Lower-back pain after training sometimes comes from breathing, not the weight
When lower-back pain shows up after a strength workout, the usual suspects are too much weight or poor technique on the lift itself. But there's a less obvious factor: your breathing pattern during the set. Holding your breath and bracing your core (a controlled version of the Valsalva maneuver) creates intra-abdominal pressure that naturally stabilizes the spine during a heavy lift.
If someone instead breathes shallowly or exhales too early, right at the hardest part of the movement, core stability drops, and part of the load that intra-abdominal pressure was supposed to absorb gets shifted directly onto the structures of the lower back — the ligaments and spinal discs.
That doesn't mean you should hold your breath through an entire set — in long, high-rep sets that's unsafe for blood pressure. But in heavy compound lifts done for 1–5 reps, a short, controlled breath-hold through the hardest part of the movement isn't a bad habit — it's a technique, and skipping it may be part of why your lower back hurts after training even with a reasonable weight.