US Patent 8991932 Backrest mechanism for chair

A backrest mechanism for a chair includes a pair of left and right side rods, each having a folded portion in a laid-V shape in a side view at a portion for supporting a sitting person around his or her lumbar, and a back plate that connects opposing surfaces of the side rods. An upper portion of the back plate above the folded portions serves as a flexible portion that is elastically deformable in a backward direction and that has smaller rigidity than the left and right side rods, and a lower portion of the back plate below the flexible portion serves as an inflexible portion that has greater rigidity than the flexible portion.