The pain when bending the calf backward, that is, when flexing the knee, can be caused by the following reasons: First, meniscus injury, when there is injury or tear of the posterior horn of the meniscus, there can be pain when flexing the knee, and then we should check the MRI of the meniscus as well as diagnose and treat it at an early stage. Second, severe osteoarthritis of the knee, severe osteoarthritis of the knee can involve the soft tissues around the knee or meniscus, cartilage, and lead to pain in flexion of the knee, which should be diagnosed by early examination of the knee X-ray film, CT. Third, patellofemoral osteoarthritis, patellofemoral osteoarthritis patients can also have knee pain when bending the knee joint, then we should check the knee joint X-ray, MRI for early diagnosis and treatment.