Abstract: A vehicle seat belt installation has a structural belt guide mounted on the seat back headrest which guides movement of the seat belt during operation between a clamping member and a clamping surface. A normally deflated inflatable bag includes a body that is mounted on a base having an inextensible fixed length with one end fixed to the structural belt guide. The other end of the bag base mounts an end belt guide loop which embraces the belt. A middle belt guide loop is secured to the bag outer surface embracing the belt to assure that occupant movement causes the belt to move along a travel route over the outer surface of the bag a distance that is substantially equal to said fixed length when the bag is deflated. When sudden vehicle deceleration is sensed, an inflator mounted in the headrest simultaneously moves the clamping member to clamp the belt to the clamping surface against movement and inflates the bag.