Lying down and lifting the left leg at the back of the thumping may be lumbar spine small joint disorder, may also be due to lumbar disc herniation. 1. Disorders of small joints of lumbar vertebrae: the small joints of lumbar vertebrae are also known as synovial joints, and the joint capsule of synovial joints is located between the upper and lower neighboring vertebrae, and the joint capsule is relatively loose under normal condition. When the articular synovial joints are disorganized, there will be misalignment of the joints or synovial embedding, etc. After lifting the left leg, the synovial membrane or misaligned joints will be reset, and the symptom of thumping at the back may occur. 2. Lumbar disc herniation: when lumbar disc herniation occurs, it may cause the tension of soft tissues around the vertebrae, the vertebrae are unstable and other pathological changes, when lying down and lifting the left leg, the force will reach the vertebrae through the conduction of the leg and hip joints, so that the vertebrae and ligaments around the vertebrae, muscles and other soft tissues move relatively, and the symptom of thumping at the back may also occur. Lying down and lifting the left leg at the back of the thumping may also have other reasons, it is recommended to go to the hospital in time to improve the examination to clarify the cause of the disease, under the guidance of the doctor to give targeted treatment or treatment.