Cold knees may be caused by physiological phenomena, poor blood circulation, synovitis, herniated lumbar discs and other reasons. 1. Physiological phenomenon: If the weather is cold or the temperature of the environment is low, and the clothes worn are too thin, the phenomenon of cold knees may occur. 2. Poor blood circulation: Due to acute injury to the knee joint area, blood flow to the knee area may be slowed down, so the knee becomes cold, accompanied by swelling and pain. 3. Synovitis: due to trauma, over-exertion or exposure to cold, etc., synovitis of the knee joint may occur, and the patient may experience symptoms such as coldness and pain in the knee. 4. Lumbar disc herniation: Patients suffering from lumbar disc herniation may have the protruding nucleus pulposus compressing the nerve root, so the lower limbs become cold, numb and painful. If the cold knees are not a physiological phenomenon, you should go to the hospital in time for examination. Normally, you should pay attention to the warmth of the knee, and carry out appropriate exercise to enhance physical fitness.