Grey-outs occur because the heart is unable to pump enough blood to the brain on its own. The likelihood of experiencing a grey-out on any given ride is mainly a function of the duration and intensity of the G-forces. The Gs on MF vary somewhat depending on where you sit, so that certainly has a lot to do with it. If, however, the G load is relatively constant from one ride to the next, that leaves only one other variable: the physiological state of the rider. Since grey-outs occur because blood pools in the lower extremeties, it stands to reason that a state of high anxiety (read high blood pressure & muscle tension) will TEND to discourage them.
It's a fact that consciously tightening the muscles in the stomach and legs during high-g loads will help prevent grey-outs. If one does this unconsciously (due to fear, elation, or experience with coasters), it will have the same effect.
That's my story and I'm sticking to it. :)
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Dave McWilliams