Abstract: A motor-driven joint of a robot arm comprising a first segment (1), a second segment (2) movably mounted on the first segment, and a motor-drive device mounted between the segments and connected to a control unit, the motor-drive device comprising an actuator (5) arranged to transmit movement in reversible manner to the second segment relative to the first segment, and blocking means (6, 7, 8, 9; 31) that are interposed between the first segment and the second segment and that are controllable to make the transmission of movement temporarily non-reversible, the joint being characterized in that the motor-drive device includes an actuating element mechanically connected to a portion of the blocking means and mounted on one of the segments to be movable, under the effect of a driving force greater than a predetermined threshold, to actuate the blocking means.