If you're middle-aged and like to play sports, exercise or work in the yard, you could be damaging your knees leading to osteoarthritis, without even knowing it.

Researchers say that injuries were more common and more severe in those who exercised more. However but they say this speaks volumes about the importance of lower impact activities, like walking, swimming, cycling, and training on an elliptical machine.

Osteoarthritis one of the most common and costliest forms of arthritis. Another new study estimates caring for osteoarthritis patients in the U.S. costs $185.8 billion a year.