Abstract: A vehicle seat belt is provided with a device to pre-tension the seat belt should an accident situation arise. Thus, in an accident situation, the person wearing the seat belt is restrained tightly in his seat. The pre-tensioning device includes a force applying means adapted to apply a tension to the seat belt and an over-dead-center linkage which normally prevents the force applying means from applying the force, there being means responsive to a predetermined deceleration to alter the condition of the over-dead-center linkage to permit the force applying means to move the seat belt.