The number of comments to your question is becoming quite large, so I thought I'd roll my comments up into an answer (of sorts).
You ask whether the seat postures could cause back pains, and whilst I have no specialist knowledge in this area, I'd have to say from a purely empirical viewpoint that the answer must be "no". A lot of people ride a lot of miles on a lot of days, without suffering long-term pain.
But it's not a blanket "no", there are caveats. For example, if you have a badly-fitting bike, this can quickly cause pain.
Plus don't forget, under the large umbrellas of "road bikes" or "mountain bikes", there are many different frame geometries which offer many levels of comfort.
For example with road bikes, at one extreme there are bikes designed to be ridden by professional racers, whose geometry will sacrifice comfort for efficiency/performance.
At the other extreme there are, for example, touring bikes which are designed with a more relaxed geometry, so as to be comfortable for riding all day, over several days, and carrying a load. There are a gozillion bikes in between, and the story is repeated for mountain bikes.
Example: two different road bikes (same manufacturer). The first (left/top?) photo shows a bike from their "road" [i.e. racing] range, the other from their "touring" range. You can see the different frame shapes involved. (The two bikes look pretty much to be on the same scale, certainly they both run 700 tyres.) But if you look at both the vertical and horizontal distances between the saddle and handlebar, hopefully you can envisage that the rider of the left bike would have their back more horizontal (efficient but uncomfortable), the rider of the right bike will have their back at maybe 45° (give or take), a far more comfortable position.