You can develop it if your foot rolls in, or pronates. Or if your calf or hamstring muscles are tight. Or your quads are weak. Or, as often happens with dancers, if the iliotibial bands on the side of the leg are too taut. Anything that exerts sideways pressure on the kneecap, pulling it out of its groove, will eventually stress the joint, wear down the smoothly gliding cartilage where bone meets bone, and trigger inflammation and pain.
Women are especially susceptible.
They may be the tougher sex, but they're weak-kneed. Women with achy, unstable knees outnumber men four to one, says Ostrowski. The wider hips that enable them to bear children also affect the knees. In men, the quad muscles run more or less straight up and down. When contracted, they exert force vertically.