In my late 20s my back started hurting. Learning how to do pull-ups and a couple other body weight exercises fixed that completely. I've now installed several pull-up bars on the doors I pass through most during the day, lowering the barrier to do quick sets during the day. The pain is gone, my posture has improved and my back has never been stronger.

If you can't do a single pull-up yet, negatives are often recommended but I found them quite difficult and unpleasant. I made good experience with mostly doing closed stance wall push ups and only a few negatives until I managed to do the first real pull-up. Always keep a close eye on form as you progress and don't rush your pull-ups.

Additionally, I highly recommend to do daily shoulder stretching exercises using bands and dead hangs for spine decompression. Your back will thank you!

I really wish they would have taught these kind of high-strength, low-rep muscle-building exercises and why they're important in PE instead of just making everyone run around and jump in circles. We spend so much time in front of our screens and it's much harder to (re-)build muscle than to maintain it. If everyone would learn how to do these exercises with proper form in their early teens and do 30-40mins of workouts 1-2 times a week at school they would enter adulthood with a much stronger and healthier body and should be able to keep it that way with minimal amounts of exercise.