Abstract: A seat comprising a first eyelet secured to the top of the seatback, on one side of the same, and inside which a first belt slides; a winding device for the first belt, secured to the rear surface of the seat; a carriage traveling substantially longitudinally and to which is secured one end of the first belt; a second eyelet secured to the seat portion of the seat and inside which a second belt slides; a second winding device secured underneath the seat for winding the second belt; and means for retaining one end of the second belt, which means are secured to the seat for enabling the connection of connecting means to which the end of the second belt is secured.