Studies have shown that patellofemoral pain syndrome comprises up to 50% of all overuse injuries. The syndrome is caused by an irritation of the under-surface of the patella (knee cap) or in the tissues surrounding the knee.
The pain is usually a dull pain that seems to come from the deep knee. It is worst with deep knee bend-type sports or activities. Usually there is pain climbing stairs and after sitting for a prolonged period of time.
The patella is a moving part that glides up and down a groove in the femur (thigh bone) as you bend and straighten your knee. The pain is caused by compression of the patella and the femur which increases as you bend your knee.
Basically, the irritation of the patella causes an inflammation which causes the pain.