Knee pain has been apart of my life for as long as I could remember. Whether it was from sitting too long, lifting (probably improperly), or running with already tight hips, pain was there. I recently took at awesome backbending class with the ledgend, David Vendetti, at South Boston Yoga. We went through various hip and quad openers and noticed I could roll on and off my knee again. This hasn't been the case since I was like, 7 years old.
Anyway, it's just another reason why I love the Pilates system. If you experience knee pain and have tight hip-flexors or quad muscles, or both, ask yourself how balanced the front of your leg is versus the back. If you feel the front has more tension than the back, perhaps try single leg deadlifts, or hamstring curls in addition to stretching the front of the leg. Anything that can help balance it out.