Gabe Mirkin, M.D.
A study from Long Beach VA Hospital in California shows why runners are far more likely to suffer knee pain than cyclists. The most common long term injury in runners is a condition called runner's knee in which the back of the knee cap rubs against the front bottom of the femur, the long bone of the upper leg. When you run, you land on the outside bottom of your foot and roll inward toward the big toe side of your foot. This is called pronation which forces your lower leg to twist inward, while your knee cap turns outward, causing the back of the knee cap to rub against the front part of the long bone behind it and hurt.
This study shows that the amount of inner twisting of the lower leg during running is related to how straight your knee is. Bending your knee decreases inner twisting and rubbing of the knee cap against the bone behind it. So people with runner's knee can pedal a bicycle with their seats set lower than normal to prevent their knees from straightening, they can wear special inserts in their shoes that restrict pronation, and they can do special exercises that strengthen the vastus medialis muscle above the kneecap that pulls the knee cap inward when you run or pedal.