A seatback frame (21) is disclosed as having an inverted U-shaped frame member (22) including an elongated and generally horizontal upper member (23) and a pair of elongated side members (24) and (25) respectively extending generally downwardly from the extremities of the upper member. A pair of side panels (28 and 29) are respectively secured to and extend generally forwardly from each side member and have inwardly facing surfaces that oppose each other. A resilient lumbar supporting panel (35) having lateral extremities is forwardly spaced from the side members proximate the side panels and is generally concave therebetween. It has a central portion that extends generally upwardly and is secured to a central portion of the upper member. The lateral extremities form retrorse portions (41) that extend generally rearwardly along the opposed surfaces of the side panels, and the rearmost margins (43) of the retrorse portions are attached to the side panels. The resilience of the lumbar supporting panel allows the retrorse portions to flex generally about their attached margins to damp any acceleration of a passenger toward the seatback frame of vice versa.