The pain in the posterior bending position of the knee after running may be caused by muscle strain, chondromalacia patellae, fasciitis and other reasons. 1. Muscle strain: after running, patients may cause fatigue or injury to the quadriceps and hamstring muscles of the thigh, which may lead to redness and swelling of the knee area, and pain in the bending position of the back side of the knee. 2. Chondromalacia patella: Chondromalacia patella refers to the osteochondrosis caused by excessive tension or pressure on the upper and lower poles of the patella, and after the patients finish running exercise, the pressure between the bones will become bigger after the muscle strength decreases, which will lead to the pain in the bending position of the back side of the knee. 3. Fasciitis: Fasciitis is a kind of aseptic inflammation of the fascia, mainly manifested as pain, muscle weakness, muscle spasm dyskinesia and other symptoms, pain in the bending position of the back of the running knee may also be caused by fasciitis. It is recommended that patients seek timely medical examination, clear causes for targeted treatment, pay attention to rest in life, avoid strenuous activities or engage in heavy labor, and pay attention to the observation of the condition, follow the doctor’s instructions for regular review.