Most of medical research will not benefit us, just our great-grandchildren. The final answer on osteoporosis and exercise is nowhere in sight. It all depends on the location of the fractures, and there is some evidence that bone density itself is not the defining factor that we can use to predict an individual's risk for a fracture. Even though it doesn't have the impact of walking, cycling may not be a total loss http://www.medpagetoday.com/PrimaryC...itness/tb/1326
I suggest that we keep up the calcium intake, take Rx if your M.D. recommends it, and try to walk the 8 to 10K steps/day that is recommended. After that there is little we can do at this point in time, but there is no reason to give up cycling, which is low impact for arthritic joints, and may strengthen the muscles that help protect us from hip fractures.