Knee weakness may be caused by knee ligament injury, knee degenerative disease, lumbar spine disease, cerebrovascular disease and other reasons. 1. Knee ligament injury: knee trauma such as anterior cruciate ligament and posterior cruciate ligament injury, knee joint instability will occur, it will occur when walking knee weakness, weakness. 2. Degenerative knee disease: degenerative knee disease can lead to changes in the structure of the knee, which will affect the activities, and the phenomenon of knee weakness and lack of strength can occur. 3. Lumbar spine disease: If the patient has lumbar disc herniation, such as intravertebral canal occupation, high lumbar disc herniation, etc., it will press on the local nerves, which will easily cause quadriceps weakness. When the quadriceps muscle is weak, the knee joint feels weak and tender. 4. Cerebrovascular disease: If the patient has cerebrovascular disease, such as cerebral embolism, it may affect the blood supply to the brain, causing problems with innervation and leading to knee weakness. Weakness of the knee can not make strength may also be other reasons, it is recommended that when the symptoms appear, go to the hospital in a timely manner, under the guidance of the doctor for targeted treatment.