Q I had a knee replacement last March and can cycle without any problems but should I run? What exercise guidelines can you give people like me?

A Patients receiving early knee replacements were supposed to be grateful for relief of pain and stiffness. Walking was considered safe but surgeons hinted at disaster if you tried anything more vigorous.

However, patients have challenged this and shown it is safe to play golf, tennis and ski. Implants are lighter, stronger and the expertise to replace them when they are worn out is now commonplace.

So how much can you do? Will the implants wear out or break loose if you run? As anecdotal evidence filters through, guidelines have slowly relented and it seems tennis and golf - if not overly vigorous - are now considered safe.

But the shock-absorbent lining of the implants is still a form of polyurethane and will not withstand the same impact as your own cartilage once did. In other words, train for a marathon every year and your normal 15- to 20-year knee replacement might only last 5-10 years.

On balance, my advice would be to enjoy your running if you must, but stick to quality not quantity. It will be a risk and you may end up having knee replacements early, which is not always straightforward. So cross-train where possible and use good, cushioned runners and shock-absorbing insoles. Don't forget you are at a crossroads and no one knows for sure, yet, which is the right direction.

• Dr Nicholas Peirce is the ECB's chief medical officer and a lead physician for the English Institute of Sport. To ask a question, email osm@observer.co.uk