A stiff mid back is why your lower back hurts

Your lumbar spine has roughly 13° of rotation available across all five segments. Your thoracic spine has around 35°. When the mid back stops rotating — which it does, in almost everyone who sits — that demand doesn't disappear. It gets taken by the joints above and below, which weren't designed for it.

The joint-by-joint idea

Working up the body, joints alternate in what they primarily need: ankles want mobility, knees want stability, hips want mobility, lumbar spine wants stability, thoracic spine wants mobility, shoulders want stability. When one link fails to do its job, its neighbours compensate — and they compensate by doing the thing they're worst at.

A stiff thoracic spine therefore shows up as two very common complaints: an aching lower back, and shoulders that can't get overhead cleanly.

Screening it

Sit on a chair with your arms crossed over your chest and rotate as far as you can each way, keeping your hips still. Roughly 45° each direction is a reasonable target. Big side-to-side differences are common and worth noting.

Second check: the cat-camel. Watch for a flat segment that refuses to move while everything around it does. That flat spot is your target.

Restoring it

Open books for rotation, with the knees genuinely pinned so you can't cheat through the lumbar spine. Cat-camels for segmental control. Then bird dogs and suitcase carries to build the anti-rotation strength that keeps the lower back stable while the mid back does the moving.

Mobility without the stability work is temporary. You want the lumbar spine strong enough to stay put, so the thoracic spine is allowed to move.