I use a great big fitness ball in the sewing room. i sometimes DO sit on it while at the machine and find it very comfy. you don't notice but supposedly it requires use of core muscles that help tone that area. but you can kind of stretch and move around more on it. can also use it to lay across to stretch and kind of massage out those back muscles that tend to tense up. when i feel really energetic, i might add a few sit ups (using the ball). They come in sizes and mine is not the largest one. in order to keep elbows above the machine as described above, i'd need the largest size. they're good for lots of toning/strenghtening exercises apart from the uses i described. but for sitting, i like it very much. i have heard of others who do use the ball for this purpose. i do have an old-school office chair that is divine but 2 machines are out so i use the chair as well as the ball. one is recessed and the other is not. i agree the recessed machine is less fatiguing and thankfully, that's the one I use for FM.