Weakness and tenderness of the left leg may be a precursor of lumbar disc herniation, spondylitis, cerebral infarction, lower extremity venous thrombosis and other diseases. 1. Lumbar disc herniation: Lumbar disc herniation is mostly caused by disc degeneration, repeated bending and pregnancy. When lumbar disc herniation stimulates the tactile and sensory fibers of the body, it will make the patient’s limbs numb. When the herniated lumbar disc damages the nerve roots, the muscle strength will be weakened in mild cases, and the muscles will lose function in more serious cases, which will make the patient’s legs weak and flaccid. 2. Spondylitis: Spondylitis is mostly caused by infection, systemic autoimmune disease, cold and fatigue, etc. Early on, patients will experience spinal shock, which is mainly manifested as double lower limb spastic paralysis and quadriplegia. 3. Cerebral infarction: cerebral infarction is usually caused by atherosclerosis of large arteries, small artery occlusion, etc. When the patient’s anterior cerebral artery is occluded, the patient will have double lower limb paralysis and weakness. 4. Lower extremity venous thrombosis: Lower extremity venous thrombosis is generally caused by venous blood flow stagnation, obesity, old age and other reasons, the onset of the patient’s main manifestation of unilateral limb swelling, generally to the left lower limb is the most common, resulting in weakness of the affected limb soft. If the symptoms of weakness and tenderness of the left leg appear, you should go to the hospital in time to consult a doctor, and under the guidance of the doctor for treatment after diagnosis.