expert advice chairs  

Herman Miller Aeron chair?
Herman Miller Aeron chair?

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I was impressed by the comfort of the inexpensive Kartell polypropylene shell chairs, intended for office use. . .I too am sensitive to proper lumbar support in seating (the orthopedic pillow added to my driver's seat has been a lifesaver). It may have been the Maiu range (see the first chair under "Habitat" chairs and tables on the site below) that I tried, several years ago.
http://www.kartell.it/

I sit on a Herman Miller...
I sit on a Herman Miller Aluminum Group chair (Eames, 1958) in my home-office - by far the most comfortable chair I have ever sat on. - And definitely one of the most handsome chairs on the market!

a little cliche'
But the Eames Dining Chairs (metal and wood) are always in the window of my local "healthy back" store. I'd say your best bet is to wander into your local vintage store (or Design Within Reach) and try out a few chairs.. see which fits you best.

sawback
Hans Wegner "Sawback" dining chairs are the most comfortable and supportive, in my opinion. I think current production model# is CH29.

Equa
I use herman miller equa chair with adjustable lumbar support.. GREAT, but i dont think i will be nice for dining chair

Kai Kristiansen chairs
Hi There, Maybe you should consider the Kai Kristiansen diningchairs as these have a flexible back support that tilts.
This allows you to sit back into the chair and have the chair back either vertical (I) or slide forward in the seat thus making the back rest tilt slightly.(/)
As that chairs have a small ledge as arm rests they should offer you support while seated but more importantly give you sturdy support when you stand up.
If your back trouble is too bak you should consider the STOKKE balans range maybe as those chairs are intentionally designed for back sufferers.
The Kai kristiansen chair though gives you some stylish danish design and collectability.
As the chairs
http://www.furnitureindex.dk

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