Patellofemoral pain is usually the result of the kneecap not tracking smoothly through the groove in the underlying bone when the leg is being bent and straightened.
Patellofemoral pain syndrome usually involves pain in or around the kneecap. People tend to notice the pain particularly when going up or down stairs, squatting, kneeling or running. It may also be aggravated by long periods of sitting with the knees bent.
A combination of factors may result in the kneecap not tracking smoothly. Tightness of the outside thigh muscles (which pull the kneecap outwards) is often accompanied by weakness of the inner thigh muscle. This results in the kneecap being pulled towards the outside of the leg. Abnormalities of coordination among these various muscle groups may also contribute.
Often the pain develops as an overuse injury seen in long-distance runners or cyclists, or it may be initiated by a twisting injury to the knee.