I always sit like this ("like a bird on a roost" in the words of my grandmother), and this, apparently, leads to sickly scoliosis. This habit is formed due to the weak muscular corset of the back, as well as as a result of the close location of the vessels to the skin (many postures become uncomfortable). The solution is to tackle the causes (back muscles) first.
As far as I know, these postures 'bend' the vessels, which is harmful for people with varicose veins.
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