I had knee problems in both knees from my ballet days as well. I had my right knee dislocate 5 times in one afternoon of riding horses when I was 15. That was pretty much the end of my ballet days. I had surgery (lateral release) on both knees when I was an undergrad and it cured me. I have had no problems since. My knees will "talk" to me a bit when I start running, but by 5-10 minutes into it, they are fine. I haven't run into any problems on the bike. Strengthing the quads is not always the solution - that is what caused my problems. It's the other muscle on the inside of your leg (name escapes me at the moment). In women, the quad wraps across your leg above the knee. Over-strengthening of that will cause improper tracking. The jumping and other movements done in ballet accentuate this. Hence, the problem in women, especially dancers. When they did my lateral release, the doc said that he could see the patella pop back to where it was supposed to be.