If immobilization was all you wanted there would be no need for fancy French crucifixion wood; just tie them to a tree and leave them. Of course, then the symbol of Christianity wouldn't be a cross, it would be a Tree
But the ability of the human body to survive even very dicey situations is quite extraordinary, as demonstrated by the infamous case of Aron Ralston who survived 127 hours pinned immobile in a Utah slot canyon and had to cut of his own arm with a knife to escape
https://www.outsideonline.com/1927626/trapped. But he's OK and got to have James Franco play him in the movie.
Which makes you wonder, how many people survived crucifixion. I presume the authorities left them up there for some set time and if nobody was moving anymore, took them down. With the medical technology of the day-no EKGs, EEGs, not even stethoscopes, it would have been very easy to think an unconscious person was dead and turn them over to their families. And it's not hard to imagine some subset of those who were merely unconscious waking up and getting on with life. Maybe that explains how Barb keeps coming back
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