Hi, being a skeptical MD (radiology), I'd like to have a physicist's view on a chiropractic mechanical device called the "Activator" (http://activator.com/, http://www.youtube.com/user/drcherok, http://www.youtube.com/watch?v=knM5dx2tFH0&feature=related), which in my view is a worthless and expensive scam. But the believers hold it in very high esteem. It's basic use is to find differences in leg length (really!) and where on the spine to correct that (indeed!) by adjusting which "misaligned" vertebra, with this gadget, that will deliver an exactly dosed mini-thrust by a little spring-loaded piston with a small amplitude. It is said to be "very gentle" (whereby they actually admit that ordinary chiropractic isn't so gentle, don't they?). If you look at the videos, the users invariably seem to place the Activator haphazardly even outside the patient's clothes, hand-held without support, so I can see no way how it could be exactly dosed at all. And even if it were, here is my question: How will the pressure wave from a couple of millimeters' thrust be propagated through the tissues towards the vertebrae? I guess it's a kind of cubic law of attenuation and scatter, as for sound. Anyways, in chiro-world, this action realigns the vertebra and corrects the leg length difference (as measured with flexed knees and shoes on... but don't laugh yet). The thing is, when you palpate your spine or neck and "feel" the various bony prominences of the vertebrae, you are actually still quite far from the vertebra itself; it's only a what-would-you-call-it? A palpation by proxy? (please give me a better term.) The most popular vertebra in chiro-world is Atlas, the first cervical vertebra (C1). You can feel it just below your skull. However, on MRI and CT scans I measured the distance from the skin surface to the actual vertebra C1, and it is about 3 (three) cm from all directions to the most prominent parts of Atlas! If, for the sake of reasoning, we suppose that it is possible to "adjust" a vertebra at all (which it isn't, but that's another discussion), how much remaining thrusting power of the delivered thrust would actually reach the bone at x cm from the body surface? /Thanks.