My day job is sorting out people’s backs, and I spent a fair bit of time working on my dad’s when he did high mileage in his NB.
Now it’s mine I have the same problem, and realise I also had it with my previous NB!
There seem to be three issues here:
lumbar support is non existent;
seat pad is too soft
lateral edges of seat do not keep legs in right position if you are a bit slimmer.
First leads to a rounded lower back- hip shape, stressing the lower back muscles.
Second puts the lumbar spine in the wrong part of the seat back.
Third causes legs to roll out sideways, which overworks a core muscle which will lead to lumbar back and / or groin pain, plus that slight stagger when you get out of the car and can’t straighten up for a few steps.
Good hydration (sipping 2 litres or so) of water a day will help with the latter, but you need to find what keeps your back / bum / legs in an L shape rather than a C shape to avoid the first two.
Happy hunting and I’m interested to hear what people find.
I’m thinking of either the perforated leather seats from another edition (leather giving less than fabric) or possibly getting these seats recovered with better seat side bolster foam and a proper shape.