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.