The absence of knee jerk reflex may be physiologic, or it may be caused by pathological factors such as acute injury to the brain and spinal cord, nerve injury, osteoarthropathy, etc. 1. Physiological factors: For example, some obese people have more subcutaneous fat, which does not easily cause knee jerk reflex when knocking the knee. When people are too nervous, it may also lead to the disappearance of knee jerk reflex, and may be accompanied by the disappearance of other parts of the reflex. 2. Pathological factors (1) Acute injury to the brain and spinal cord: such as acute inflammatory demyelinating encephalopathy, acute cerebral infarction and other diseases may affect the low-level reflex center of the body, thus causing the knee reflex to weaken or even disappear. (2) Nerve injury: such as peripheral neuritis, anterior horn of the spinal cord gray matter inflammation, radiculitis, etc. may cause nerve or nerve root injury, which may further cause the peripheral nerves to be unable to produce innervation, resulting in the absence of the knee-jerk reflex. (3) Osteoarthropathy: Severe osteoarthropathy may cause atrophy of the muscles around the knee, which may also lead to the disappearance of the knee jerk reflex. After the disappearance of the knee jerk reflex in physiological conditions, especially when accompanied by symptoms such as numbness and weakness of the limbs, it is recommended to go to the hospital in a timely manner to find out the cause of the knee jerk reflex and then treat it actively in order to avoid delaying the condition.