A seat structure comprises a base configured to be a chair base assembly; a seat support located above the base, wherein a front part of the seat support has rotary connection with the front part of the base through a transition link; a chair back assembly having rotary connection with a rear part of the base through a pivot; a lever driving member linked between the chair back assembly and the seat support, and having rotary connection with a rear part of the seat support, wherein the lever driving member and the seat support are approximately on the same surface. When the chair back assembly rotates rearward around the pivot from the initial position, the rear part of the seat support is raised diagonally to the rear through the lever driving member, meanwhile the transition link and the front part of the seat support are pulled up.

A device for adjusting the tilt of a chair backrest having a backrest frame rotatable with respect to the base frame of the chair is provided. The device includes elastic means for returning the backrest and/or countering the rotation thereof, in addition to means for controlling the preload of the elastic means. The means for controlling the preload include at least one first connecting rod and at least one first crank. The first connecting rod is hinged to the first crank and constrained to the elastic means so as to move the point of application of the returning and/or countering force with respect to the backrest. Adjusting means to adjust the position of the first crank determine at least two distinct stable positions of the first crank corresponding to two distinct positions of the mentioned point of application of the returning and/or countering force with respect to the base frame.

A tiltable chair includes a flexible seat body including an integrally formed seat back and seat surface supported on a seat frame disposed on upper parts of legs, and the seat back connected to a back frame allowing tilting rearwards, the position of the connection being in an area around the upper part of the pelvis of a person seated thereon. A guide rail having an arc-shaped groove is fixed to the seat frame, and guide members which engage and roll in the arc-shaped grooves are pivotably supported on a shaft bracket of the rear surface of the seat surface. The arc-shaped groove has a high center so that the seat surface undergoes curved motion forward and upward such that the ischium is positioned lower than the lower part of the knees when the seat back tilts rearwards.

A chair control assembly includes a base structure having first and second pivot points spaced from one another, a seat support structure coupled to the first pivot point, a back support structure coupled to the second pivot point and adapted to move between first and second positions, wherein the base structure does not move as the back support structure moves between the first and second positions, and a control link pivotably coupled to the seat support structure for rotation about a third pivot point and pivotably coupled to the back support structure for rotation about a fourth pivot point, wherein the third pivot point is rearward of the fourth pivot point when the back support structure is in the upright position, and the third pivot point moves forward relative to the fourth pivot point as the back support structure moves from the upright to reclined positions.

A chair assembly includes a back support arrangement including a back support arrangement, a seat support arrangement and a four-bar arrangement that includes a rearward end and a forward end that is adjustable between a raised and lowered positions, and an arm rest assembly adapted to support the arm of a seated user thereon and supported on the forward end of the four-bar arrangement, wherein the rearward end of the four-bar arrangement is pivotably supported from an arm support structure for pivotable movement about a substantially vertical pivot axis such that the forward end of the four-bar arrangement is moveable between a first position and second position located laterally outward from the first position, and wherein the substantially vertical pivot axis is located forwardly of a rearmost edge of an upwardly-facing surface of the seat support arrangement.

A control assembly for a chair includes a base structure having a first pivot point and a second pivot point spaced from the first pivot point, a seat support structure having a forward portion pivotably coupled to the base structure for rotation about the first pivot point, a back support structure having a forward portion pivotably coupled to the base structure for rotation about the second pivot point, wherein the back support structure is adapted to move between upright and reclined positions, and wherein the seat support structure reclines rearwardly as the back support structure moves from the upright to reclined positions, and a control link having a first end pivotably coupled to a rearward portion of the seat support structure for rotation about a third pivot point, and a second end pivotably coupled to a rearward portion of the back support structure for rotation about a fourth pivot point.

A chair back assembly includes a substantially rigid back frame assembly, a back shell member operably supported by the back frame assembly and including a laterally extending top portion, a laterally extending bottom portion and a pair of longitudinally extending side portion extending between the top portion and the bottom portion and cooperating therewith to define an open space therebetween, wherein the pair of side portions are substantially rigid in a lateral direction, and wherein the back shell member is substantially rigid in a lateral direction and substantially flexibly resilient in a fore-to-aft direction, and a cover having a first surface adapted to support a seated user and a second surface opposite the first surface, wherein the cover is positioned over the back shell member to cover at least a portion of the open space.

A chair assembly includes a back support arrangement that includes a back frame member and a forwardly-facing surface configured to support a user, a four-bar linkage assembly including a first linkage member, a second linkage member, a third linkage member and a fourth linkage member each pivotably coupled to one another such that the four-bar linkage assembly includes an upper end that is adjustable between raised and lowered positions, and an arm rest assembly supported on the upper end of the four-bar linkage assembly, wherein a lower end of the four-bar linkage assembly is pivotably supported from an arm support structure for pivotable movement such that the upper end of the four-bar linkage assembly is movable between a first position and second position located laterally outward from the first position, and wherein the fifth pivot point is located forwardly of the back frame member.

A chair assembly includes a base structure defining upper and lower portions, and a support structure, a seat support structure having a forward portion pivotably connected to the upper portion of the base structure for rotation about a first pivot point and a rearward portion, a back support structure having a forward portion pivotably connected to the lower portion of the base structure for rotation about a second pivot point that is located below and rearward of the first pivot point, and an upwardly extending back portion that moves rearwardly and downwardly as the back support structure pivots and the back portion moves, and a control link pivotably coupled to the seat support structure for rotation about a third pivot point and to the back support structure for rotation about a fourth pivot point each located rearward of the first and second pivot points.

A chair assembly includes a seat support structure, a back support structure, a back tensioning assembly adjustable between low and high tension positions, an actuator assembly adapted to adjust the back tensioning assembly between the low and high tension positions, a control input assembly actuable by rotating the portion of the control input assembly, and a control link assembly operably coupling the control input assembly with the actuator assembly to adjust the back tensioning assembly between the low and high tension positions upon rotation of the portion of the control assembly by a seated user, wherein a number of rotations of the portion of the control input assembly is not limited by the control link.