A vehicle seat, comprising: a foundation adapted for attachment to a vehicle floor; a seat bottom mounted on the seat foundation; a support frame extending upwardly from the seat bottom; a deformable seat (18, 118, 518) shell adapted to assume various shapes between an initial lordosis position and a final kyphosis position in response to a variable rearward load applied by the torso of a passenger seated on the seat bottom; and a compliant shell-motion controller (20, 120, 520) that couples the deformable seat shell (18, 118, 518) with the support frame, the shell-motion controller (20, 120, 520) comprising two stationary mounts (261P, 262P), two upper motion-control links (24U, 124U, 524U) that extend between the stationary mounts and the deformable seat shell (18, 118, 518), and two lower motion-control links (24L, 124L, 524L) that extend between the stationary mounts (261P, 262P) and the deformable seat shell (18, 118, 518), wherein the lower motion-control links (24L, 124L, 524L) each have a flexural portion that bends during shape-changing deformation of the deformable seat shell (18, 118, 518) caused by the rearward load applied by the torso of the seated passenger, wherein each lower motion-control link (24L, 124L, 524L) has the flexural portion at a location between the deformable seat shell (18, 118, 518) and one of the stationary mounts (261P, 262P), and wherein each upper motion-control link (24U, 124U, 524U) comprises a joint coupling the deformable seat shell (18, 118, 518) to one of the stationary mounts (261P, 262P), said joint being either a spheroidal joint or a revolute joint (J1, J2'). a foundation adapted for attachment to a vehicle floor; a seat bottom mounted on the seat foundation; a support frame extending upwardly from the seat bottom; a deformable seat (18, 118, 518) shell adapted to assume various shapes between an initial lordosis position and a final kyphosis position in response to a variable rearward load applied by the torso of a passenger seated on the seat bottom; and a compliant shell-motion controller (20, 120, 520) that couples the deformable seat shell (18, 118, 518) with the support frame, the shell-motion controller (20, 120, 520) comprising two stationary mounts (261P, 262P), two upper motion-control links (24U, 124U, 524U) that extend between the stationary mounts and the deformable seat shell (18, 118, 518), and two lower motion-control links (24L, 124L, 524L) that extend between the stationary mounts (261P, 262P) and the deformable seat shell (18, 118, 518), wherein the lower motion-control links (24L, 124L, 524L) each have a flexural portion that bends during shape-changing deformation of the deformable seat shell (18, 118, 518) caused by the rearward load applied by the torso of the seated passenger, wherein each lower motion-control link (24L, 124L, 524L) has the flexural portion at a location between the deformable seat shell (18, 118, 518) and one of the stationary mounts (261P, 262P), and wherein each upper motion-control link (24U, 124U, 524U) comprises a joint coupling the deformable seat shell (18, 118, 518) to one of the stationary mounts (261P, 262P), said joint being either a spheroidal joint or a revolute joint (J1, J2').