Bone density: are muscles or gravity more important?

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Earlier this year, I wrote a series of posts about bone density (see here, here and here), as well as a Jockology column on the topic. I started out with the assumption that maintaining strong bones is all about weight-bearing activity (which clearly makes no sense for, say, your arm bones), but after talking to some researchers, I learned that it’s our muscles, in fact, that place the greatest stress on our bones (which explains why our arm bones don’t wither away). But there were still a lot of conflicting studies about what type of activity is most important for bone health — whether, for instance, the smooth motion of an elliptical trainer or bicycle can help as much as the jarring motion of running.

So I was interested to see in the latest issue of Medicine & Science in Sports & Exercise a series of papers debating this very question, under the heading “Muscle Forces or Gravity–What Predominates Mechanical Loading on Bone?” (Intro here.) Basically, the answer is that we don’t know, and researchers are still arguing about it. It seems that gravity was long assumed to be the key, dating back to studies of bed rest in the 1920s and studies astronauts in the 1960s and 1970s. But there was a big shift in the 1980s:

Since 1987 when Harold Frost first proposed the “mechanostat theory” and began to assert that “Bone strength and ‘mass’ normally adapt to the largest voluntary loads on bones. The loads come from muscles, not body weight,” the notion has increasingly pervaded the literature.

Now, apparently, there’s a bit of a backlash from researchers arguing that gravitational loading — and in particular, sudden jarring like you get from hopping — is more important than previously suspected. I’m afraid there isn’t a nice, neat take-home message here. The researchers still don’t know the optimal way to stimulate bone growth, but it looks increasingly as if both muscular strength and weight-bearing activity have a role to play. So for now, cover your bases by doing both!