Pain below the knee after running has more causes, the more common are patellar tendonitis, infrapatellar fat pad inflammation, chondromalacia patellae, ligament and meniscus injuries, etc. It is recommended that the patient be further examined to clarify the cause of the disease. 1. Patellar tendonitis: it is mostly aseptic inflammation of the tendon between patella and tibia due to trauma and strain injury, which may cause pain below the knee and aggravate after activity. 2. Infrapatellar fat pad inflammation: mainly due to trauma and strain injury, resulting in infrapatellar fat pad aseptic inflammatory reaction, pain below the knee after activity, can be relieved after rest. 3. Chondromalacia patella: it is related to overloading of the knee joint, malnutrition and developmental abnormalities, etc., which can lead to knee weakness and anterior knee pain, especially after exercise. 4. Ligament and meniscus injuries: because of running, the possibility of knee ligament and meniscus injuries cannot be ruled out, such as anterior cruciate ligament injuries can lead to pain below the knee. It is recommended that patients go to the hospital in time to improve the auxiliary examination, in order to clarify the condition, and the professional physician will give the appropriate treatment for the condition.