Seth Hosmer
Your bones, like all structures in the body, are dynamic. They respond to the conditions placed on them, and adapt accordingly. Bone density loss occurs in the general population when there is not adequate stress on the musculoskeletal system. There are also nutritional (e.g. Vitamin D deficiency) and disease states that contribute to bone density loss.
The stresses that most effectively maintain bone density are compressive and bending forces, though torsional forces can also contribute. Any activity that provides these types of forces of adequate magnitude will maintain bone density.
There are different types of bone in the body. Your femur, for example, is primarily cortical (compact) bone. This type of bone does not respond as quickly to loading (or lack of loading) as the other type of bone, which is cancellous (spongy) bone...found in places like the spine.
If you want to maintain your bone mass, you must have weight-bearing of adequate magnitude in the upper and lower body. Running etc would be good for leg bone mass, but does not help spinal bone mass. For that, you would have to do something like strength training.
Shameless plug - we offer Kettlebell classes at HPChiro that can help you maintain your bone mass and build strength and endurance. Kettlebells are an excellent way to strengthen your core and make you a "complete" athlete rather than just all heart lungs and legs.
Back to bone density (mass). Peak bone mass is built by your 20s. After that it is largely a race to stop bone loss as it is generally accepted that you can't exceed the peak bone mass that you built by your 20s. If you are over 30, that makes it even more important to maintain.
Bottom line - like most everything else in your body, use it or lose it. If you do not do dynamic weight bearing activity of a certain magnitude, your bone mass will gradually decline making you more susceptible to fractures. The most effective way to add dynamic and adequate-magnitude forces to your bone structure is strength training.
Seth Hosmer, DC, CSCS
HPChiro.com
blog: http://www.hpchiro.com/blog/