Every doctor I've ever seen about back pain has specifically said do _not_ use an exercise ball as a chair, especially without lumbar support. n ~= 4, so it's a

Every doctor I've ever seen about back pain has specifically said do _not_ use an exercise ball as a chair, especially without lumbar support. n ~= 4, so it's anecdata, but just wanted to put it out there: this may not be the right solution for everyone.