Back support for seating unit

A back support includes a base, a seat, and a back frame pivoted to the base. The seating unit has a flexible back (i.e., back shell) supported on the back frame and having stiff upper and lower sections connected by a forwardly protruding flexible lumbar section for providing lumbar support to a seated user. A spring mechanism is connected to the back frame and to a center lower edge of the flexible back. The spring mechanism biases the flexible back to bias the lumbar toward a forwardly protruding shape to provide optimal lumbar support when in the upright position and also during recline of the seated user.