Foot and knee flexion pain may be caused by meniscus injury or patellofemoral arthritis, osteoarthritis. 1. Meniscus injury: it may be caused by trauma, or it may be caused by degenerative disease. When meniscus injury occurs, timely treatment is needed to avoid untimely treatment, resulting in aggravation of meniscus injury, bending pain in the knee area, limitation of activities, swelling, joint popping and other symptoms. 2. Patellofemoral arthritis: it is a kind of aseptic arthritis, which is an inflammation formed in the joint area between patella and femur, and it is considered to be related to trauma, heredity and body weight, and the patients will have pain in the knee area, and the pain will be aggravated with the development of the disease, especially when laboring or bending the knee, the pain will be aggravated. 3. Osteoarthritis: When degenerative osteoarthritis occurs in the elderly, there can be obvious pain in the knee when bending the leg, due to the degenerative osteoarthritis with cartilage damage, and meniscus damage, there can be symptoms of pain when bending the leg. It is recommended to consult a doctor in time to clarify the cause of the disease and then give targeted treatment.