I know the problem. The seats in my opinion are the weakest part of the Prius. I rented one for 3 days and almost decided not to get one because of that. It took several days for my neck and back to recover. The problem is that the seat back has no lumbar support. It looks like it does but it does not. The seat back sinks in after you have been sitting in it for awhile. It should be firmer to give proper support. Anyway I decided there must be a solution, and came up with a small lumbar support that works very well. I went through trying about three different ones. The others where either too bulky or too stiff or hard to adjust. The seat just needed a little help. Both my wife and I use one. I have had my Prius since May and I can say that with that lumbar support in place I have no back problems. I drive the Prius about 82 miles a day. Have 16k miles on it. We took it on vacation sitting in the car for about 11 hours and had absolutely no back problems. See the WEB link to see what it looks like. I did not get it there but from a local Relax the Back Store.