A dull, aching pain behind the kneecap in the center of the knee that may be accompanied by popping sounds when climbing stairs or when rising after sitting; usually begins gradually; common among healthy, young athletes, especially females; may be caused by inflexibility of the thigh muscles supporting the knee joint, problems with knee or hip alignment, overdoing sports activities, or using improper sports training techniques or equipment.

Source: American Association of Orthopaedic Surgeons