A system for adjusting the firmness of a substrate configured to support a subject includes a first rod configured to be movable by a mechanism, a second rod parallel to and spaced from the first rod a distance that spans a majority of a dimension of the substrate, and flexible straps extending between the first rod and the second rod and attached to the first rod and the second rod at respective ends of each flexible strap. The mechanism is configured to move the first rod in a first direction to increase tension on the flexible straps and move the first rod in a second direction to decrease tension on the flexible straps. The mechanism can be manually operated by the subject or can be a motor that is controlled by a controller.