Low-impact exercise such as using an indoor rowing machine or swimming could help prevent osteoarthritis, according to new research.

Middle-aged men and women who do a lot of exercise could be damaging their knees, the University of California, San Francisco study shows.

But some activities are worse than others, with some said to offer benefits for the joints.

"Low-impact activities, such as swimming and cycling, may protect diseased cartilage and prevent healthy cartilage from developing disease," said Dr Christoph Stehling of the university.

Osteoarthritis affects 8.5 million people in the UK, according to the NHS and mostly occurs in the knees, hips and small joints of the hands.

It usually develops n over 50s and is more common in women, although it is not an inevitable part of getting old.

Symptoms vary and exercising and wearing suitable footwear can help ease the condition, although there is no cure.