Abstract: An office chair has a seat (6) supported by a base (2). A seat back (7) is resiliently attached to the seat. A pair of axles (36a, 36b) support upper cams (31a, 32a) and lower cams (31b, 32b), respectively. A stiff, flexible sheet (20) is attached (210) to the seat back and extends over and is supported by the upper and lower cams. As a mechanical linkage which interconnects the cams with each other and an operator controlled knob (500) coordinately rotates the cams, the curvature of the flexible sheet (20) continuously changes its contour. More specifically, the contour flows such that a salient point (S) of maximum contact with the lumbar region of the user shifts in a vertical direction (h) and in a horizontal direction (e) simultaneously and concurrently.