Ergonomic chair backrest

A chair comprising a base, a seat supported on the base and a back coupled to at least one of the seat and base, wherein the back comprises a backrest having a normal curved contour as seen from the side including a lower, lumbar region for contact by the lumbar area of a person seated on the chair and which is forwardly curved, and an upper region for contact by the upper thorax of the person, the backrest being biased to said normal contour and flexible to vary said contour, and the back further comprising at least one upright strut spanning the lumbar and upper thorax regions of the backrest, the strut having a lower end which is fixed with respect to the seat and an upper end which is coupled to said upper region of the backrest for back and forward movement therewith in response to rearward pressure exerted by said person leaning back against the backrest and return to said normal contour, the strut comprising a parallelogram linkage operative to increase the curvature of said lumber region while at least substantially maintaining the contour of said upper region of the backrest in response to said rearward pressure.