It has been a long standing myth amongst non-runners that running (specially long distance running e.g. marathons) can cause arthritis and damage the knees. Infact one of my friends who is an ergonomist and occupational therapist also said the same thing to me. She is an ardent cyclist and swears that pounding the knees on the roads is not her idea of exercise as its going to have a significant impact on her knees and leave her with early onset of osteoarthritis in the knee. She has even encouraged me to consider giving up running and take up cycling as an exercise instead as it is potentially less strenuous for the knees.
One can be forgiven for believing these contentions. It is easy to imagine ballistic forces moving through our knees when running and since the knees of long distance runners undergo these mechanical forces repetitively, it can be assumed that it would lead to degeneration and therefore knee osteoarthritis.