A coil spring mattress and adjustable foundation assembly includes a coil spring mattress； and an adjustable foundation including a mattress support surface including a head and back section hingedly connected to an intermediate section at one end and a leg and foot section hingedly connected to the intermediate section at another end, wherein the intermediate section includes a first portion and a second portion, wherein the first portion is hingedly connected to the head and back section and the second portion is hingedly connected to the leg and foot section. The intermediate or seat section is configured to increase in length upon articulation of the head and back section and/or the leg and foot section from a flat position or an increase in inclination of any section. Likewise, the intermediate or seat section is configured to decrease in length upon articulation of the head and back section and/or the leg and foot section from an inclined position to a flat position or a decrease in length upon declination of any section.
