The causes of the sudden inability to walk in the legs of the elderly include acute herniation of the lumbar disc, acute tear of the meniscus of the knee joint, etc., as follows: First, acute herniation of the lumbar disc: it can cause severe compression of the nerve, the patient will experience severe edema of the nerve, the sudden loss of innervation of the leg muscles, which will lead to the inability to walk, generally the patient will have severe lumbar pain, lumbar spine activity is limited, taking MRI or CT scan can Second, acute tear of the meniscus of the knee joint: it can cause the leg to be unable to walk, mainly due to the symptom of interlocking knee joints, and the patient cannot walk with weight, usually with severe knee pain and limited activities, and the patient can find the site and severity of the lesion through MRI examination. The patient can detect the location and severity of the lesion through MRI.