Muscle · Hand

Abductor pollicis brevis

Musculus abductor pollicis brevis

The most superficial muscle of the thenar eminence — the fleshy pad at the base of the thumb. It lifts the thumb straight up out of the plane of the palm, which is the movement that opens the hand wide enough to take hold of a cup or a doorknob. Because it sits at the very end of the median nerve, it is usually the first muscle to weaken and the first to visibly waste when that nerve is compressed at the wrist, which makes a flattened thenar pad one of the more telling signs in a hand examination.