A positioning system includes a deflection surface disposed within a seat, configured to support an occupant in the seat, and movable in response to motion of the occupant. The positioning system also includes a lock movable between an unlocked position in which the lock permits motion of the deflection surface relative to motion of the occupant and a locked position in which the lock restrains motion of the deflection surface relative to motion of the occupant. The lock is configured to move from the unlocked position to the locked position in response to receiving a signal from a controller, the signal including information indicative of a vehicle event.