A vehicle occupant sensing system for detecting a condition of a vehicle seat assembly. The vehicle occupant sensing system includes a base, an upper slide member supported for movement toward and away from the base, and at least one guiding rib. The guiding rib extends from one of the upper slide member and the base toward the other of the upper slide member and the base so as to guide movement of the upper slide member toward and away from the base. The vehicle occupant sensing system may be employed in a vehicle seat assembly to detect a condition of the vehicle seat.