Runner's knee (patellofemoral stress syndrome) is a condition in which the kneecap (patella) rubs roughly against the end of the thighbone (femur) when the knee moves.

The kneecap is a circular bone that is attached to ligaments and tendons around the knee. The kneecap normally moves up or down slightly without touching the thighbone during running.

Runner's Knee Normally, the kneecap (patella) moves up or down slightly without touching the thighbone (femur) during running. If the feet roll in excessively (pronation), the lower leg twists inward, pulling the kneecap inward, while the quadriceps muscles pull the kneecap outward. These opposing forces cause the back of the kneecap to rub against the end of the thighbone, resulting in injury and pain.

Runner's knee may be caused by a structural defect, such as a kneecap located too high or too low in the knee joint, off-center insertion of the muscles into the kneecap, tight hamstrings, tight Achilles tendons, and weak thigh muscles—which normally help stabilize the knee. Weak thigh muscles are the most common treatable cause of runner's knee; these weak muscles allow the kneecap to move sideways and rub against the thigh bone. A second common treatable cause is rolling of the feet onto the outside (pronation) excessively when walking or running while the front thigh muscles (quadriceps) pull the kneecap outward. Together, these forces cause the kneecap to rub against the end of the thighbone.

Pain and sometimes swelling usually start during running and are concentrated on the undersurface of the kneecap. At first, only running downhill is painful, but later any running and eventually even other leg movements, particularly walking down steps, are painful.

Refraining from running until it can be done without pain is important. Other exercises, such as riding a bicycle (if not painful), rowing, and swimming, can be continued to maintain physical fitness. Exercises to stretch the muscles in the back (hamstrings) and front (quadriceps) of the thigh and to strengthen the vastus medialis, an inner thigh muscle that pulls the kneecap inward, are helpful. Commercially available arch supports placed in both exercise and street shoes may help. Sometimes, shoe inserts have to be custom-made.

Strengthening the Vastus Medialis (Inner Thigh) Muscle Stand with both knees straight. Contract the quadriceps muscles (in the front of the thighs), raising the kneecaps. Hold this position for a count of 10, then relax. Repeat frequently throughout the day. Sit on the floor with both knees straight and the legs far apart. Rotate legs outward so that the toes point as far to the side as possible. Slowly raise the injured leg from the hip (with the knee straight), hold for 10 seconds 10 inches from the ground, and then lower it, keeping the knee straight. Do 3 sets of 10 every other day. Sit on the floor with two or more pillows under each knee so that it is flexed at a 135° angle. Place a 5-pound weight on the ankle. Slowly raise the foot by straightening the knee, then slowly lower the foot. Do 3 sets of 10. Progress by increasing the weight, not the number of repetitions.

Last full review/revision February 2003