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Extension is the opposite of flexion, describing a straightening movement that increases the angle between body parts. For example, when standing up, the knees are extended. When a joint can move forward and backward, such as the neck and trunk, extension is movement in the posterior direction. Extension of the hip or shoulder moves the arm or leg backward. Even for other upper extremity joints - elbow and wrist, posterior movement of the distal segment results in extension. Except for knee, ankle, and wrist, where the distal end has to move in the anterior direction for it to be called extension. Hyperextension is any extension that goes beyond 180 degrees and becomes reflexive.[from Wikipedia]
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