The most common causes of pain in the middle of the waist to the left accompanied by numbness in the legs are bad posture, lumbar disc herniation, urinary system diseases, etc. If the symptoms persist, it is recommended to seek medical treatment in a timely manner. 1. Poor posture: If patients maintain a poor posture for a long time, it is easy to bring pressure to the muscles of the waist and legs, which in turn causes muscle pain in the middle of the waist to the left and numbness of the nerves. 2. Lumbar disc herniation: when the patient’s lumbar spine due to trauma or degenerative diseases can easily cause lumbar disc herniation, the lumbar disc herniation pressure to the surrounding soft tissues, which will produce lumbar pain and numbness in the legs. 3. Urological diseases: If the patient has kidney stones or bladder stones, when the stones irritate the surrounding mucosa can be involved in causing pain and numbness in the lower back and legs. Caused by the middle of the waist to the left side of the pain accompanied by numbness in the legs of the causes are more, but also may be associated with ankylosing spondylitis, bone fracture, osteoporosis and other reasons, if there is no relief after resting, it is recommended to go to the hospital in a timely manner to find out the cause of the disease, for targeted treatment.