Hi...
Circulars make sense for anyone w/ any kind of muscle or pain issue. And all of us who knit/ crochet/ etc have that !!! I started using circulars yrs ago making Lopi sweaters + discovered they were great for larger items such as baby blankets + lap robes. Then came the realization that you could knit longer w/ less pain using them ( I knit A LOT).
When I was diagnosed w/ Fibromyalgia some yrs back...that was the end of ever knitting w/ straights. I occasionally use them as st holders or to stretch an edge straight while blocking...but that's it. If I make a scarf or dishcloth I'll use a pr of dpns w/ an elastic wrapped around 1 end of each.
And I agree...posture is SO important. As well as learning to actually relax your shoulders + neck while sitting "well". Knitting is supposed to be fun...not torture...so relax + have fun!!!
Take + Care + happy knitting to all......Andrea