Middle-aged men and women may be risking arthritis if they overdo their exercise regime, research suggests.

A US study of more than 200 people aged 45 to 55 and of "normal" weight found those doing the most exercise were the most likely to suffer knee damage.

Running and jumping may also do more damage to cartilage and ligaments than swimming and cycling, researchers said.

One arthritis charity said it was important to keep fit and most people would not have any problems.