The invention relates to a supple member with adjustable stiffness for lying and/or sitting furniture. This member includes one or more adjustable helical springs and a rotary part. The rotary part includes a hub able to rotate about the central axis and one or more elastically flexible fingers. Each finger extends from a first end secured to the hub of the rotary part, over an arc of circle about the central axis, to a second end, and cooperates with a corresponding end of one or more of the adjustable helical springs.