The reason why the elbow cannot be straightened is usually due to the impaired extension function, which may be caused by long-term labor, trauma, degenerative changes and so on. 1. Long-term labor: Usually, the cause of the disease is long-term labor, which triggers inflammation in the humerus area and local adhesion pain, thus making the elbow unable to straighten. 2. Trauma: Trauma such as acute injury and chronic strain can also cause symptoms. Acute injury may cause joint fracture or even joint dislocation, so that the elbow can not be straightened. Chronic strain injuries can lead to soft tissue injuries, and long-term injuries can lead to adhesions that limit elbow extension. 3. Degenerative changes: Inflammation may occur within the elbow joint, resulting in narrowing of the joint space and osteophytes, which may limit elbow extension. In addition to the above, muscle disuse atrophy, free bodies in the elbow joint, etc. may also cause symptoms of elbow hyperextension. If you cannot straighten your elbow, you should go to the hospital in time for treatment under the guidance of your doctor.