Back construction for seating unit having inverted U-shaped frame

A back construction includes an inverted U-shaped back frame and a back shell supported on the back frame and having stiff thoracic and pelvic regions connected by a flexible lumbar region in a manner adapted to ergonomically support a seated user. The lumber section includes a plurality of vertically spaced slots to increase its flexibility. The back shell has a complex curved shape that is forwardly concave in horizontal cross section and forwardly convex in vertical cross section. A force-generating mechanism biases the lumber section forwardly for lumbar support to a seated user.