Hidden pain in the left posterior side of the lower back is usually caused by pathological factors, and the causes of lower back pain may be urinary system diseases, lumbar intervertebral disc herniation, and musculoskeletal inflammation of the lumbar back. 1. Urological diseases: the presence of pyelonephritis, kidney stones, renal tumors and other urological diseases may lead to an increase in the pulling force on the renal peritoneum, thus triggering spasms of the renal pelvis and ureter, resulting in lumbar back pain. 2. Herniated lumbar intervertebral disc: If there is herniated lumbar intervertebral disc, the nerves are compressed by the nucleus pulposus, which will induce persistent low back pain, and also cause pain and numbness in the lower limbs. 3. Myofasciitis of the lumbar back: it means that the myofascia of the lumbar back has chronic strain aseptic inflammation due to over exertion, etc., which may be manifested as lumbar left posterior hidden pain. In addition, trauma, local tumors, etc. may also cause lumbar back pain. If the condition of lumbar back pain occurs and the pain is difficult to be relieved by oneself, it is recommended to consult a doctor in time and take appropriate treatment according to the cause of the disease.