Loss of the knee jerk reflex means that the patient has a problem with the central nervous system such as a brain hemorrhage or peripheral nerve damage such as lumbar 2-4 nerve damage due to a herniated disc. 1. The knee jerk reflex is innervated by the lumbar 2-4 nerve. The quadriceps muscle contracts when the lower limbs are completely relaxed and the tendon at the lower edge of the patella is struck with a percussion hammer. 2. Loss of knee jerk reflex may be due to lumbar 2-4 nerve injury, because it will appear tendon reflex weakened or disappeared, the common causes are spinal stenosis or disc herniation. 3. Patients with cerebrovascular disease, such as cerebral hemorrhage, may also have absent knee reflexes. Loss of knee reflex may also be caused by improper operation or cooperation during examination, etc. It is recommended that the patient go to the hospital for consultation in time.