OK guys and girls just stop and think for a sec. Inverted coaster...you are sat on your bum, which is padded and designed to support you, distributing your weight through your spine. Now think Flying-Dutchman, you are laying on your front, only certain parts of you are in contact with the ride. If you imagine going into a loop in the "prone" position...
1) As you enter it, the tops of your shoulders are thrust into the harness, increasing the pressure on them and your collar bones, the top of your spine is compressed and your head is likely to be pulled down adding unbelieveable tensile & compressive forces on your neck.
2) As you start to head up in the air, these forces ripple down your body (like a wave on the ocean) canusing the bones in your spine to grate together with a high possability of popped/slipped discs.
3) You crest up into the apex of the loop, if you neck was pulled forward it will now be wrenched back and you'll likely give yourself a concussion. You spine is arched over but you either are pushed up into the harness or drop down into your seat (depending on velocity).
4) You pass over the top of the loop and the ride begins to accelerate again, not a problem here really.
5) You start your decent, again, your body weight is pressed down onto your shoulders against the restraints (covered that part already).
6) Vertically down/curve under. You come away from the seat and are once again pressed into the harness, your spine being pushed into your body by the G's and your head gets pulled forward again.
7) Bottoming out/exit the loop. The G forces pull your head down like someon hanging off it, again wrenching at your neck and spine, your velocity is high and the forces once again ripple throough your spine as they balance from the exit.
Now imagine all that happening to your body in a matter of seconds. The amount of change in the size and direction of force/pressure would be painful to say the least.
Well, that's how I understand it anyway (just watch AIR proove me wrong!).
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"'cos you know, it's strange! You stand in a library and go "AAAAARRRRGGGHHHH" and everyone just kinda stares at you, but if you do the same thing on a aeroplane everyone joins in!"