Sudden weakness of the knee can be caused by many reasons, such as overwork, spinal cervical spondylosis, meniscus injury, lumbar disc herniation and so on. 1. Overexertion: Due to excessive exercise or heavy physical labor, excessive exertion leads to weakening of muscle strength, which may cause sudden weakness of the knee. 2. Spinal cord cervical spondylosis: due to severe cervical disc herniation, caused by compression of the spinal cord, which may cause weakness of both lower limbs, walking like stepping on cotton, walking instability, and sudden weakness of the knees. 3. Meniscus injury: due to meniscus injury, the injured meniscus will also be squeezed during exercise, which may lead to the symptoms of sudden weakness of the knee. 4. Lumbar disc herniation: As the herniated disc compresses the nerves innervating the quadriceps muscle, the strength of the muscle is weakened, which may also cause sudden weakness of the knee. Sudden weakness of the knee should be taken seriously, and go to the hospital as soon as possible for appropriate examination and clear diagnosis.