Abstract: An apparatus (20, 220, 420) for tensioning a vehicle seat belt and tightening the seat belt against a vehicle occupant comprising a buckle (22) which is connectable with a tongue of a seat belt. An anchor (80, 280, 444) is connectable with a part of a vehicle. A tubular housing (42, 242, 442) is fixed to one of the buckle (22) and the anchor (80, 280, 444), has a rectangular inner periphery taken in a plane extending normal to a longitudinal axis (B) of the housing, and includes a first wall (64, 264, 446). A second wall (84, 284, 484) is fixed to the other of the buckle (22) and the anchor (80, 280, 444), has a rectangular outer periphery taken in the plane, and is received in the housing (42, 242, 442) opposite the first wall (64, 264, 446) to at least partially define a chamber (102, 302, 502) which is expansible. A connector (82, 282, 482) is fixed to the second wall (84, 284, 484) and to the other of the buckle (22) and the anchor (80, 280, 444).