I've had terrible posture for as long as I can remember. When I get really "in the zone" coding or making music, or whatever that fascinates me, the awareness of my body just seems to float away, and I have no idea what posture I'm in for hours at a time. It's not unusual for me to not notice until I get a bad cramp, or pins and needles because I've cut off my circulation.
I remember as well, many years ago as a child, I joined the Boys Brigade, and something about the way I held myself and moved really wound up the drill instructor, and the other Boys taunted me for it. I play drums and bass, so I know I could march "in time" - but there was something postural I was doing wrong that they never managed to describe to me (this was long before cell phones were common, so they couldn't use a video to show me).
I'm not sure that bad posture is an autistic thing particularly - but I think that not noticing a bad posture when you are doing it might be more common for autistic folks.
I'd encourage you to try and improve your posture if you can. I arrange my den so that I can use my PC standing up sometimes - I've found that really helps. Once you get to my age (45), years of bad posture can really take its toll - as I am now learning the hard way.
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