling, to no results. Im now slowing down on training, havent climbed more than a few yards for the last weeks, am doing fisiotherapy and hitting the gym regularly, with emphasis on the legs. after only a couple of weeks I can feel some improvement, but honestly a real test would be to climb a few ascents.. I will perform an isocinetic test today, since my (second) orthopedist feels that muscle imbalances were the cause to the chondro - which shouldnt be a surprise after what Ive learned here, but would be nice to measure those imbalances.
Ill also consult with a second fisiotherapeust, who is also a cyclist, to ask about possible links on speedplay and my pain. and I have just started taking a supp of Glocosamine/MSM/chondroithin, which is said to only be helpful after 3 months on it..
p.s. for newcomers, I also found many related info in this topic in particular, among others:viewtopic.php?f=8&t=80445&hilit=chondromalacia