Do invest in a decent quality adjustable chair, preferable one with a forward tilt motion so that the back will come forward to support you when you lean forward to sew. Your leg should be flat from your hip to your knee. THen , if your feet don't reach the floor, use a foot prop of some sort (any kind of box or even books work just fine). It is also very important to make sure you have your arm/wrists supported properly when you are sewing and that the machine is the right height for your neck.
Can you tell that I used to sell office furniture for a living and we dealt with ergonomic issues every day??? Myself, I use an adjustable chair with adjustable arms that swivel around out of the way if necessary...love it..from Haworth. (Pricey, but I got it when I was a rep for them)