There are 3 main reasons for numbness in the back of the left foot: 1. Due to the lumbar 4 and lumbar 5 discs protruding posteriorly and pressing on the lumbar 5 nerve root, it may lead to numbness in the nerve distribution area of the lower extremity, which is the location of the back of the foot. This is caused by the herniated disc compressing the nerve root, resulting in ischemia of the nerve root.2. Due to the extremely lateral type of lumbar disc protrusion in lumbar 5 and sacral 1, it will compress the lumbar 5 nerve root, which may also lead to numbness in the left dorsum of the foot.3. Due to focal stimulation of the superficial peroneal nerve, such as local hyperplastic calcification, as well as inflammatory lesions, or injury to the location of the peroneal head, it may lead to the superficial peroneal The nerve is squeezed and stimulated, resulting in ischemia of the nerve bundle, which can lead to numbness in the dorsal position of the left foot. Therefore, after this situation, you can go to the hospital for a lumbar MRI to see the specific protrusion, and then if necessary, you can take an electromyography to further clarify what kind of nerve is responsible.