The sudden weakness of the legs may be caused by the following reasons: 1. It is caused by overworking, many patients do excessive exercise or overwork for the lower limbs, causing excessive fatigue of the lower limb muscles, which may suddenly appear during walking weakness of the lower limbs. 2. It may be caused by inflammation related to the lower limbs, for which the most common is the presence of osteoarthritis of the knee joint. This is the most common cause of osteoarthritis of the knee, or meniscus damage, patellar tenderness and other diseases, which can cause patients to have symptoms of weakness, that is, sudden weakness of the lower limbs or even fall. 3, may be due to peripheral neuropathy, such as in severe lumbar disc herniation, sciatica attack, in addition to causing radioactive pain in the lower limbs, but also cause the symptoms of numbness and weakness of the lower limbs. 4, this condition is due to central neuropathy, such as brain infarction, cerebral hemorrhage. For example, when there is a cerebral infarction, cerebral hemorrhage or acute spinal cord injury, it may also suddenly produce symptoms of weakness in the lower limbs.