An underactuated prosthetic hand (1, fig 1) has openable and closable fingers (3-6, fig 1) extending from a metacarpal part of the hand. A thumb 7 is attached to one side of the metacarpal part of the hand and is mechanically movable between an open position and a closed position by means of a thumb operating mechanism. The thumb is attached by way of a thumb swivel element 11 that is rotatable between positions in which the thumb is in an adducted lateral grasp terminal position (fig 4) and an abducted power grasp terminal position (fig 5). A locking mechanism 16 is provided for locking the thumb swivel element in each of the two terminal positions in a releasable manner and optionally in intermediate locking positions. The locking mechanism may be a resiliently biased plate with teeth that engage with a corresponding formation 15 on the thumb swivel element.
