I had to keep still when thy did an MRT scan on my knee. I involuntary twitched and jerked all the time (unless I don't notice I'm still). The report later said something about objects on the scan that are naturally caused by moving the limb in question while scanning.
I already succeeded on making my arms and head twitch instead of my leg. But it was hard and not working 100%.
It's as if when somebody tells me to remain still my body suddenly decides to get all twitching and jerking and ticcing.
I can keep still though, when I don't pay attention to that I do it. I often stay entirely still like a real stature at the bus stop. As soon as I notice, I start twitching again.
So if I'm unaware that I'm motionless, I can be. If that has registered, I twitch and jerk. Pretty stupid and annoying.
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Autism + ADHD
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The trouble with having an open mind, of course, is that people will insist on coming along and trying to put things in it. Terry Pratchett