The kneecap (patella) lies over the top of the knee joint. When you straighten and bend your knee, the kneecap slides up and down over the joint. Irritation can occur between the kneecap and the knee joint, causing pain under or around the patella. This is known as patellofemoral pain syndrome (femoral means 'of the upper leg').
Patellofemoral pain mainly occurs in teenagers and young adults, and more often in girls than boys. The pain is worsened with
- sitting for long periods with bent knees
- kneeling down
- going up or down stairs
- cycling (against resistance)
The pain diminishes at rest and when the knee is straight. The knee may feel stiff or swollen. Most individuals have symptoms in only one knee, but it can be present in both.