An apparatus for dynamically stiffening a vehicle seat back may include a vehicle seat including a seat bottom mountable within a vehicle, a seat back extending upwardly away from the seat bottom, and a bight defined at a forward-facing interface between the seat bottom and the seat back, an occupant restraint harness coupled to the vehicle seat and engaging a top end of the seat back, and a seat back stiffening apparatus having a seat back stiffening member and an actuator coupled to the occupant restraint harness adjacent to or near the bight of the vehicle seat. The actuator may be responsive to a first force applied thereto by the occupant restraint harness in excess of a first threshold force to deploy the seat back stiffening member to couple to and between the seat bottom and the seat back to add stiffness to the seat back frame.