Any of the muscles that increase the angle between segments of a limb, E.g. straightening the elbow or knee or bending the wrist or spine backward. The movement is usually directed backward, with the notable exception of the knee joint. In humans, certain muscles of the hand and foot are named for this function. In the hand these include the extensor carpi radialis brevis, extensor carpi radialis longus, and extensor carpi ulnaris, which run from the humerus along the back of the forearm to the metacarpal bones at the back of the hand. [http://www.britannica.com/EBchecked/topic/198909/extensor-muscle]
a skeletal muscle whose contraction extends or stretches a body part.