Soreness and distension of the left posterior back is usually caused by lumbar muscle strain, lumbar spine degenerative disease, left pyelonite and other reasons. 1. Lumbar muscle strain: patients due to the left side of the lumbar muscles, tendons, ligaments and other soft tissues, long-term strain caused by inflammation, resulting in left back pain symptoms. 2. Lumbar degenerative diseases: when patients have lumbar disc herniation, lumbar spondylolisthesis, lumbar spinal stenosis and other diseases, lumbar spine degeneration compression of the posterior nerve root can cause the left posterior lumbar soreness, swelling and pain, radiating pain in the lower limbs, numbness and weakness of the lower limbs and other symptoms. 3. Left renal pelvis stones: patients with left renal pelvis stones, due to the movement of stones in the left renal pelvis ureter inside the ureter caused by the irritation of the renal pelvis ureter, which caused the left lower back pain. When the patient left back pain symptoms, need to go to the hospital in a timely manner, under the guidance of the doctor for standardized treatment, so as not to delay the condition.