A self-adjusting seat stiffness system that uses a tension member and tensioning device to maintain or increase tension in a seat component during a perceived vehicle dynamic event. The system may use an inertial latching spool for the tensioning device as an example of a way to maintain tension at a target acceleration of the spool by locking up the tension member, or may have a controller to control the tensioning member to provide variable tensions depending on varying perceived vehicle parameters. The tensioning of the tension member helps to avoid submarining of an occupant under a seatbelt due to unacceptable seat compression during a vehicle dynamic event.