Purring to maintain bone density!!!

I just read a fascinating article in the January 2009 edition of BBC Focus magazine. On P48 the question Do Lions Purr? was asked. The answer was yes, the reason given for all cats purring is fascinating. 

There is a suggestion that purring may have evolved as a compensation mechanism to maintain bone density and muscle tone despite the cats leading largely sedentary lives.

Ok, so this hasn't been proven and a quick google search was fruitless but the concept that either vibrations or sounds at a certain frequency could maintain bone density and mucle tone is fascinating. 

If true there may be no benefit to humans but it is known that exercise that helps improve bon density generally requires some impact. For example walking doesn't greatly improve bone desity but running can and jumping definitely can.

Just thought I'd log it here in case it's useful for the future.