Two physiotherapists told me I had patella tendonitis, but luckily, a 3rd guy diagnosed some grinding under the knee cap, and told me it was chondromalacia. My condition cleared up in 3 days (!!!) after he told me to do some straight leg kicking exercises with very light weights to tighten one (or both) the smaller, middle quad muslces (vastus intermedius and rectus femoris).
The physio told me that, when we are fatigued, and/or muslce imbalances develop over a period of time, the firing order of the quads is disrupted. Normally, during a quad contraction, the rectus femoris quickly pulls the patella up and out of the way from the end of the femur as the leg flexes. When the rectus femoris gets a bit slack, it doesn't pull the patella up quick enough, so it therefore starts grinding on the femur, causing the pain, etc.
I have this, I had my knee scoped to smooth out some of the edges. I'm not sure this didn't make it worse. The problem for me was finding an exercise to build the quad without aggravating the problem. Cycling seems to be doing this for me. I get a little sore after a long ride, but I find that every day walking around is actually improved since I started riding. I think the only thing you can do is try to build that inner quad up as much as possible, just be careful how you do so (as the above site suggests). That will prop up the patella and keep it from rubbing.
I originally got the condition from not rehabbing an ACL reconstruction properly. My quad became underdeveloped and over the years this wore the cartilage down to the bone in spots.
I've had this for years! It's the reason why I started cycling. I continue to do my straight leg exercise (leg lifts and step ups) 2-3 times per week. Stretching is very important. I build up my riding over time, ie, I don't go out and ride a 60 miler if I haven't ridden in several months. Be sure your saddle height is correct. In general pain is nominal but I can tell if I've overdone it.
A doctor told me that I will have periods with no pain and other times I will have pain and don't know why. That's pretty much the nature of the disease.
This winter I spent a lot of time painting and working on my house getting it ready to sell. I wasn't riding or lifting weights, maybe it was all of the stooping and such but my knees felt worse then than they do now with all my skiing and biking.
Also, spinning your gears, not mashing them keeps the stress off the knees.
l listen when my knees start telling me I'm overdoing it.
I also wear orthodics in all my shoes, including my cycling shoes. I had some made by a pedothorist
BTW, I rode 4,500 miles last year, did 4 bike tours, which included lots of hills in Maine, Virginia, and the hilly part of Indiana and didn't have many complaints.
If you haven't done so get to a good PT who understands sports (mine is a PT for the Cincinnati Reds and worked on Pete Rose, Jr.) and listen to what they have to tell you because it is something you don't "get over" unless you retire from exercising (I actually had a doctor tell me this about 10 yrs. ago).
Okay - From everything my sports med ortho tells me...you never "get over" this...it is a tear in the cart that is permanent. They can clean up the rough edges, etc, and you can do pt to help the knee track better but that is about it. It also depends on the grade. I have grade IV in my left and grade II in my right. (V means no cart. left I believe...but dont quote me.)
the doctor i talked to said you could potentially get over it, especially sincei caught it fairly early, but if i stopped the exercises, it would probablly come right back. He also said my body may cope with it a little better because i'm fairly young. My pseudo-coach and teamamte is a PT, so i am going to talk to him about making an appointment. I figured since he is an experienced cyclist, he will also be able to tell me bike specific adjustments to my setup, training, etc... Thanks to everyone for your replies so far.