Runner’s Knee – or Patellofemoral Pain Syndrome (PFPS) – has its name for one reason as it is an ailment most commonly found in runners. Stress from running may cause irritation where the kneecap (patella) rests on the thighbone and the resulting pain can be sharp and sudden or dull and chronic. The pain may disappear while you're running, only to return again after the workout.
Patellofemoral pain syndrome can affect one or both knees and is found in mostly younger, recreational runners and twice as many women as men, as women tend to have wider hips, resulting in a greater angling of the thighbone to the knee, which puts the kneecap under additional stress. Symptoms include tenderness behind or around the patella, usually toward its center. Pain may be felt toward the back of the knee, a sense of cracking or that the knee's giving out. Steps, hills, and uneven terrain can aggravate PFPS.
There is no single cause of runner's knee. The cause can either be biomechanical—the patella may be larger on the outside than it is on the inside, it may sit too high in the femoral groove, or it may dislocate easily. Worn cartilage in the knee joint reduces shock absorption. High-arched feet provide less cushioning, or flat feet or knees that turn in or out excessively can pull the patella sideways.
The cause can also be muscular in nature. Tight hamstring and calf muscles put additional pressure on the knee, and weak quadriceps muscles can cause the patella to track out of alignment. Just the repetitive force of a normal running stride alone can be enough to bring on pain.