As far as I can tell crappy mattresses and deciding to sleep on the floor instead, the springs don’t poke you when you sleep on the floor. There was probably other things like not being a very athletic child that worsened it. I distinctly remember in school staying in the class room sitting in a chair rather than going for break because my back hurt too much. Moving a cupboard while my Dad dragged me up the stairs really fucked it… Then my Mum giving me whip lash at the go kart track 3 years ago.
I can see how my comment makes it out that it was the gym causing back pain rather than the gym and lifting heavy things later in life helping my back not hurt every day.