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.
Osteoarthritis – the most common form of arthritis – is a degenerative joint disease that causes pain, swelling and stiffness and affects 8m people in the UK.
It is more common in women, and the risk increases with age and weight.
Presenting the findings at the annual meeting of the Radiological Society of North America, the researchers said their study included people who had not reported any previous knee pain.
Based on a questionnaire designed to work out how much exercise they do, participants were split into low-, middle- and high-activity groups.
A typical high-activity individual would do several hours of walking, sports or other types of exercise per week, as well as gardening and other household chores.
They then underwent MRI scans of the knee, looking for tears, lesions and other abnormalities in the cartilage and ligaments.
The damage seen was associated solely with activity levels and was not age or gender specific, the researchers said.
And it also seemed to be linked to the type of exercise a person did, although the researchers said this needed to be looked at in other studies.