Hyperextension is an excessive joint movement in which the angle formed by the bones of that joint is opened, or straightened, beyond its normal, healthy, range of motion. This movement may make the joint unstable and increase the risk for dislocation or other potential injuries.
Occasionally, hyperextension is referred to as a normal movement, or exercise, that positions a body part or limb posteriorly (towards the back) of the anatomical position. An example of this type of exercise is the superman exercise where the back is hyperextended compared with a normal anatomic position.
The opposite term is flexion, or bending. Flexion shortens a joint angle and extension increases it.
Also See: extension