Middle-aged men and women may be risking arthritis if they overdo their exercise regime, research suggests.
Can exercise be too much of a good thing?
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.
Study leader Dr Christoph Stehling, a researcher at the University of California, San Francisco said: "Our data suggest that people with higher physical activity levels may be at greater risk for developing knee abnormalities and, thus, at higher risk for developing osteoarthritis.
"This study and previous studies by our group suggest that high impact, weight-bearing physical activity, such as running and jumping, may be worse for cartilage health.
"Conversely, low-impact activities, such as swimming and cycling, may protect diseased cartilage and prevent healthy cartilage from developing disease."
A spokeswoman for the Arthritis Research Campaign said that the gains of exercise far outweighed any potential risks.
"We have known for years that certain high impact sports and jobs are associated with an increased risk of osteoarthritis, particularly of the knee, but for the vast majority of people exercise is good, not only for the cartilage but for total body health.
"Most people can exercise without any problems, but if you have had a joint injury or torn cartilage or ligaments you should be cautious about weight-bearing exercise, and swimming and cycling may provide a better option for you."
Kate Llewelyn, of the charity Arthritis Care, said: "Osteoarthritis used to be considered wear and tear arthritis, but it's now thought that there are many more factors than age and use that contribute to its development.
"Many people wrongly assume that exercising when you already have arthritis is a no no, but in fact appropriate exercise is one of the best ways to help control pain, boost energy, keep you mobile and strengthen your joints."
I wonder if they controlled for previous activity, i.e. if all the high active people had been high active their whole lives, or became exercise fanatics later in lilfe. I've been an athlete since I was a teenager, and at age 46 have never had a knee problem in my life, though my mileage has often exceeded 50- miles per week. My husband is the same way. But I have friends who have taken up running 15-20 years later than I did, and they seem to struggle more with injuries.