Runner's knee can result from a number of different causes. The most common is an overuse injury. As its name suggests, repetitive activities that use the knees, such as running, can cause this condition. Direct force or blunt trauma to the area can also cause runner's knee. Weak muscles around the knee can also be a factor in runner's knee.
The most common symptom of runner's knee is pain around or behind the kneecap. This pain may be accentuated when the knee is bent during walking, kneeling, squatting, running, or going down stairs or downhill. Sitting for long periods of time may also cause aching. In some cases, there may be swelling and even a popping or grinding sensation in the knee.