I think a whole good back would be better than half a bad back. In all seriousness though they strongly advise against flying if you have back or shoulder issues.
The air pressure does cause a 'pushing' on your extrameties which causes arching.
From their web site: Folks with recent back, neck and heart problems should check with a doctor before flying. iFLY recommends people with prior shoulder dislocations DO NOT FLY!
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