(Disclaimer: I am not a medical professional and this is for entertainment purposes only)
So I recently learned that I am autistic and hyper mobile. Hyper mobile means that my joints are more flexible than average. Autism and hyper mobility are commonly comorbid. Another factor that can affect people with autism is that autistic people miss social cues. So that can include watching how other people walk, sit, stand etc., and missing some of the details of these processes.
Anyways, when I was watching Downton Abbey, I was really impressed by how long the actresses sit up straight on the edges of their seats for. Here’s one example:

This isn’t the best example. The best example is when everyone is sitting in the living room, having a long conversation, sitting on the edges of their seats with their backs ramrod straight.
I would watch this and think, “If I had to do that I would be in so much pain!”
And you would think that I would know how to sit up straight because I’ve been in dance classes my whole life. I remember when I was five, we would all sit in a circle with our legs straight out in front of us and the teachers would show us to sit up straight instead of slouching back into a c-shaped spine.
But recently, after several months in physical therapy, I have come to understand that what I have always thought of as sitting up straight is incorrect. Here is a picture of what felt like sitting up straight to me (apologies for the sneeze face. It was the best screenshot I could pull out of the video I made about this. I’m not actually sneezing, just mid sentence).

And here is a picture of what sitting up straight actually looks like:

In the first way, I arch my back too far forward because I thought that whatever is the direct opposite of slouching must be sitting up straight.
In the second picture, I’m imagining pulling muscles inside of the middle of my lower torso toward the floor. I’m imagining pulling my sits bones away from my shoulders and creating as much space between the top and bottom of my torso as I can.
I’m the first version, it hurts, and rightly so because it is doing damage to my spine.
In the second, version, it feels like work, but it feels good, like I have muscles to hold onto and like I’m building strength.