i have had numerous shoulder injuries from years of competitive sports and can easily suffer pain when i push to hard.
i sew about 3-4 hours a day 6 days a week and i've had this schedule for about 8 years.
i have my sewing platform high in relation to where my bottom sits in the chair. i also have a lengthy and wide sewing platform.
i believe by having this set up i don't have to hunch down and forward to see my work, my shoulders aren't over extended, and i'm able to have my forearms fully supported - thereby taking some of the pressure off the shoulders and back.