I have a theory on the shape of the seats. I don't think that the seats have a general lack of lumbar support, but they are narrow at the shoulder which forces you to sit/lean forward, albeit somewhat supported.

I'm 6'1" and only 165 pounds, but I have an athletic build. I have a narrow waist but somewhat broad shoulders and back. While I can get my back into the lower part of the seat (lumbar), my upper back (through the shoulders) is not touching the seats. I'm hoping as my seats break-in, the area at the shoulders will become more compliant.

Note: You cannot say the "seats suck" although, as you say, just for you. The seats were not designed for "bad backs".

I should call custom upholstery place and see if they can insert some foam in the lumbar area.

I do the same thing. I also keep the seat closer to vertical than I did with other cars. No discomfort at all, even on long drives.

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Not to beat a dead horse, but from my previous cars, s2k, impreza, g35, miata and various other non sporty sedans none of them lacked the lumbar support as the Alcantara seats on the BRZ. Even with a bad back I can ride my motorcycles, scooters, bikes, kayaks just fine... The seats and interior on the brz limited just plain suck i's not even worth a debate...

At 5'10" and 155lbs I can tell you the seats are tight around the lats. Not uncomfortable, but definitely hugging you. My main complaint is the seat rake and the fact that the seat raises from the back, effectively eliminating any rake.

Sorry to bring this topic back from the dead, but I'm a UK BRZ owner since November 2015. I have posted on a UK forum but no one there seems to have hit on this problem which I encountered several days into car ownership. This has proven to be one of the most uncomfortable cars I have driven (due to the back ache) and I think the main reason is that unlike most car seats, the BRZ has no lumbar support. Initially I was getting pains in my lower back and legs, but now the pain is mainly in my back. I've tried a couple of cushions and also a lumbar support with a small pump but I have not been happy with any of these solutions.

I have been experimenting with the seats, moving it forwards etc (which is not ideal as I'm 6 foot 4) and I see some of you on here have suggested declining more, which I'm now trying. I have a 500 mile round trip tomorrow and Wednesday and I think if I don't get any improvement in this trip I'm sadly done with the car.

Lumbar support
 

I had same problem and I solved this problem with buying LUMBAR SUPPORT PILLOWS . Our generation spends half of the life sitting. We have become careless about our posture and completely ignore the effects it can have on our body in the long run. A healthy posture is one where the spine is properly aligned. A posture, which offers optimum comfort, including improved concentration, even through long hours of work.

Sitting, they say, is a mechanical interaction between six important body parts: pelvis, vertebrae, muscles, discs between the vertebrae, and the skin. Hence, it becomes all the more important to check and improve our sitting posture. The act of sitting down from a standing position begins with a slightly forward lean of the body (to keep the body balanced) and a subsequent bending of the hips and knees. It is found that About 65% of the total change of angle takes place in the hip joint. If a chair provides you with the appropriate lumbar or pelvic support, there is lesser stress on the lower back muscles. Therefore, it becomes important for us to choose the right chair.

Best advice right there. Butt all the way to the back, steering wheel pulled closer towards you, and recline the chair a bit.

And put your left foot in and take your left foot out put your left foot in and you shake ita all about.