Abstract: Passive knee extension systems and related methods of promoting knee extension. The system comprises a shell comprising a pliable base portion and a rigid top portion. The base portion comprises first and second ends, first and second lateral sides, a top side and an interior cavity. The system further comprises a first and second mounting straps extending between the first and second lateral sides proximate to the first and second ends, respectively. The system also comprises a rigid container with an open top positioned within the interior cavity, and a plurality of base weights positionable within exterior weight pockets of the first and second lateral sides. The system further comprises a compressible support cushion positionable on the top portion. The top portion is movable between a closed position extending over the top of the container, and an open position with the top of the container being exposed.