A support diaphragm for a seat or backrest of a chair includes a support frame having laterally spaced first and second support portions. A support material includes a flexible sheet extending from and generally tensioned between the support portions to form a front portion, and extending from and generally tensioned between the support portions to form a back portion spaced from and in underlying relationship to the front portion. A strut is coupled at lateral ends to the front and back portions of the support material. The strut lengthwise extends along the support material in at least one predetermined direction relative to a direction of tension in the support material. The strut provides generally no resistance to movement of the front and back portions toward each other, the width of the strut between the lateral ends provides a desired amount of contouring of the front portion of the support material.
