Extensor Pollicis Brevis Muscle

The extensor pollicis brevis muscle is a short muscle originating from the radius to insert on the thumb and serves to straighten it out. Muscle extension occurs when the angle between the bones is increased - as when the forearm is aligned with the arm. An extensor, then, is any muscle serving to extend a bodily part (usually a limb) away from the body or when a digit is extended from its base.