People with patellofemoral pain syndrome (PFPS) often have knee pain and stiffness when climbing, going down stairs, or while squatting. Popping or stiffness after sitting with the knee bent is called the movie theatre sign.

Most of the symptoms of PFPS depend on the up and down movement of the kneecap and the pressure it places on the cartilage and bone underneath. Scientists aren't sure what causes the stiffness. Symptoms go away when the person avoids any of these activities.

Changes in the joint, ligaments, and joint capsule may occur after a long period of pain. Messages of pain and stiffness may be sent to the brain sooner or more often than in the normal knee.

A recent study at the University of Illinois found that subjects with PFPS may be misinterpreting their pain as stiffness. Future treatment of PFPS may be centered on pain relief in order to decrease the sensation of stiffness.