Abstract: A support device including a plurality of cells, each cell being at least partially defined by a cell wall, a chamber, and a plurality of valves. Each valve is movable between an open position where the valve provides fluid communication between an associated cell and the chamber, and a closed position wherein the valve generally blocks fluid communication between the associated cell and the chamber. Each valve is configured to be biased to a closed position during use of the device. The device is configured such that when a predetermined force is applied to each cell during use the associated valve moves to the open position.