Possible reasons for not being able to straighten the leg include knee injury, osteoarthritis of the knee, and fracture of the knee joint. 1. Knee injury: the leg is not straight, may be due to strenuous exercise, external collision, long-term strain and other reasons, resulting in knee ligament damage, resulting in leg straightening pain, activity is limited. 2. Osteoarthritis of the knee joint: Osteoarthritis of the knee joint is often caused by meniscus or soft tissue injuries, overloading of the joints, etc., resulting in pain, swelling, limited range of motion and other symptoms of the knee joint, which can lead to the phenomenon of the leg not being straightened. 3. Knee fracture: Prolonged immobilization and braking of the fractured part of the knee joint may lead to stiffness of the knee joint, and the phenomenon of the leg not being straightened may occur. If the fracture heals abnormally, the range of motion of the knee joint will be limited and the leg will not be straight. There may also be other reasons for the lack of leg extension, it is recommended to go to the hospital in a timely manner, improve the examination to clarify the cause of the disease, and then under the guidance of the doctor to give targeted treatment or therapy.