I am 20 and fit but my left knee is extremely painful after running. The discomfort can last for days. A previous injury to the same area healed after I took glucosamine sulphate but now even walking hurts. What could it be?
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The anatomy of this joint is extremely complex, as the knee needs to bend and twist while supporting heavy loads. The knee is therefore very sensitive to small problems in alignment, training, and overuse. Pain at the front and centre of the knee is frequent in active, healthy young individuals, and more common in girls than boys.

The term "anterior knee pain" encompasses a wide spectrum, from a totally benign condition where no actual pathology is encountered, to severe damage to the articular cartilage. In general, though, having anterior knee pain does not imply that you will damage your knee by continuing to run: your knee will just hurt more.

To help in making the diagnosis, one should know when your knee pain started, and greater details about your sports participation and training: were there any recent changes to the duration, frequency or intensity of your activities? Did you change your shoes or the surfaces you run on?

In general, though, pain at the front of the knee can have many causes: the articular cartilage may have been injured, the tendons that connect to the kneecap may have developed tendinopathy, the bubble of fat between the patellar tendon and the knee joint itself may be inflamed, one of the menisci (the fibrous cartilages that act as stress dampeners between the shin and thigh) may be torn.

The good news is that, at your age, the condition is more likely to be transient. You need assessment by an experienced health-care professional, with appropriate physiotherapy, and, possibly, activity modifications.

· Professor Maffulli is a consultant orthopaedic and sports injury surgeon at Keele University medical school. If you have a question for him, email fitness@theguardian.com