The front supporting surface of the backrest is defined by an array of flexible elements (7) located side by side and having a generally S-shaped configuration with a lower forwardly convex curve (4) for supporting the lumbar region of the back and an upper forwardly concave curve (5) for supporting the upper dorsal region. Adjustment means (10, 11) enable the curvature of the flexible elements (7) to be altered selectively so as to obtain the best adaption of the backrest (1) to the characteristics of the person occupying the seat.
