There are several common causes of knee pain when bending the leg: 1. It may be caused by a simple cold in the knee, causing muscle spasms in the knee area and pulling on the muscles during activities such as bending the leg. 2. It may be caused by synovitis in the knee joint after strenuous exercise or physical work, which can cause pain in the knee joint when moving. 3. The pain may be accompanied by swelling of the soft tissue of the knee joint and painful pulling when the joint is moved. 4. Degenerative changes in the knee joint may also occur in the elderly, who are prone to osteophytes, and the pain in the knee joint may suddenly increase when walking up and down stairs or squatting and standing up. 5. Tumors in the knee joint may occur, such as aneurysms 5. There may be tumors in the knee joint, such as aneurysm and chondroid osteoma.