My son and I are just starting out cycling and he's been complaining of soreness in the back after a 20mile ride.
He's a shorter human at 5'3" and we got him a late 80's puch in a 48 to 49cm range. Top bar gives him 1.5in clearance and I've repositioned handlebar height so that he isnt riding too horizontal. The boy is fit and bikes to school daily, so me at 45 y.o and 3 herniated disc felt no pain. What other factors might cause back soreness?