Oh the dreaded 'W' position. A way of sitting that has scared parents, doctors and teachers for generations. Children have found it comfortable whilst the adults around them shift and squirm. What's so big about sitting W?
From what I understand, sitting W puts stress on the knees, ankles and hips and is typically easier for some children because it requires less trunk strength. It also provides more area to spread weight, making it easier for children with less body strength to sit up. At the same time, it forces the hips to swing out in an unnatural position. There have been cases of hip dislocation and other injury resulting from it, in fact I even had a cousin who's knee popped out after sitting in the W position all day at school.
I was a W sitter. I figured out how comfortable it was when I was about two or three years old. I only stopped when I was about ten years old after a doctor yelled at me to stop. It was the most comfortable for me. I didn't have to struggle for balance and my hips didn't hurt. My femurs (thighs) are inverted, and W sitting is often a symptom of it. No, it is not a result of sitting W style.
Sitting criss cross hurt a lot. My hips didn't stretch like that and I felt like I was sitting on my lower back. Ouch. Sometimes I tried pulling my knees up to my chest but that resulted in a painful spine. Sometimes I tried to sit to the side, but I didn't have the body strength to support myself up. And forget putting legs out straight: unless I was against the wall, I really couldn't stay like that for long. It really affected me in school when I was little- I had a hard time enjoying books, assemblies and games because it was painful unless I sat W. So, W was my only option.
I'm not saying you should let a child sit W or that you should correct it. I am also not saying that they have arthritis as well. I'm just saying I was a W sitter, and I had arthritis since age two.