Patent Publication Number: US-6669292-B2

Title: Ergonomic chair

Description:
RELATED APPLICATIONS 
     This application claims the benefit of an earlier filing date and is a continuation-in-part of pending U.S. application, Application No. 09/882,237, filed Jun. 15, 2001, entitled  ERGONOMIC CHAIR , the disclosure of which is incorporated herein in full by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a chair of the type suitable for use in an office environment and, more particularly, to a reclining office chair having several structural and operating features which offer a number of ergonomic and other advantages over the prior art. 
     2. Description of the Related Art 
     Over many years attempts have been made to design chairs for use in office environments which are comfortable to use and thereby avoid user fatigue over prolonged use. In one simple form a chair may be provided with a swivel base for ease of turning and include a control mechanism which permits the chair to rock. A disadvantage of these relatively simple chairs is that conjoint rocking motion of the chair seat and back naturally lifts the user&#39;s feet off the floor, which can create stability problems and place upward force on the front of the user&#39;s thighs which can reduce fluid circulation in the user&#39;s legs. 
     To improve on the foregoing chair construction, chair controls are known which provide for synchronous movement of the chair seat and back. Where office chairs are concerned, a “synchronous control” means the arrangement of a combined or dependent back adjustment and seat adjustment, that is to say the adjustment of the back inclination fundamentally also results in an adjustment of the sitting surface. An example of a synchronous chair control is disclosed in U.S. Pat. No. 5,318,345, issued to Olson and assigned to the common assignee herein. With the aforementioned Olson control, the chair back is designed to tilt at one predetermined rate of recline while the seat tilts synchronously at a much lesser rate. The result is that the user&#39;s feet are not lifted from the floor when the back is reclined. Also, fluid circulation in the user&#39;s legs is not interrupted by substantial upward movement of the forward end of the seat. Another advantage of this control is that undesirable “shirt pull” is minimized by the strategic location of the tilt axis. Other examples of synchronous chair controls are disclosed in U.S. Pat. Nos. 5,366,274 and 5,860,701, to name a few. 
     In U.S. Pat. No. 6,125,521, it is disclosed to be desirable to provide a chair having a seat and backrest which pivot generally about the axis of the hip joints of the user. A disadvantage of that chair is that as the chair back reclines, the rear of the seat also tilts downwardly, having the effect of changing the user&#39;s gaze angle. Further, in that chair, the arms also tilt with the chair back, thus displacing the user&#39;s arms away from any work surface. In U.S. Pat. No. 5,979,984, the seat is arranged to both slide forwardly and the rear portion of the seat moves downwardly as the back reclines. 
     Another feature embodied in recently designed office chairs that offers considerable ergonomic advantages is a tilt limiter feature for the chair back. With such a mechanism built into the chair control, the user may selectively set the degree of back recline at a predetermined angle thereby adding to comfort as the chair is used. An example of such a tilt limiter mechanism is disclosed in U.S. Pat. No. 6,102,477 issued to Kurtz and assigned to the common assignee herein. This particular mechanism offers the advantage of providing for infinitely variable angles of tilt within a predetermined overall range. The mechanism is also highly cost-effective to construct. 
     Yet another feature of current ergonomically designed chairs is the provision of height and pivot adjustable arm pads. Such a feature is particularly advantageous in providing the user with additional support to the arms, forearms, wrists and shoulders in order to minimize repetitive stress injuries when the user is keyboarding, for example, while seated in the chair. An example of such an adjustable arm pad using a gas cylinder is disclosed in U.S. Pat. No. 5,908,221 issued to Neil. 
     Yet another feature of current ergonomically designed office chairs includes an adjustable lumbar support mechanism for providing preselected chair back tension in the region of the user&#39;s lower back. An adjustable lumbar support allows the chair user to select a comfortable level of pressure on the lower back depending upon the specific office task being performed. Such a mechanism is disclosed, for example, in U.S. Pat. No. 5,797,652. 
     Still another feature of certain ergonomically designed office chairs, particularly of recent vintage, is the incorporation of fabric mesh into the construction of the chair seat, and/or back. These materials ostensibly offer the advantage of enhanced air circulation for and consequent heat transfer from the chair user&#39;s body, which can improve the comfort of the chair. An example of the use of such fabric mesh in an office chair is disclosed in aforementioned U.S. Pat. No. 6,125,521 issued to Stumpf et al. 
     Yet another feature of certain ergonomically designed chairs is the provision of a seat cushion having the capability of effecting heat transfer from the chair user&#39;s buttocks area while at the same time offering comfort to the user while seated, together with adequate support. Known seat cushions having such capability may involve a passive or active air flow circulation feature of the type disclosed, for example, in U.S. Pat. No. 6,179,706. 
     SUMMARY OF THE INVENTION 
     The present invention provides a totally redesigned ergonomic chair that incorporates simple but improved functional and esthetic aspects in all areas of a modular chair construction and in its use, including synchronous tilt of back and seat; tilt limit control; separate seat adjustment; arm adjustment; adjustable lumbar support; cushion airflow; mesh attachment and modular base frame assembly. 
     The various subfeatures of these various components are the subject of the following individual applications, the parent applications of each of which were filed on the same date as the parent application of the present case, the continuation-in-part applications being filed on even date herewith, all commonly assigned, the disclosures of all of which are incorporated herein in fall by reference: 
     Multi-position Tilt Limiting Mechanism U.S. Ser. No. 09/882,500, filed Jun. 15, 2001 
     Locking Device for Chair Seat Horizontal Adjustment Mechanism U.S. Ser. No. 09/881,896, filed Jun. 15, 2001 and Adjustable Chair Seat Locking Mechanism Continuation-in-part application, Application No. 10/077,313, filed on even date herewith 
     Height and Pivot-Adjustable Chair Arm U.S. Ser. No. 09/881,818, filed Jun. 15, 2001 and Vertically and Horizontally Adjustable Chair Armrest—Continuation-in-part application, Application No. 10/077,073, filed on even date herewith 
     Lumbar Support for a Chair U.S. Ser. No. 09/881,795, filed Jun. 15, 2001 
     Body Support Member U.S. Ser. No. 09/882,503, filed Jun. 15, 2001 Continuation-in-part application, Application No. 10/172,699, filed Jun. 14, 2002 
     Chair Back Construction U.S. Ser. No. 09/882,140, filed Jun. 15, 2001 and Chair Back Construction—Continuation-in-part application, Application No. 10/077,540, filed on even date herewith 
     Chair of Modular Construction U.S. Ser. No. 09/881,897, filed Jun. 15, 2001 
     In each of these cases, features combine to provide an overall chair that is a significant improvement over the prior art. 
     Thus, for example, the present invention provides a reclining chair having a four bar linkage system that causes the rear of the seat to elevate as the back is reclined lending an unusual and comfortable balance during reclining. A very simple and economically constructed tilt limit control conveniently and effectively limits the degree of chair back tilt to one of several reclined positions by manual movement of a lever. Horizontal positioning of the chair seat cushion may be accomplished using a simple but positive locking device that allows the chair user to select a preferred horizontal seat cushion position. Height and pivot adjustable chair arms are simply and positively actuated with the push of a button or simple rotation, lending convenient adjustment to suit a specific work task. A lumbar support is easily height adjustable, by providing tension to the back frame and requires no screws or adjustment knobs in its adjustment mechanism, and also does so by avoiding direct contact of the lumbar support with the back of the user. A modular cushion seat includes a comfortable thermal air flow layer and gel layer which is vented uniquely for air circulation and stress and pressure management. The back of the chair is of fabric mesh construction and includes a novel attachment system for superior comfort. The base and back of the chair are of modular construction that provides for ease of assembly and lends rigidity to the chair construction, and in which an open skeletal frame structure displays both the simplicity of the chair structure while adding to its esthetic appeal. 
     A primary object of the present invention is a tiltable chair wherein users of substantially all weights and sizes may be continuously balanced in the chair at any selected reclination position, therefore enhancing both “fit” and “comfort.” The present invention improves over the prior art by providing an ergonomic chair having a four-bar linkage arrangement wherein a lower frame member is provided with a rigid front support and a rigid rear support with a seat member pivotably connected to the front support. A back rest has a first pivot point connected at an upper end of the rear support of the lower frame member. A link member pivotably connects at a first end to a rear support of the seat member and at a second end to a second, lower pivot point on the back rest. This novel arrangement permits tilting movement of the backrest rearwardly relative to the lower frame member while concurrently causing elevation of a rear portion of the seat member, permitting the feet to remain on the floor and alleviating pressure on the user&#39;s thighs. This is accomplished by a linkage mechanism creating an instantaneous center of rotation of the chair seat and back that is approximately at the user&#39;s hip, so that the movement of the seat and back reduces undesirable “shirt pull.” This arrangement also is more responsive to the user and provides correct back support throughout tilt. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other novel features and advantages of the invention will be better understood upon a reading of the following detailed description taken in conjunction with the accompanying drawings wherein: 
     FIG. 1 is a left front perspective view of an ergonomic chair constructed in accordance with the principles of the invention and incorporating all of the improved modular components; 
     FIG. 2 is an exploded perspective view of the ergonomic chair shown in FIG. 1; 
     FIG. 3 is an exploded perspective view providing more details of a central support module of the ergonomic chair shown in FIG. 1; 
     FIG. 4 a  is a left side view of the ergonomic chair of FIG. 1; 
     FIG. 4 b  is a right side view of the ergonomic chair of FIG. 1; 
     FIG. 5 is a front view of the ergonomic chair of FIG. 1; 
     FIG. 6 is a rear view of the ergonomic chair of FIG. 1; 
     FIG. 7 is a top view of the ergonomic chair of FIG. 1; 
     FIG. 7 a  is a partial top view of a seat of the ergonomic chair of FIG. 1 with a cushion assembly removed; 
     FIG. 8 is a bottom view of the ergonomic chair of FIG. 1; 
     FIG. 9 is a bottom view with the ergonomic chair of FIG. 1 with a base removed; 
     FIG. 10 is a partial left side view illustrating the ergonomic chair of FIG. 1 in a fully upright position; 
     FIG. 10 a  is a side schematic view showing the preferred dimensional relationships between components of the ergonomic chair of FIG. 1 with the chair back in a fully upright position; 
     FIG. 11 is a partial left side view of the ergonomic chair of FIG. 1 shown in a partially reclined position; 
     FIG. 12 is a partial left side view of the ergonomic chair of FIG. 1 shown in a fully reclined position; 
     FIG. 12 a  is a side schematic view showing the preferred dimensional relationships between the ergonomic chair components when the chair back is in a fully reclined position; 
     FIG. 13 is a side schematic view showing the linkage arrangement of the chair; 
     FIG. 14 is a side schematic view showing the kinematics of the chair; 
     FIG. 15 is a perspective view of the preferred chair back assembly of the present invention; 
     FIG. 16 a  is an exploded perspective view of the preferred chair back assembly of the present invention; 
     FIG. 16 b  is a perspective view of the assembled back frame without the backrest thereon; 
     FIG. 17 is a perspective view of the chair back assembly illustrating the adjustability of the preferred form of adjustable lumbar support; 
     FIG. 18 is a cross-sectional view taken substantially along the line  18 — 18  in FIG. 15; 
     FIG. 19 is an enlarged view of the circled section  19  in FIG. 18; 
     FIG. 20 is a partial cross sectional view showing the relationship of the lumbar member of the mesh carrier taken substantially along the line  20 — 20  in FIG. 15; 
     FIG. 21 a  is an enlarged view of one form of mesh arrangement for use in the invention; 
     FIG. 21 b  is an enlarged front view of a second preferred embodiment of mesh arrangement for use with the present invention; 
     FIG. 21 c  is an enlarged rear view of the mesh of FIG. 21 b;    
     FIG. 22 is an enlarged exploded sectional view showing one form of assembly of the mesh to a carrier; 
     FIG. 23 is a view similar to FIG. 22 showing an adhesive bonding method of fastening the mesh to a carrier; 
     FIG. 24 is an enlarged partial sectional view showing the carrier mounted to the top of the back frame; 
     FIG. 25 is an enlarged partial sectional view showing the carrier mounted to the bottom of the back frame; 
     FIG. 26 is an enlarged partial bottom view showing detail of a tilt limit mechanism, with the base, arms and seat of the chair removed for ease of understanding; 
     FIG. 26 a  is a cross-sectional view taken along the line  26   a — 26   a  in FIG. 26, showing details of the link and tilt limit mechanism with greater clarity; 
     FIG. 27 is a bottom exploded perspective view showing further detail of the tilt limit mechanism; 
     FIG. 28 is an enlarged perspective view of a stop plate of the tilt limit mechanism; 
     FIG. 29 is a top perspective view of a seat plate; 
     FIG. 30 is a cross-sectional view of the seat plate taken along the line  30 — 30  in FIG. 29; 
     FIG. 31 a  is an exploded perspective view looking up into an assembled seat plate and seat pan; 
     FIG. 31 b  is an exploded perspective view looking down on the assembly of the seat pan and seat plate; 
     FIG. 32 is an exploded perspective view of a preferred seat cushion assembly of the present invention; 
     FIG. 33 is a front view of a preferred form of arm assembly for use with the present invention; 
     FIG. 34 is a cross-sectional view of the preferred arm assembly taken along the line  34 — 34  in FIG. 33; 
     FIG. 35 is an enlarged sectional view, broken away of a portion of the arm assembly as illustrated in FIG. 34, with the armrest in a locked position; 
     FIG. 36 is an elevation view of a guide tube; 
     FIG. 37 is a plan sectional view taken along line  37 — 37  of FIG. 36; 
     FIG. 38 is an elevation sectional view taken along lines  38 — 38  of FIG. 36; 
     FIG. 39 is a plan sectional view taken along line  39 — 39  of FIG. 36; 
     FIG. 40 is an enlarged exploded isometric view of the top of the guide tube and an activator nut; 
     FIG. 41 is an enlarged elevation view of a rod; 
     FIG. 42 is a downward looking isometric view of an armrest base; 
     FIG. 43 is an upward looking isometric view of the armrest base; 
     FIG. 44 is a sectional elevation view taken along line  44 — 44  of FIG. 42; 
     FIG. 45 is a top plan view of a cap; 
     FIG. 46 is an elevation view of the cap; 
     FIG. 47 is a bottom plan view of the cap; 
     FIG. 48 is an enlarged isometric view of a lever; 
     FIG. 49 is a plan sectional view taken along line  49 — 49  of FIG. 33 showing the armrest in a locked position; 
     FIG. 50 is a view similar to that shown in FIG. 49 except that the armrest is shown in an unlocked position. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, and initially to FIGS. 1 and 2, an improved ergonomic chair constructed in accordance with the numerous principles of the invention is shown and designated generally by the reference numeral  10 . The chair  10  comprises as its principal components a back  20  and seat  30 . Suitable armrest assemblies  40  having upper armrest pads  408  may be provided. The chair  10 , in a conventional manner, may be supported on a spider base  15  movable on casters  16 . 
     As shown in FIGS. 10-12, the chair  10  is so constructed as to have synchronous movement of the back  20  and seat  30 . The modular chair construction generally is described in greater detail in aforesaid copending application Ser. No. 09/881,897, incorporated herein by reference. The chair  10  receives the upper end of a gas cylinder  17 . The gas cylinder  17  is preferably a two-stage type available from Stabilus GmbH of Germany. This cylinder  17  is operable by a manually pivotable lever arm  18 . FIGS. 2 and 4 b , which activates the cylinder  17  for height and adjustability of the chair  10  in a manner well-known in the art, the advantage in this case is that movement of the arm  18  in either up or down direction effects movement of the cylinder to either direction. 
     For purposes of clarity of discussion only, the chair  10  will be described and claimed with reference to up, down, left and right and forward and rear as though a user were seated on the chair, but without intent to limit the claims except where apparent. Further, because many of the parts are identical but are mirror images in arrangement, the same part number will be used to describe like parts but with an “L” or “R” designation for the left or right side used only as and when necessary. 
     The chair  10  is formed of modular construction. It has a central base or support module  100 , FIG.  3 . The module  100  has a mounting section  101  extending transversely. The mounting section  101  forms a support which receives elongated seat and back frame support members  110 . Armrest modules or assemblies  40  are attached at the ends of the central base member  100  by bolts  490  and hidden nuts  491 . The lower curved end portion  402  of each of the arm rest modules  40  is positioned on the mounting section  101  and extends past each end of the first and second elongated seat and back support modules  110 . 
     The armrest modules  40  have upper end portions  401  to which armrests  408 , FIG  4   a , may be attached, as hereinafter described. 
     FIG. 3 is a front isometric view of the central base module  100  illustrating the top  102  and bottom  103  thereof as well as the opposing ends  104  thereof. A hub opening  105  is provided for securing the central base to the top of the piston/cylinder arrangement  17 . 
     Each elongated seat and back frame support member  110  has a front seat member support end  130  and a rear backrest frame support end  120 . As can be observed, the rear ends  120  are separated at a greater lateral distance than the front ends  130  and the front ends  130  are at a lower elevation than the rear ends  120 . Each of the ends  120  and  130  form pivots at apertures  121  and  131 , respectively, therethrough for receiving appropriate pivot pins  122 ,  123 ,  132 ,  133 , FIG.  2 . 
     As can be best seen in FIGS. 2,  6 ,  10  and  16   a , a backrest module or assembly  200  which is a skeletal frame in arrangement, comprises a generally splayed outwardly U-shaped frame formed of a pair of spaced uprights  201  and having a lower connecting bight segment  206  (FIG.  9 ). An upper transverse member  207  and a lower transverse member  208  are both fixedly connected to uprights  201  to provide rigidity thereto. The lower member  208  is affixed to each upright  201  by an appropriate “C” shaped member  209  held in position by threaded fastener  202  fed into threaded bosses  203  one each upright  201 . The members  209  serve to space the lower transverse member  208  forwardly of the uprights  201  for reasons which will be apparent. The lower transverse member  208  and the upper end assembly  205  of the uprights  201  receive and support a carrier  220  in which a mesh fabric  290  is positioned. The uprights  201  also are configured to support an adjustable lumbar member  250 , all as described hereafter. 
     The right and left outer ends  210  on the lower transverse member  208  of the backrest assembly  200  provide pivotal attachment to the rear ends  120  of corresponding ones of the first and second elongated seat and back support modules  110  via the pivot pins and bearing sleeves (shown in FIG. 2) through apertures  121  in the support member  110  and openings  211 , FIGS. 15,  16   a ,  16   b , in the ends  210 . 
     Centrally positioned lower bight portion  206  has at its forward end a clevis arrangement  212 FIG. 15, defined by a forwardly extending bracket  212   a  on which are formed a pair of spaced tabs  213  having openings  214  thereon for receiving a pivot pin. The forward end of the bracket  212   a  has a stop plate engagement member or projection  215  thereon that cooperates with the tilt limit mechanism as hereinafter described (see FIGS.  15  and  26 ). 
     With reference to FIGS. 29,  30 ,  31   a  and  31   b , a seat member assembly or module  300  is illustrated in detail and includes generally a seat plate  330 , a seat pan  301  and a fastener  303 . Details of the seat assembly  300  are provided hereinafter and in copending U.S. Patent application entitled “Adjustable Chair Seat With Locking Mechanism,” filed on even date herewith, Application No. 10/077,313, commonly assigned, the disclosure of which is incorporated herein in full by reference. 
     Briefly, the assembly  300  includes the seat plate  330  having a front end portion  331  having pivot pin apertures  332  and a pocket or housing style rear end portion  333 . A pivot attachment means such as the pivot pins  132 ,  133 , FIG. 2, pass through the apertures  131  in the seat support members  110  and into the apertures  332 , FIG. 31 a  and are used to couple the front end portion  331  of the seat plate  330  to the front ends  130  of the seat and back support members  110 . 
     A housing type structure is provided at the rear end portion  333  of the seat plate  330 , and has depending outer walls  346 . FIGS. 29,  31   a , which include a lower bottom wall portion  347  extending partially across the width of the seat plate  330  to define the bottom of the housing. Spaced inwardly from the outer walls  346  are a pair of inner walls  348 , provided with apertures  349  therethrough for purposes of receiving pivot pins  361 ,  362 , FIG. 2, therethrough. A large opening  345  for receiving the clevis and projection members  215  and is disposed between the inner walls  348 . At the top of the very rear of the seat plate  330  is a tab-like projection  350  having apertures  351  therethrough for reasons later explained. Positioned in a portion of the housing at the rear end portion  333  of the seat plate  330  are a pair of spaced pivot links  216 , FIG. 2. A first lower end  217  of each of the links  216  is pivotably attached to the lower portion of the rear end portion  333  of the seat plate via pins  361 ,  362  at apertures  349 . A second upper end  218  of each of the links  216  is attached to one tab  213  of the clevis  212  on the back rest frame module  200 , as is described in more detail hereafter in relation to FIGS. 7 a ,  10  and  26   a.    
     The rear end portion  333  of the housing section of seat plate  330  is constructed for pivotal coupling to the clevis arrangement  212 . This pivotal coupling comprises the pair of laterally spaced link members  216 , each having the lower end  217  for pivotal coupling to the respective sides of the seat plate  330  on the inner walls  348  and the upper ends  218  for pivotal coupling to the respective sides of the aperture/tabs  213  of the clevis  212 . 
     A pivot pin  219 , FIGS. 2,  7   a , extends through the openings  214 , FIG. 15, in the tab members  213  of the clevis, and passes through openings in the upper ends  218  of each of the link members  216 . The pin  219  extends over spaced apart curved surface ledges  352  at the top of the inner walls  348  for defining movement of the link members  216 . As seen in FIG. 30, the curved ledges  352  of each inner wall supports roller ends  353 , FIG. 7A, of the pin  219  and the upper ends  218  of the link members  216 , and provides the guide path and limit stops for movement of the upper ends of the link members and thus of the link members  216 . The lower end  217  of each link member  216  is pivotally connected to the bottom portion of inner walls  348  via pins  361 ,  362  that are passed through the apertures  349  disposed near the bottom wall  347  of each inner wall  348 . The upper ends  218  of the links  216 , when the chair is unweighted by a user (or if weighted but not reclined), will be at the upper ends of the ledges  352  as shown in FIG.  30 . As the back  20  reclines or tilts, the pin  219  will move along the curved descending ledges  352  toward the bottom thereof. Because the link also is pinned to the seat  330  at the aperture  349 , it will cause the rear portion  333  of the seat plate  330  to rotate and rise about the front end portion  331 . 
     The relative positions of the seat  30  and back  20  of the chair  10 , during reclining of the back  20 , can be seen in the side views of FIGS. 10-12. The chair seat plate  330  is pivotably connected via the pins  132 ,  133  at pivot points P 30  to the forward end  130  of support members  110  (only one of which can be seen) and is pivotably connected at rear pivot points P 32  to the lower ends  217  of the links  216  at the apertures  349  by the pins  361 ,  362 . Each link  216  in turn is pivotably connected at its upper end  218  at pivot point P 34  to the clevis  212  on the back frame assembly  200 . The back frame assembly  200  also is pivotably connected via the lower transverse member  208  at pivot point P 20  to the two laterally spaced support ends  120  of the frame support members  110 . 
     As shown in the dimensional schematic of FIG. 10 a , when the chair back  20  is in a fully upright position the seat  30  in one preferred form is inclined to the rear and forms an angle of about 10.0 degrees from horizontal, and the angle of the seat plate  330  forms an angle of about 15.7 degrees from the horizontal although the this angle of the seat elate can be in a range of between about 10 and 20 degrees. The preferred distance between the pivot points P 30  and P 32  is about 12.889 inches and the distance between the pivot points P 32  and P 34  of the links  216  is approximately 2.01 inches, although these distances can be in ranges of between about 10 and 15 inches and about 1.5 to 2.5 inches, respectively. Further, the preferred distance between the pivot points P 20  and P 34  is approximately 4.71 inches while the horizontal distance between pivot points P 30  and P 20  is about 14.5 inches. The distance between the pivot points P 20  and P 34  may be in the range of between about 3 and 6 inches while the distance between the pivot points P 30  and P 20  may be in a range of between about 12 and 17 inches. 
     As shown in the three stages of back tilt illustrated in FIGS. 10-12, as the back  20  reclines rearwardly, the links  216  move in a counterclockwise direction of rotation about pivot P 32  causing the rear end portion  333 , FIGS. 2,  29 ,  30  of the seat plate  330  to pivot about the pins  132 ,  133  at the front end portion  331  and elevate the rear end portion relative to the front end portion  331 . In the fully reclined position of the back  20  as shown in the schematic of FIG. 12 a  the seat plate  330  preferably reduces its angle of inclination with the horizontal from about 15.7 degrees (FIG. 10 a ) to about 8.7 degrees and the seat  30  reduces its angle of inclination from about 10 degrees to about 3.1 degrees, while the afore-described distances between all pivot points remain constant. The reduced inclination angle of the seat plate  330  may be in a range of between about 6 and 10 degrees. This synchronous motion of the seat plate  330  and the seat  30  with the back  20  provides for an exceptionally comfortable reclining motion of the chair user to aid in avoiding fatigue as the user is performing various work-related tasks. The ride motion is achieved by a simple seatback-seatrest four bar mechanism, which immediately responds to a user exerting a back force and/or self-weight. A back force applied by the user against the back  20  induces a lift in the rear part of the seat, during reclining, in a synchronic-type motion and vise versa. 
     The chair link mechanism restores ride stability, and attains a desirable ride quality, equally well for a wide range of users, by varying the back force reaction in the mechanism. The back force reaction results from users imposing their own back force and/or self-weight on the chair mechanism. By proportioning linkages lengths, and selecting the locations of the rotationally-free hinges, i.e., the pivots, the back force reaction is designed to vary in order to achieve balance. This continuous force balancing process, which characterizes the ride motion at all positions, establishes and maintains an equilibrated ride. As a result, the need for a user to consciously adjust a back tilt tension knob, to feel comfortably balanced when reclining, is replaced with this more adaptive and dynamic feature. By doing so, the chair design is taken one step further toward conformance to all users without involving them in unnecessary conscious feature-adjustment efforts. 
     The mechanism configuration determines the location of the instantaneous center of chair rotation. As the configuration changes as a result of changing the sitting posture and position, the chair design allows the locus of its instantaneous center of rotation to generally coincide with the user&#39;s rotation center, i.e, hip joint, at all locations within the ride range (FIG.  13 ). This feature is calibrated for equal performance to many users, where the instantaneous center of chair rotation is set to move along the locus, i.e. trajectory. By maintaining this quality in the ride, abdominal, back, and other muscuoleskeletal straining, are all eliminated in the full ride range. A more uniform foot reaction is maintained in the ride, therefore, enhancing the ride quality even further. Also, the opening of the torso-legs angle enhances fluid circulation and other ergonomic factors. 
     Shown in FIGS. 13 and 14 are schematic views of the synchronous seat and back tilt feature employing the four-bar mechanism which allows the rear of the seat  30  to elevate as the backrest  20  is reclined. The mechanism is designed to immediately respond to the users weight and provide the correct back support throughout the range of back reclining. This function allows for reclining of the chair  10  about an instantaneous center point C that, as noted, is very closely coincident with the pivot axis of the user&#39;s hips and avoids undesirable “shirt pull” of the user. Also, because the front of the seat  30  is not elevated during back reclining, no additional pressure is applied to the front underside of the user&#39;s thighs, and also a relatively constant gaze angle is maintained during reclining. 
     To accomplish the foregoing advantages, the chair  10  schematically comprises four basic members and four rotationally-free pivots. The basic members include a floor supported member  60 , a seat rest  62 , a linking member  64  and a backrest  66 . The floor supported member  60  has an upwardly directed portion  68  that terminates at an end defining pivot point P 30  to which the seat rest  62  is pivotably connected at its forward portion. The member  60  also has an upwardly directed portion  70  which terminates at an end defining pivot point P 20  to which the backrest  66  is pivotably connected. A lower portion  72  of the back rest  66  is pivotably connected at point P 34  to the upper end of linking member  64  and a downwardly extending portion  74  of the seat rest  62  is pivotably connected at point P 32  to the other lower end of the linking member  64 . 
     The kinematics of the chair  10  are illustrated in FIG.  14 . As force F is applied on the backrest  66 , the back tilt angle β increases, eye location shifts backwards an amount ΔH 1 , and eye elevation decreases by an amount ΔV 3 . The change in back tilt angle β transmits motion by way of the upper and lower back pivots P 20  and P 34 , respectively, to the linking member  64 . As a result of the position of the linking member  64 , the rear seat pivot P 32  moves in coordination with pivot P 34  in a composite rotational and translation motion. As the seat rest  62  rotates about pivot P 30 , a lift ΔV 2  is caused in the rear part of the seat rest  62  relative to its front edge ΔV 1  in the amount ΔV 2 -ΔV 1 , therefore introducing a seat rest angle α. During back reclining an increasing portion of the user&#39;s weight supported by the chair is transferred from the seat support to the back support while the mechanical advantage of the mechanism lifting the seat support decreases. The user will therefore sense a static balance position in any position of recline and will require little effort to move to a new position. 
     As the user sits in the chair, a back force, F, and/or weight, W, is exerted on the chair mechanism and, therefore, sets it in a self-equilibrating motion, with ride qualities that are designed beforehand. These forces may only be balanced by a proper back force reaction, and by the chair reconfiguring its geometry to the level required by the exerted force. By doing so, the motion response parameters appearing in FIGS. 13,  14 , are varied in known proportions and rates to achieve the ride quality. The design ride range subtends a backrest angle from about 90° to about 120°. 
     To fine-tune the ride qualify, and to provide for static overall stifthess, the mechanism may also be equipped with external elements, including springs. The addition of these external devices would further calibrate the ride quality towards its desirable, pre-designed features, while, at the same time, maintaining all other functional qualities intact. Thus, in order to assist the chair linkage mechanism in allowing the backrest member  20  to maintain a fully upright position when the chair  10  is not in use, as shown in FIG. 10 one or more extension springs  75 , FIG. 7 a , may be connected between pivot shaft  219  and the rear edge portion  350  of the seat plate  330 . The pivot shaft  219  essentially also defines pivot point P 34  and will cause a return force to be exerted on back frame assembly  200  by the springs  75 . 
     In order to relate the explanation of the schematic linkage as described in FIGS. 10 a ,  12   a ,  13  and  14 , to the chair  10 , the respective parts on the chair correspond to the parts as numbered on the schematic linkage 
     
       
         
           
               
               
               
             
               
                   
               
               
                 CHAIR 
                 LINKAGE SCHEMATIC 
                   
               
               
                   
               
             
            
               
                 110 
                 60 
                 Floor support member 
               
               
                  30 
                 62 
                 Seat 
               
               
                 216 
                 64 
                 Link member 
               
               
                  20 
                 66 
                 Backrest 
               
               
                 130 
                 68 
                 Front upward member on 
               
               
                   
                   
                 60 
               
               
                 120 
                 70 
                 Rear upward member on 60 
               
               
                 212 
                 72 
                 Lower portion of backrest 66 
               
               
                 348, 349 
                 74 
                 Downward portion of seat 
               
               
                   
                   
                 rest 62 
               
               
                   
               
            
           
         
       
     
     FIGS. 26-28, a very simple and economical seat back tilt limit control module  150  is associated with the seat assembly  300  using a lever handle  155  attached to the tilt limit module for adjusting the amount of permissible tilt of the seat back  20 . As can be seen in FIG. 27, the seat plate  330  has a threaded stub  152  on the bottom side  335  thereof to which a stop plate  154  and the lever handle  155  can be attached by a shoulder bolt and washer  156  that is threadedly inserted into stub through aligned orifices  157  in the handle, stop plate and stub. The lever handle  155  may be attached to the stop plate  154  from either side of the chair  10 , and herein is shown on the left side. It will be noted that stop plate  154  has an outer edge  158  with a series of steps of different radii for interacting with the engaging member  215  carried by the end of the clevis  212  at the bottom of the back rest frame  200 , as will be seen more clearly in FIG.  16 . In the preferred embodiment, the engagement member  215  is in the form of a nose-like protrusion. 
     The outer edge  158  of stop plate  154  has specific steps  159 ,  160 , and  161 , as can be best seen in FIG. 28, and each is at a different distance from the central axis defined by aperture  157  with respect to the other. A locking means  163 , in the form of a spring-biased plunger is mounted in a boss  164  on the bottom of seat plate  330  and engages detents  165 ,  166  and  167  found in the upper portion of stop plate  154 . It will be noted that the outer edge  158  of stop plate  154  could be a smooth cam having an edge of a continuously changing radius. The number of detents determines the number of fixed chair back tilt positions. 
     The indexing detents  165 ,  166  and  167  are of concave shape and are adjacent each other. Since the spring-loaded plunger is in the form of a spring-loaded ball  163 , the plunger is enabled to freely move from one concave detent to another by compressing the spring and rotating the lever  156  and thus the stop plate  154  to permit the plunger to be positioned in any selected detent and by releasing the plunger to cause the stop plate to hold the back frame in a selected tilt position. 
     Because the projection  215  is below the pivot axis P 20  formed at ends  120  and  210 , as the back frame  200  pivots about axis P 20 , the protrusion  215  moves inwardly towards the seat stop plate  154  thus allowing adjustment of the tilt mechanism by moving handle or lever  155 . The handle  155  is fixed from rotation relative to stop plate  154  via a number of matching spring fingers  168 , engaging recesses in the handle. When the lever  155  and the stop plate  154  rotates, it causes the various steps  159 ,  160  and  161  on the outer edge thereof to be in selective engagement with the projection  215 . The details of the tilt limit module are disclosed and claimed in commonly assigned co-pending patent application Ser. No. 09/882,500 filed Jun. 15, 2001, and entitled “Multi-Position Tilt-Limiting Mechanism,” the disclosure of which is incorporated herein in full by reference. 
     Turning now to FIGS. 15,  16   a ,  16   b  and  17 , the complete backrest assembly  200  of the chair  10  is illustrated in perspective and shows the novel feature of the lumbar support construction and carrier assembly which will be described in detail. 
     Yet another novel and highly functional feature of the chair  10  that offers ergonomic advantages over the prior art is the construction of the chair back  20 . As previously noted, the back  20  is designed to include a panel of fabric mesh  290  which is preferably of an open weave type known in the art. The construction of the fabric mesh  290  may have a variety of weave configurations. One configuration that has proved to be advantageous is shown in FIG. 21 a  comprising vertical strands  291  of multifilament yarn and horizontal monofilaments  292 . The monofilaments  292  in this construction can be seen to cross over the strands  291  and also crisscross over each other thereby locking the strands  291  in place. 
     A preferred mesh weave is shown in front and back views in FIGS. 21 b  and  21   c , respectively. In this version, the horizontal multifilament or weft material  295  is simply interwoven in steps with a vertical monofilament warp  296 . In the illustrated embodiment the warp does not cross the weft at each adjacent position. In this pattern for each warp cross over a weft, there will be seven warps between that do not cross the same weft before a repeat. Thus warps “0” and “8” cross weft C, while warps  1  and  9  cross weft D. The warp material preferably is made of Hytrel® monofilament of 730 Denier. This provides structure and a relatively “shiny” look to the back of the chair. The weft  295  is a multifilament which includes a polymer yarn and an amount of Hytrel®. The multifilament is a suitable synthetic resin and provides a relatively soft and comfortable feel to the front surface of the chair back. Because of the pattern, it also has a more finished and pleasing esthetic look as compared to open mesh of different patterns. 
     In order to support the mesh  290  around its edges, the aforementioned carrier  220  is used. The physical connection of the carrier  220  to the mesh  290  may be performed in a number of ways. However, a most reliable connection is disclosed in co-pending U.S. patent application Ser. No. 09/656,491, filed by Timothy P. Coffield on Sep. 6, 2000 and titled “Bonding Strip for Load Bearing Fabric.” FIGS. 15,  20 ,  22  and  23  illustrate a carrier  220  comprising two halves  221  and  222  disposed on opposite sides of the edge portion of mesh  290 . The two halves  221  and  222  may, in one form, be formed with internal grooves  224 . The halves are placed in a fixture  225  together with an adhesive  226 . The adhesive extends through warps and wefts of the fabric  290  and into pockets formed by the grooves  224  and, once cured, creates a mechanical interconnection that is of high strength and durability, and also helps hold the two halves  221 / 222  of the carrier  200  together. Additional features (not shown) such as screws or the like also may be used to assure the parts to not separate. While halves  221 ,  222  are shown as approximately equal size in the preferred form, as illustrated in FIGS. 24 and 25, the one side is larger than the other, providing a more pleasing esthetic look to the mounted carrier. 
     The carrier  220  is formed as a generally rectangular semi-rigid member of resilient, stretchable material. In order to support the carrier  220  with mesh  290 , in accordance with the invention and referring once again to FIGS. 2,  6 ,  20  and  16   a  and  24 , the main back frame uprights  201  each has spherical end assemblies portions  205  thereon which are “snap fit” received within circular apertures  227  formed in the upper right and upper left hand corners of the carrier  220 . These joints allow upper edge  231  of the carrier  220  to flex allowing the chair back  20  to comfortably conform to the position of the user&#39;s shoulders. The back may be secured along bottom edge  232  to the lower transverse frame member  208  by a series of five outwardly depending tabs  238  formed on the bottom of transverse member  208 , which tabs fit into complementary recesses  223  formed in lower surface  232 . Details of the upper ball and socket connections may be seen in the cross-sectional view of FIG. 24, while the lower attachment construction can be seen in detail in FIG.  25 . In assembly, the lower edge  232  is first assembled to the tabs, tension applied to the carrier  220  and the upper openings  227  snapped into the spherical ends  205  of uprights  201 . 
     It can now be appreciated that a chair back construction as just described offers considerable ergonomic advantages. The use of open mesh  290  allows the chair back  20  to not only breathe, but to flex in conformity with the back of the user. The back  20  is also highly cost effective to manufacture and assemble. Further, the back member is positioned on the frame assembly  200  in a manner that keeps the carrier and mesh in tension, providing both flexibility of the back surface but sufficient rigidity for the carrier that it maintains its shape. Because the lower transverse member  208  is curved and set forward of the uprights  201  (via C members  209 ), the lower end  232  of the carrier assumes a curved configuration that also is spaced forwardly of the uprights  201 , so the user never feels the back frame. 
     As observed the carrier  220  has a slight curvilinear shape from top to bottom so as to assume compressing tension in the mesh fabric. When a user&#39;s back contacts the fabric at various locations and during casual movement in the chair. The curvilinear shape also is intended to provide support in the general lumbar region, as best seen in the side view of FIGS. 4 a  and  4   b.    
     Furthermore, a major advantage of this tensioned structure is the capability to provide a unique adjustable lumbar support. The back assembly  200  includes a transverse lumbar support tube  250  having gripping means  251  on each of its opposed ends, together with a pair of spaced slide members  253 . A cross-section of the gripping means  252  can be seen in FIG. 20 wherein the carrier  220  is provided with a pair of opposed recesses  254  into which opposed projections  255  of the gripping means  252  are slidably received, with the carrier trapped between the projections  255 . Thus, the lumbar support tube  251  is slideable on opposed edges of the carrier  220 . 
     FIG. 18 illustrates a cross-sectional view of the support tube taken substantially along the line  18 — 18  of FIG.  15 . There, it can be seen that slide members  253  are configured to engage vertical supports  201 . As shown in FIGS. 16 b  and  19 , the engagement arrangement of the slide members  253  includes simple vertical grooves  256  formed in the supports  201  and engaging a central rib  257  on the slide member. It can now be appreciated, particularly with reference to FIG. 17, that the lumbar support tube  251  is vertically movable between upper and lower positions as it slides on edges of the carrier  220  by means of the gripping means  252  and also slides on the vertical supports  201  by means of the slide members  253 . The result of such movement is to allow the chair user to adjust the vertical height of the lumbar support tube  251  by simple manual manipulation. The lumbar tube  251  is held in proper connection to the supports  201  by the tension of the carrier  220  and mesh  290  without the need for screws, adjustment knobs or the like. In this tension mode the lumbar tube  251  causes the carrier  220  and mesh  290  to be forced forwardly of the chair uprights in the lumbar region of the user, while direct contact of the lumbar support tube  251  with the back of the user is avoided. An in-depth description of this assembly may be found in aforementioned copending application Ser. No. 09/881,795, filed Jun. 15, 2001, incorporated in full by reference. 
     The vertically adjustable lumbar support member  250  is intended to change the lineal curvature of the carrier  220  as the tube  251  is slide up or down between the carrier  220  and uprights  201 . By changing the carrier configuration, no high pressure contact points are placed on the user&#39;s back; rather, a taut but flexible mesh is properly positioned for preferred support and comfort, even as the chair reclines. 
     Turning now to other aspects of a preferred seat  30  developed for use with the ergonomic chair, various aspects of the horizontal seat adjustment and unique cushion arrangement will be described in detail with reference to FIGS. 29-31. The seat plate  330  has been generally described heretofore. The plate  330  is intended to cooperate with a seat pan  301  which has the seat cushion assembly  500  affixed thereto. 
     The seat pan  301  may be made of any suitable material such as a synthetic resin which may be molded as an integral piece. The seat pan  301  includes an upper portion  304  and a lower portion  305 . The upper portion is covered with a cushion assembly  500  described hereinafter and forms the seating surface of the chair  10 . The seat pan also includes four spaced L-shaped fingers,  306 ,  307 ,  308  and  309  depending from the lower portion  305 , a fastener receiving opening  310 , a rim  318  around the fastener receiving opening and two abutment elements  312 ,  313 . The seat pan  301  also includes a front portion  314 , and a rear portion  315 . A large opening  316  is located in the rear portion  315  of the seat pan to accommodate movement of the links  216 . Peripheral fastener openings, such as the openings and bosses  317  are provided to fasten the seat cushion assembly  500  to the seat pad  301 . The seat pan further includes a number of strengthening ribs such as the rib  318  in the upper portion. Flanking the fastener receiving opening  310  are two depending bumps  319 ,  320 . The fastener receiving opening  310  and the two bumps  319 ,  320  are formed on a beam  321 . Because the material of the seat pan  301  is a synthetic resin, the beam  321  is resilient and thus able to flex in response to any applied force. Such a force may come from the fastener  303  being received through the fastener receiving opening as well as from contact with the seat plate  330  which may provide forces on the two bumps  319 ,  320 . The abutment elements  312 ,  313  are also formed at the end of respective beams  322 ,  323  for added flexibility. 
     The seat plate  330  may be formed as an integral unit of any suitable strong material, such as aluminum. As noted, the seat plate includes a front portion  331 , a rear portion  333 , a top portion  334  and a bottom portion  335 . The seat plate includes four guide slots  336 ,  337 ,  338 ,  339 , each with a large head opening and a narrow body opening (FIG.  29 ), which cooperate with the L-shaped fingers  306 ,  307 ,  308 ,  309 , respectively, of the seat pan  301  for restraining and guiding the seat pan  301  in movement in a horizontal direction relative to the fixed seat plate  330 . 
     The seat pan  301  also includes an elongated fastener receiving opening  340  and two abutment limit slots  341 ,  342 . These limit the horizontal movement of the seat pan by limiting the movement of the abutment elements  312  and  313 . At the rear portion of the seat plate is a large opening  345  that receives the clevis  212  and links  216 . On either side of the elongated fastener receiving opening  340  is a set of recesses  336 ,  337  in the top portion. The pair of sets of recesses form a detent with the pair of depending bumps  319 ,  320  on the lower portion of the seat pan. The limit slots  341 ,  342  receive the abutment elements  312 ,  313 , respectively and limit movement of the seat pad so that the L-shaped fingers do not disengage from the guide slots. The torque limiting fastener  303  is provided for seat adjustment. The design of the fastener  303  is such that once inserted it cannot easily be removed. Details of the fastener are disclosed in the above-mentioned copending application, Application No. 10/077.313, filed on even date herewith, entitled “Adjustable Chair Seat With Locking Mechanism”, incorporated herein in full by reference. 
     In operation the seat pan  301 , seat plate  330  and fastener  303  are all formed using well known techniques. Assembly is simple and easy. The seat pan  301  and the seat plate  330  are aligned to allow the L-shaped fingers  306 - 309  to pass through the enlarged openings at the end of the guide slots  336 - 339  and for the abutment elements  312 ,  313  to be pressed into the limit slots  341 ,  342 . Thereafter, the fastener  303  is threaded into the fastener receiving opening  310  of the seat pan  301  with the seat plate  330  sandwiched between. Once the seat pad and the fastener are engaged, they will not separate; however, the fastener  303  may be loosened or tightened simply by rotating the handle clockwise or counterclockwise. When the fastener is loosened, the seat pan may be adjusted generally horizontally relative to the seat plate for the user&#39;s comfort. Because of the bumps  319 ,  320  and recesses  336 ,  337 , a detent is formed which is easily heard and felt by the chair user and this helps in the adjustment process. When the adjustment is complete, the fastener is counter rotated to squeeze the seat pan and the seat plate together. 
     Another improvement in the ergonomic chair is a highly effective seat cushion assembly. 
     FIG. 32 is an exploded view of one embodiment of a seat cushion assembly  500  preferred for use with the chair  10  of the present invention. Seat cushion assembly  500  comprises foam body  512  which can be formed of materials typically used in such seat cushions, such as open-celled or closed-celled polyurethane foam. The foam body  512  has an upper surface  513  and a plurality of vertical columns  515  disposed substantially centrally in the foam body  512 . In the illustrated embodiment, the columns  515  extend upwardly such that the top surface of the columns  515  define a curved surface substantially parallel to upper surface  513 . In the embodiment illustrated in FIG. 32, the columns  515  are formed integrally with foam body  512 . The foam material of which foam body  512  and columns  515  are made will be resiliently deformable to some extent. The columns  515  are structured such that, in the absence of other structural elements of the assembly  500 , each column  515  is capable of deflecting substantially independently of the other columns  515  in response to compressive forces applied by a chair user. 
     The vertical columns  575  are structured so as to define a plurality of air spaces  516  therebetween, which together define a plenum or air reservoir  518 . A plurality of channels  520  is disposed within foam body  512  and extend from air reservoir  518  toward the periphery of foam body  512 . In the illustrated preferred embodiment of a seat cushion, the channels  520  are directed to the front and lateral sides of the foam body  512 . 
     Design parameters of vertical columns  515  include their number, planar spacing, depth, aspect ratios, and material density and stiffness. Depending on their size and shape, the number of vertical columns is preferably about 40-80. The columns preferably can have a diameter at their lower end in the range of about 1.0-2.0 inches. The columns can have a diameter at their upper end in the range of about 0.9-2.0 inches. The height of the columns can range up to about 4 inches, and most preferably will be in the range of about 0.5-1.5 inches. The columns  515  in the seat cushion  512  can be of different sizes and shapes. The number of air channels  520  will depend upon their size. The total volume capacity of channels  520  will be a function of the volume capacity of air reservoir  518 . 
     The seat cushion assembly  500  further comprises an elastomeric layer  530  that overlays the upwardly extending columns or risers  515 . In the illustrated embodiment, the periphery  531  of elastomeric layer  530  is seated within a fitting edge  514  at the upper surface  513  of foam body  512 . Elastomeric layer  530  comprises a top surface  532  and a bottom surface  534 . The bottom surface  534  of elastomeric layer  530  defines the top surface of air reservoir  518 . The elastomeric layer  530  comprises a material having significant resilience and flow properties. Suitable materials for elastomeric layer  530  include, for example, a gelatinous sheet and a polymeric membrane, or other gelatinous materials with variable viscoelastic properties. One suitable material includes a gel sold under the trademark LEVAGEL® by Royal Medica of Italy. Information about this material is available at www.royalmedica.it. The elastomeric layer  530  and upwardly extending risers or columns  515  are each characterized by both an elastic stiffness value and a dissipative stiffness value In a preferred embodiment of the invention, the ratio of elastic stiffness to dissipative stiffness of the vertical columns  515  is greater than the ratio of elastic stiffness to dissipative stiffness of the elastomeric layer  530 . 
     Depending on the material selected and the properties of the seat cushion desired, elastomeric layer  530  can have a thickness in the preferred range of about 0.2-0.4 inches. The area of elastomeric layer  530  can be less than the area of top surface  513  and most preferably in the range of about 30-55%. In one embodiment, elastomeric layer  530  is about 0.25 inches thick, and has an area of 210 sq. in., relative to a total area of top surface  513  of 392 sq. in. 
     In use, the foam body  512  of seat cushion assembly  500  is supported by a stiff seat pan  301  rigidly fixed on the seating system assembly  300 . The seat pan  301  comprises an inner pan  526  fastened to the seat pan  301 . 
     When a user is seated on a seat cushion of the instant invention, the user&#39;s weight is transmitted as vertical compressive forces and transverse shear forces to the user/seat cushion interface. These forces are transmitted through elastomeric layer  530  to vertical columns  515 . Elastomeric layer  530  and vertical columns  515  function cooperatively with one another to achieve a self-limiting mechanical response to obtain desired mechanical qualities. 
     The redistribution of applied forces can be further enhanced by an air-permeable layer  535 , disposed above elastomeric layer  530 . The air-permeable layer  535  may comprise an open-cell or non-woven viscoelastic material having specified thickness and viscoelastic properties, which air-permeable layer  535  can function to further dissipate applied forces before such forces reach elastomeric layer  530 . Optionally, an intermediate foam layer  531  can be placed between air-permeable layer  535  and elastomeric layer  530 . A cover fabric, not shown, can overlie the entire seating structure. The layers  512 ,  530 ,  535 , and the cover fabric can be pre-bonded to one another such as with adhesives. Alternatively, the layers can simply be stacked on top of each other, in which case there should be a sufficient amount of friction between the layers to prevent slippage of the layers with respect to one another in response to shear forces applied during use. 
     The structure of the instant invention will transmit shear forces emanating at the user/seat interface across the interfaces between each of the layers until elastomeric layer  530 . Elastomeric layer  530  will deform viscously in response to applied shear forces, thereby counteracting the shear component of the user&#39;s weight by dissipative means, such that the user&#39;s skin will not experience the shear component. As a result, the user&#39;s tissues will experience substantially only compressive stresses in the normal direction. This reduction in shear stress can reduce the potential for the development of pressure ulcers, and reduce undesirable interference with blood vessel activity in the vicinity of these tissues. Moreover, the unique arrangement and air pressure helps to avoid heat build-up which frequently occurs in chairs and has an advantage over mesh seats which may feel cool and drafty. 
     When the user leaves the seat cushion of the instant invention, the resiliency of the foam body  512 , vertical columns  515  and the elastomeric layer  530  allows fill recovery of both shear and compression deformational mechanisms of the cushion. The passive air pump depressurizes, allowing outside air to pass through the outside cover, the air permeable layer, and optional intermediate foam layer to enter the air reservoir and channels, and open cells in the foam body  512  if open-celled foam is used. The elastomeric layer  530  will also return to its original shape prior to the application of compression and shear forces by a user. 
     The preferred seat cushion assembly and the advantages thereof are more particularly described in copending application Ser. No. 09/882,503, filed Jun. 15, 2001, entitled “Locking Device for Chair Seat Horizontal Adjustment Mechanism,” the disclosure of which is incorporated herein in full by reference. 
     As previously noted, another aspect of the improved ergonomic chair includes improved arm assemblies  40  that are both vertically adjustable and in which the armrests  408  are rotatable generally in a horizontal plane. 
     The disclosure herein concentrates on the armrest assemblies which are simply constructed and reliable and allow adjustment both vertically and horizontally. Referring now to FIGS. 33-37, the armrest assembly  40  includes an upstanding support  400  which has an open upper end portion  401 , a curved lower end portion  402  and a longitudinally extending opening  403  extending downwardly from the upper end portion. Within the support opening  403  are mounted oppositely disposed liner racks  404 ,  405 , each having a plurality of notches extending in a longitudinal direction. As viewed in FIG. 34, the longitudinal direction is generally vertical. 
     A guide tube  407  is positioned in the support opening  403  such that the guide tube is generally vertically movable relative to the support. An armrest  408  is mounted to the guide tube so that relative movement of the guide tube causes vertical adjustment of the armrest. 
     Within the movable guide tube is a elongated element  409  in the form of a rod, the rod being mounted within the guide tube to be rotatable only. As will be explained below, the rod  409  does not slide vertically or longitudinally relative to the guide tube  407 . Mounted to the elongated rod  409  is a locking element  410  which is rotatable with the rod to selectively engage and disengage the opposed notches  406  of the liner racks  405 ,  405 . Mounted at an upper end portion  411  of the rod is an activator nut  412  which engages the rod and causes the rod to rotate, the engagement surface of the nut moving between raised and lowered positions (raised in FIG.  35 ). A spring  413  is mounted between the activator nut and the guide tube and biases the activator nut to the raised position whereby the rod is rotated to and maintained in a locked position. The activator nut is moved by a lever  414  which is operatively connected to the upstanding support by being pivotally mounted to an armrest base  415 . A cap  416  is also mounted to the guide tube and is operatively connected to the armrest base to allow the base to rotate in a generally horizontal direction relative to the cap. 
     The upstanding support  400  has a generally cylindrical shape extending in a generally vertical direction. The lower end portion  402  of the upstanding support curves to a generally horizontal disposition allowing it to be attached to the central base member  100  of the chair  10 . The support is made of any suitable material, such as aluminum. 
     Formed around the upstanding support is a second cylindrical element  417  often referred to as a shroud. The shroud slides along the outside surface of the upstanding support and provides a pleasing aesthetic appearance to the armrest assembly. An upper part  418  of the shroud  417  includes a first horizontal annular bearing surface  419 , a vertical annular bearing surface  420  and a second horizontal annular bearing surface  421 . These bearing surfaces engage corresponding bearing surfaces of the armrest base  415 . The shroud also includes a top flange  422  having fastener receiving openings  423 ,  424 . 
     As mentioned, within the upstanding support  400  are the two oppositely disposed liner racks  404 ,  405 , with each rack including the plurality of notches  406 . The racks have small tabs  425 ,  426  which engage openings  427 ,  428  in the upstanding support. In addition to the notches, the racks also include bearing surfaces  429 ,  430  for the vertically sliding guide tube  407 . 
     The guide tube is generally cylindrical in shape and includes a central opening  431 , FIGS. 36-40. The guide tube includes an upper end portion  432  including two fastener openings  433 ,  434 , an annular groove  435  for receiving the spring  413  and two oppositely disposed keys  436 ,  437 . Along an outside surface  438  of the guide tube are grooves, such as the grooves  439 ,  440 , for limiting the upward travel of the guide tube. Toward a lower portion  441  of the guide tube, there are two circumferentially extending slots  442 ,  443 . The slots each extend about an arc of about forty-five degrees. As will be explained below, the locking element  410  extends through the slots to make engagement with the notches  406 . When engagement is made, the guide tube  407  and the attached armrest  408  are locked relative to the support  400 . The slots  442 ,  443  also enable the locking element to rotate out of engagement with the notches through the forty-five degree arcs and thereby disengage the guide tube from the support allowing the guide tube to be moved vertically within the support opening  403 . In this manner the armrest may be vertically adjusted. 
     Mounted to the guide tube  407  is the activator nut  412 , FIGS. 33,  34 ,  36 ,  38  and  40 . The activator nut includes an annular flange  444 , having a spring retaining surface  445  and key slots, of which one key slot  446  is shown in FIG. 10, to accommodate the keys  436 ,  437  of the guide tube. The actuator nut also includes a top surface  447  to engage the lever  414  and a central threaded opening  448 . The threaded opening engages the rod  409  causing the rod to rotate. 
     Mounted within the guide tube is the elongated rod  409 , FIGS. 38,  39  and  41 . The rod is mounted for rotational movement only and does not slide longitudinally in relation to the guide tube. At an upper end portion  411  of the rod is an external screw thread  449  which engages the internal screw thread  448  of the activator nut. In the lower portion of the rod is a snap fit connector  450  for engaging the locking element  410 . The locking element has opposed extending arms  451 ,  452  and a central opening  453 . The locking element central opening receives the snap fit connector of the rod, which deforms and then snaps back into place to make a connection. 
     The spring  413  is mounted within the annular groove  435  at the upper end portion  432  of the guide tube  407 . An upper end of the spring bears against the surface  445  of the annular flange  444 . Because of the keys and key slots, the activator nut moves between raised and lowered positions in a linear fashion as shown in FIG.  35 . When the activator nut is moved to its lowered position, the spring is compressed and forms a biasing force against the activator nut tending to return it to its raised position. 
     Referring now to FIGS. 42-44, the armrest base  415  is shown in more detail. The base is an integral element having a large oval opening  454  at an extended end portion to accommodate a touch pad  455 , FIGS. 33 and 48, of the operating lever  414 . At the opposite end of the armrest base is a cuplike structure including a bottom bearing surface  456  and a downward projecting flange  457  forming a vertical bearing surface  458  for rotation about the shroud. The armrest base also includes a cup upper surface  459 . The upper surface  459  includes two sets of recesses  460 ,  461 , each in a curved format. These recesses form a detent with the cap  416  as will be explained below. The cup bottom also includes an opening having a central portion  462  and two end portions  463 ,  464 . The end portions are curved slots to accommodate two fasteners  465 ,  466 , FIG. 4, allowing the base to pivot until the ends of the slots abut the fasteners. The central portion  462  allows the lever to engage the top surface  447  of the activator nut  412 . 
     The cap  416  includes two side openings  467 ,  468 , FIGS. 45-47, for receiving the fasteners  465 ,  466  and a central opening  469  for passing the lever. The side openings are aligned with the end portion slots  463 ,  464 . The cap also includes two sets of resiliently mounted tabs  470 ,  471 . The tabs align with and engage the two sets of curved recesses  460 ,  461  on the armrest base to form a series of detents. This allows horizontal movement of the armrest base from one detent to another within about a forty-five degree arc determined by the arcs of the end portion slots  463 ,  464  and depending shoulders  472 ,  473  surrounding each opening  467 ,  468 , respectively. 
     Mounted to the armrest base  415  is the lever  414  which extends from the oval opening  454  at one end of the armrest base to the central opening  462  at the other end portion of the armrest base. At the extended end of the lever is the touch pad  455 , FIG. 48, while at the other end is a depending projection  474 , FIGS. 35 and 48, that makes contact with the upper surface  447  of the activator nut  412 . The lever is mounted to pivot about pivot point  475  such that an upward force on the touch pad  455  causes the projection  474  to move downwardly. The downward movement of the lever projection causes the activator nut to move from its raised position to its lowered position thereby causing the rod  409  to rotate and disengage the locking element  410  from the notches  406 . 
     Referring now to FIGS. 49 and 50, the pivoting movement of the locking element  410  is clearly shown. In FIG. 49 the armrest is locked with the locking element engaging the notches  406 . When the lever is activated, the locking element is pivoted out of engagement with the notches as shown in FIG.  50 . 
     It can now be appreciated that the armrest assembly is simply constructed, easy to form and assemble and easy to use. In operation, the relative vertical positioning of the armrest, the guide tube and the connected shroud to the upstanding support and the notched liner racks determines the height of the armrest relative to the seat of the chair. Usually the armrest is locked by the locking element engaging a pair of notches. Depressing the lever touch pad causes the projection end to bear down on the top surface of the activator nut. Since the nut cannot rotate, it is depressed causing the threadedly engaged rod to rotate. Rotation of the rod causes the locking element to rotate 45 degrees out of engagement with the pair of notches. The armrest may then be manually adjusted upwardly or downwardly. Once the force on the lever is released, the spring mounted to the guide tube causes the actuator nut to return to its raised position. This linear movement of the activator nut causes reverse rotation of the rod and the locking element causing the locking element to engage a new pair of notches. When this occurs, the armrest is locked in its new position. 
     Adjusting the armrest in a horizontal direction requires only the movement of the armrest to pivot it outwardly or inwardly within an arc of about 45 degrees. The resiliently mounted tabs of the cap move from one pair of recesses to another pair in the armrest base. This detent mechanism allows the armrest to pivot between six discreet positions. Movement occurs when the force on the armrest is sufficient to move the resilient tabs out of engagement with a pair of recesses. 
     What has been described is a simply constructed and reliable armrest assembly that is adjustable both vertically and horizontally. These adjustments may be easily made through simple manipulation of portions of the armrest assembly. 
     It can now be appreciated that a chair  10  constructed according to the invention offers considerable advantages in user comfort by virtue of its synchronous linkage construction particularly where it is used for prolonged periods of time. The chair  10  is also cost effective to manufacture and assemble. 
     While the present invention has been described in connection with a preferred embodiment, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from the true spirit and scope of the present invention. Accordingly, it is intended by the appended claims to cover all such changes and modifications as come within the spirit and scope of the invention.