I had a PT show me exactly the best way to sit and sew so I wouldn't hurt. I sew sitting rim rod straight. I never hunch my shoulders or bend forward toward the machine. I only bend my neck down but never jut my head forward. My machine is sitting low. Bring your hand up until your elbows are 90º to your upper arms, that's the height your sewing machine needs to be. About the height of a pull out keyboard on a computer desk. I tilt the machine forward if I'm sewing for longer then a couple of hours. I took the back off my office chair and that's what I sit on. It swivels and rolls easy, even on carpet. I don't hurt anymore even if I sew all day.
P.S. I go to PT for exercise instead of a gym. I feel so much better then the hard work out I was getting from a trainer. I have better balance and feel limber. The exercises from the PT three times a week has made a big difference in my quality of sleep and how great I feel every day.