A chair structure, comprising: - a supporting frame ; - a flexible member ; - a movable member ; wherein said supporting frame comprises at least one rigid member ; wherein said flexible member is designed to be anchored to said frame in proximity to said at least one rigid member , said flexible member having at least one first end designed to be anchored to the frame and a second end designed to support the lumbar area of the user's back; wherein said movable member is slidably mounted on said flexible member and/or on said at least one rigid member in order to promote the selective variation of the mechanical stiffness provided by said flexible member for the user's back; and wherein said movable member can be directly seized by a user to promote the selective sliding thereof on said flexible member and/or on said at least one rigid member , said movable member having a first outer surface designed to directly contact the outer surface of said flexible member and a second outer surface designed to directly contact the outer surface of said rigid member ; characterized in that said flexible member has at least one elongated stem comprising said first end . - a supporting frame ; - a flexible member ; - a movable member ; wherein said supporting frame comprises at least one rigid member ; wherein said flexible member is designed to be anchored to said frame in proximity to said at least one rigid member , said flexible member having at least one first end designed to be anchored to the frame and a second end designed to support the lumbar area of the user's back; wherein said movable member is slidably mounted on said flexible member and/or on said at least one rigid member in order to promote the selective variation of the mechanical stiffness provided by said flexible member for the user's back; and wherein said movable member can be directly seized by a user to promote the selective sliding thereof on said flexible member and/or on said at least one rigid member , said movable member having a first outer surface designed to directly contact the outer surface of said flexible member and a second outer surface designed to directly contact the outer surface of said rigid member ; characterized in that said flexible member has at least one elongated stem comprising said first end . wherein said supporting frame comprises at least one rigid member ; wherein said flexible member is designed to be anchored to said frame in proximity to said at least one rigid member , said flexible member having at least one first end designed to be anchored to the frame and a second end designed to support the lumbar area of the user's back; wherein said movable member is slidably mounted on said flexible member and/or on said at least one rigid member in order to promote the selective variation of the mechanical stiffness provided by said flexible member for the user's back; and wherein said movable member can be directly seized by a user to promote the selective sliding thereof on said flexible member and/or on said at least one rigid member , said movable member having a first outer surface designed to directly contact the outer surface of said flexible member and a second outer surface designed to directly contact the outer surface of said rigid member ; characterized in that said flexible member has at least one elongated stem comprising said first end . <EOS>