The causes of back pain when lifting the left leg include lumbar muscle soft tissue injury, lumbar intervertebral disc herniation and renal abscess. 1. Soft tissue injury of lumbar muscle: muscle strain or impact on the left side of the waist can lead to soft tissue injury of lumbar muscle, and when lifting the left leg, it can pull the soft tissue of lumbar region and cause lumbar pain. 2. Lumbar intervertebral disc herniation: when patients with lumbar intervertebral disc herniation lift their legs, they can stimulate lumbar spinal nerve fibers and sciatic nerves, which will cause lumbar pain and lower limb pain and other symptoms. 3. Kidney abscess: upper respiratory tract infection or urinary tract infection, bacteria spread through the bloodstream or urinary tract to the left kidney, can lead to kidney abscess, manifested as left-sided low back pain, chills, high fever and so on. Lifting the left foot to cause back pain may also have other pathological factors, it is recommended to go to the hospital in a timely manner, improve the examination to clarify the cause of the disease, under the guidance of the doctor to give targeted treatment or treatment.