Let’s examine the possibility. You don't work out or participate in sports. You're not a frontline worker. You still experience back and shoulder pain often. How can this be?
It all gets back to the idea of postural muscles. Sitting all day long can be very bad for your back - particularly if you have bad posture. It’s true that this primarily involves your lower back and core muscles.
However, bad posture can be a slippery slope. If poor posture makes you uncomfortable and you compensate for it by contorting yourself in your seat, or walking and moving in unnatural ways, it can throw off those muscle groups as well.
Of course, one solution - or one part of the solution - is correcting your posture. Doing stretches before and after long sitting spells can also help to loosen your back muscles so that they become less tight. Limiting how long you sit for by taking breaks to walk short distances - even if it’s just pacing around the room - can also help.
However, taking up some physical activity can do the most good. Strengthening and developing your back muscles can make them more resistant to injury, but having a more developed muscular system can also help to passively improve your posture so that these issues are less likely to become issues in the first place.