Runner’s knee, also known as patellofemoral syndrome, is the most common overuse injury among runners. It is most frequent in adolescents and young adults, and is more common in females. It often results from excessive running, particularly downhill, but may also occur in athletes who do a lot of knee bending, such as jumpers, skiers, cyclists, weight lifters, and soccer players. It is felt as a dull, aching pain around the front of the knee and deep to the patella. The pain may be felt when walking or running, going up or down stairs, kneeling or squatting, or after sitting with the knee bent for an extended period.

Patellofemoral syndrome may be initiated by a variety of causes, including individual variations in the shape and movement of the patella, a direct blow to the patella, or flat feet or improper shoes that cause excessive turning in or out of the feet or leg. These factors may cause in an imbalance in the muscle pull that acts on the patella, resulting in an abnormal tracking of the patella that allows it to deviate too far toward the lateral side of the patellar surface on the distal femur.

Because the hips are wider than the knee region, the femur has a diagonal orientation within the thigh, in contrast to the vertically oriented tibia of the leg. The Q-angle is a measure of how far the femur is angled laterally away from vertical. The Q-angle is normally 10–15 degrees, with females typically having a larger Q-angle due to their wider pelvis. During extension of the knee, the quadriceps femoris muscle pulls the patella both superiorly and laterally, with the lateral pull greater in women due to their large Q-angle. This makes women more vulnerable to developing patellofemoral syndrome than men. Normally, the large lip on the lateral side of the patellar surface of the femur compensates for the lateral pull on the patella, and thus helps to maintain its proper tracking.

However, if the pull produced by the medial and lateral sides of the quadriceps femoris muscle is not properly balanced, abnormal tracking of the patella toward the lateral side may occur. With continued use, this produces pain and could result in damage to the articulating surfaces of the patella and femur, and the possible future development of arthritis. Treatment generally involves stopping the activity that produces knee pain for a period of time, followed by a gradual resumption of activity. Proper strengthening of the quadriceps femoris muscle to correct for imbalances is also important to help prevent reoccurrence.

Pain around the kneecap is often a sign of overuse. In other words, the knee might have been exposed to too much or too frequent strain without being able to adapt to it fast enough. This is particularly common when doing sports such as jogging, mountaineering or intensive cycling. The condition is often referred to as “runner’s knee” because of this.

There are currently no treatments that directly target the cause of pain at the front of the knee. But good-quality studies have shown that regularly doing exercises to strengthen the thigh and hip muscles can help.

People who do a lot of sports and develop acute knee pain are also advised to take a break from sports or do less for a while so that their knee can recover.

The main risk factor for pain around the kneecap is overuse. Activities that put a lot of strain on the front of the knee include jogging, mountaineering, going up and down stairs, and doing squats. Cycling sports can also lead to knee pain – especially if you cycle on hills, mountains or use high gears.

Overuse is more likely to occur if the activity is done for too long or too often, and if the intensity is too great or increased too quickly. Research has shown that pain at the front of the knee often develops in beginners, competitive runners and long-distance runners.

In some people the front knee pain is work-related – for instance, because they spend too much time in a squatting position or doing heavy lifting.

Overuse of the knee can lead to small injuries around the kneecap – for instance, in the bands of tissue that hold it in place, in the bones and in the small nerves. It’s not clear why some people develop knee problems and others don’t. One factor that plays a role is how the kneecap moves along the trochlear groove in the joint. But specialists are also looking into other possible influences, including:

• Weak thigh and hip muscles

• Anatomical factors such as having one leg that is longer than the other

• Foot deformities

• Unusually short muscles

• Being knock-kneed or bow-legged

Although having a larger Q angle (the angle at which the upper leg bone meets the lower leg bone) is often thought to lead to knee pain, research so far hasn’t found a link between the Q angle and patellofemoral pain.

It’s still not clear how much of an influence the other factors have because past studies have produced conflicting results. So far, research has only suggested that having weak thigh muscles can increase the risk of knee pain.

In many cases knee pain is likely to be the result of a combination of various factors.

Pain at the front of the knee usually occurs just behind or next to the kneecap. Doctors then call it patellofemoral pain or patellofemoral pain syndrome (PFPS). “Patellofemoral” means that it affects the area between the kneecap (patella) and the thigh bone (femur).

The pain is often mild at first and then gets worse over time. It tends to be a dull, aching pain and is particularly noticeable when you put strain on the knee – especially when going up or down stairs or bending the knee a lot. The knee may also hurt and feel stiff after sitting for a long time.

A lot of people who have knee pain can feel or hear crackling, crunching or creaking when they move the affected knee. But this isn’t a reason for concern: There’s no link between knee sounds and pain or the function of the knee. Many people have “noisy knees” without any pain or other knee problems. One reason why joints sometimes make sounds is because small bubbles of gas form in them and you can hear the bubbles bursting.

People whose knees make sounds can – and should – still move. The knee joint can only stay healthy if it is moved enough. If no pressure is applied to it, the cartilage in the joint isn’t supplied with enough nutrients. The bones need to have strain put on them too, to stop them from becoming weaker.

Patellofemoral pain (runner's knee) is diagnosed based on the symptoms. In other words, if

• the knee hurts just behind or next to the kneecap,

• the pain occurs when you put stress on your knee, for instance when climbing stairs, jogging or sitting in a squatting position, and

• no other cause can be found.

The doctor first asks about the symptoms and feels the knee. Then she or he asks you to squat down. A different test involves slowly stepping off a step with the healthy leg. If that makes the pain worse, it could be patellofemoral pain. Some medical professionals also use special questionnaires to get a more accurate description of the symptoms.

If necessary, further knee tests are done to rule out other causes. These include things like a torn meniscus, osteoarthritis of the knee, and tendon-related problems. But osteoarthritis is very rare in people under the age of 40. Before the end of puberty, certain growth-related problems can cause knee pain too. These can also be ruled out by examining the knee.

Imaging techniques like x-rays, ultrasound scans or MRIs do not help to diagnose runner's knee. But it can make sense to use them if something else is thought to be causing the pain – such as a tendon overuse injury, osteoarthritis of the knee, or a bone fracture.

Good-quality studies have shown that acute pain can be relieved by resting the knee and – in the long term – regularly doing strengthening exercises.

• Resting the knee: People who do a lot of sports are advised to do less or stop for a while, to allow the knee to recover.

• Strengthening the thigh and hip muscles: Strengthening these groups of muscles takes some of the load off the knee joint. Regularly doing exercises can relieve the pain and improve the joint function.

Acute pain can also be treated with pain-relieving and inflammation-reducing gels or creams containing the drugs ibuprofen or diclofenac. They are applied to the knee 2 to 4 times a day. Gels and creams have fewer side effects than tablets with the same drug in them.

It is not a good idea to take painkillers just so you can do endurance sports again without pain. There are two main reasons for this: On the one hand, you need to rest the knee and, on the other, the medication reduces your kidney function. This can be more of a problem if you get hot, don’t drink enough, and overexert yourself because the kidney already has to work harder.

If your feet roll too far inward when you walk (overpronation), shoe inserts can be used too. Inserts probably help in some people, at least in the short term. But there aren't many good-quality studies on the effectiveness of shoe inserts.

Pain at the front of the knee is one of the most common knee problems. It mainly affects teenagers and physically active people.

The pain can develop differently over time. About half of those affected only have pain for a few weeks or manage to get it under control within a few months. In the other half, the pain lasts for several years or keeps coming back.