A device for adjusting the relative angular position of two mutually articulated structures comprising a first and a second element (5, 6) respectively connected to said structures, that rotate mutually around the axis of articulation of said structures and that define two chambers between them whose volume varies inversely as a result of the relative rotation of said elements. Said variable-volume chambers are connected together by conduits (3). The chambers and the conduits are filled with a fluid that flows from one chamber to the other through the conduits as a result of the relative rotation of the two elements around their axis of articulation. Valves are provided for interrupting communication through said conduits (3), so as to prevent the passage of fluid from one chamber to the other and consequently lock the two elements in any relative position to which they have been adjusted.