What causes lower back pain and when is it serious?

The overwhelming majority is mechanical: muscles, ligaments and discs objecting to load, posture marathons or sudden effort, and it improves within weeks with movement, not bed rest. Staying active is the treatment; bed rest prolongs it.

Heat, simple pain relief and gradually resuming life outperform anxiety and scans, which usually show incidental findings even in the pain-free.

The rare urgent versions announce themselves: numbness in the saddle area, new bladder or bowel trouble, leg weakness, fever, unexplained weight loss, cancer history, or pain after significant trauma. Those need immediate care; the rest need patience and walking.