They've been saying this since the late 19th century. And it has some truth, but it's not common.
I've heard two pieces of advice I agree with:
1. Since the problem is crushing a blood vessel if you feel numbness often adjust your seat, adjust your position, or buy a new seat.
2. Don't sit for hours straight, give yourself occasional standing breaks. This will force you to reposition and give yourself a break.
Seats are supposed to fit to your butt. It's crucial they fit.
If you're having no discomfort (and don't be a tough guy about it) then you're probably not hurting yourself. Amazing how that works
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