5 of a harness and lateral parts 2 2 u2032 extending below the seat and serving for the lateral holding of the legs 3 3 u2032 of the passenger the locking of the legs 3 3 u2032 is achieved by means of two flaps 10 10 u2032 mounted articulated at the end of the support of the seat base when they are in the closed position these flaps cooperate with the base of the seat and in particular with its lateral parts 2 2 u2032 so as to encircle the legs 3 3 u2032 of the passenger seated in the seat a first example embodiment of the opening closing mechanism of the device is depicted in fig3 to 6 this mechanism can be mounted for example inside a beam disposed in line with the support of the seat base at its middle part so as to project between the legs of the passenger this mechanism comprises a pedal 11 fixed to a lever 12 mounted for articulation about a horizontal shaft 13 fixed to the support 6 of the seat base 7 the bottom end of the lever 12 is fixed to a horizontal shaft 15 which can slide inside the support 6 of the seat base so as to actuate the movement of closing opening of the flaps 10 10 u2032 the lever 12 has a lug 14 projecting towards the rear and mounted for articulation at the end 21 of.