The mechanism has an inner ring (6) and an outer ring (2) that are carried by an articulation piece (1) secured to a thigh of a subject. A control wheel indexing coupler i.e. cable (12), has an end that is fixed to a tilting tab (14) of another articulation piece (4) secured to a leg of the subject by a spindle drive tape roller (19) so as to allow supporting of a heel of a foot on a floor for folding the tab to block articulation of an orthosis and allow supporting of a tip of the foot on the floor for moving away the tab from the body to unblock articulation of the orthosis.
