Pain when bending the knee after running can be due to chondromalacia patella, thigh muscle strain. It is caused by knee ligament injury, different causes, different treatment methods. 1. Chondromalacia patella: chondromalacia patella is a degenerative lesion of the femoral patellar joint cartilage due to strain or injury, and patients with this disease may have the performance of pain when bending the knee after running. Treatment should limit the knee to strenuous exercise to avoid further damage to the femoro-patellar joint cartilage, and can also be treated with medications such as ibuprofen, glucosamine, and sodium vitrate. Conservative treatment is ineffective patients can consider surgical treatment. 2. Muscle strain: if the thigh muscle is strained when running, the pain when bending the knee after running can appear, the treatment should avoid the movement of the affected limb, and can be treated by cold compress, pressure bandage, elevating the affected limb. 3. Knee ligament injury: If the cruciate ligament or the medial collateral ligament of the knee joint is injured during running, the symptom of pain when bending the knee after running will also appear, and the treatment is also based on rest, ice compresses and compression bandages, and for the patients whose ligament injuries lead to the decrease of the stability of their knee joints, they can consider surgical treatment. If the symptom of pain when bending the knee after running continues to be unrelieved, you should consult a doctor in time to find out the cause and dispose of it.