The symptoms of sudden weakness when walking may be due to the following diseases: first, the most common disease is patellar ligament injury, most patellar ligament injury symptoms are caused by labor injuries, especially people who repeatedly do squatting movements or athletes who repeatedly jump and so on. Due to multiple strains and local damage caused by impaired blood circulation, there will also be palpable fixed pain points, up and down the stairs may induce pain symptoms; especially in the downstairs, half squatting when the pain is most obvious, walking can appear when the lower limbs suddenly weak symptoms. Second, patients with knee meniscus injury may also experience sudden weakness of the lower limbs and pain in a certain position of the knee joint, and the joint cannot be fully extended or bent; pain may be induced when the joint moves to a certain angle, and serious patients may experience swelling of the knee.