A locking system to be fitted on a joint device (10) is equipped with means of control of the relative range of motion (R.O.M.) between two rods or uprights (11, 12) connected to it said joint consists of a pair of plates (13, 14) connected to each other by means of rivets (15) and to the rods or uprights (11, 12) protruding from the joint. This locking system comprises at least one locking device consisting of a stop member (16) which is inserted between a rod (11) and the plate (13) connected to a fixed central element of the joint. Said stop member is activated to move from a retracted position to an inserted position in the joint, at right angles in respect of the plane of the plate (13), by means of a cursor element (18) sliding on a slide (19) positioned in the upper part of a bridge element (20), said cursor element comprising a sloping surface (17) which causes the movement of the stop member (16) from a retracted to an inserted position.
