Reasons for not straightening the leg include: 1. If you are an elderly person, it may be caused by degenerative osteoarthritis of the knee; 2. If there is a fracture at the knee joint, plus the fixation time is too long, it is easy to cause adhesion of the muscles here, which will also make the leg not straighten; 3. Suffering from rheumatoid arthritis, which will lead to deformation of the joints, and if it is involved in the knee, it will also make the leg not straighten; 4. Excessive exertion, or Excessive fatigue, or too long in the state of bending the knee, will also make the leg can not straighten, often transient, usually rest for a while will be better.