If knee flexion pain occurs after a knee sprain or strain, consider pain caused by tissue damage within the knee, such as damage to the internal and external ligaments of the knee, damage to the cruciate ligaments, and damage to the meniscus, which is particularly noticeable when hiking and when going up and down stairs. If there is no obvious history of trauma, the pain may also be due to long-term chronic strain on the knee, resulting in synovitis or bursitis, which may be accompanied by significant joint cavity effusion. If middle-aged or older patients have flexural knee pain, it may be due to osteoarthritis or pain caused by degeneration or hyperplasia of the knee joint. A small number of patients with a history of rheumatic or rheumatoid arthritis may also experience painful flexion of the knee. The specific cause of the pain needs to be diagnosed in the orthopedic department of the hospital through knee radiographs or MRI.