A shoulder hyperextension is also known as a shoulder dislocation. When you hyperextend your shoulder, the upper arm bone -- or humerus -- has become dislocated from the ball-and-socket joint of the shoulder known as the glenoid. A fall onto an extended arm, trauma such as an auto accident or a direct collision in sports can lead to shoulder hyperextension. This condition is painful and requires immediate medical attention.
Left untreated, shoulder hyperextension can cause permanent damage to the tendons and nerves of your shoulder and upper arm.