Abstract:
A “task” chair intended primarily for a person working at a computer terminal has a back provided with a pair of upright struts that incorporate respective parallelogram linkages operative to increase the curvature of the lumbar region of the seat back while at least substantially maintaining the contour of the upper region of the backrest when a person leans back in the chair.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates generally to chairs and is concerned in particular with the chair backs that are designed in accordance with ergonomic principles.  
       BACKGROUND OF THE INVENTION  
       [0002]     The invention has been devised in the context of chair designs that are intended to address problems with respect to the posture and comfort of individuals who are required to spend long periods of time in a seated position, for example, working at a computer. However, the invention is not limited in this respect.  
         [0003]     In modern office environments, people who work at computers naturally adjust their seating positions from time to time, for example, by leaning back in their chairs. Some people even work leaning back. Such a sitting posture places significant strain on the upper thorax and neck regions of the person. In other words, if the person leans back, while continuing to keep their eyes on the computer screen, they must inevitably compensate by tilting their head forward, causing strain in the neck and/or upper thorax region.  
         [0004]     An object of the present invention is to address this problem by providing an improved chair back structure.  
       SUMMARY OF THE INVENTION  
       [0005]     A chair in accordance with the invention comprises a base, a seat supported on the base and a back coupled to at least one of the seat and base. The back includes a backrest having a normal contour which includes a lower, lumbar region for contact by the lumbar area of a person seated in the chair and which is forwardly curved, and an upper region for contact by the upper thorax of the person. The backrest is biased to the normal contour and is flexible to vary the contour. The back further includes at least one upright strut spanning the lumbar and upper thorax regions of the backrest, the strut having a lower end which is fixed with respect to the seat and an upper end which is coupled to the upper region of the backrest for back and forward movement therewith in response to rearward pressure exerted by said person leaning back against the backrest, and return to said normal contour. The strut includes a parallelogram linkage operative to increase the curvature of the lumbar region while at least substantially maintaining the contour of the upper region of the backrest in response to said rearward pressure.  
         [0006]     In other words, as the person leans back, the contour of the backrest changes to support the lumbar area and the upper thorax and/or neck of the person and thereby at least partly alleviate the strain on the upper thorax and neck region that would otherwise occur, for example, when the person leans back while continuing to look at a computer screen as discussed previously.  
         [0007]     Preferably, the normal contour of the backrest follows a gentle S-shaped curve in the upright direction of the backrest (top to bottom) including a forwardly curved lumbar region and an upper region in which the top of the backrest curves slightly forwardly so as to tend to augment the support function in the upper thorax and neck region.  
         [0008]     While a single strut may be provided, for example, generally centrally of the backrest, a pair of struts preferably is provided, one at each side of the backrest. The strut or struts may be external to the backrest and therefore visible in use, or may be incorporated into the backrest itself or into a supplementary cover at the rear of the backrest. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     In order that the invention may be more clearly understood, reference will now be made to the accompanying drawings which illustrate a particular preferred embodiment of the invention by way of example, and in which:  
         [0010]      FIG. 1  is a three-quarter perspective view from the rear and to one side of a chair in accordance with the invention;  
         [0011]      FIG. 2  is a vertical sectional view generally on the centreline of the back of the chair in its normal rest position;  
         [0012]      FIG. 3  is a view similar to  FIG. 2  showing the contour of the backrest when a person leans back in the chair;  
         [0013]      FIG. 4  is an exploded perspective view illustrating the struts at the rear of the backrest; and,  
         [0014]      FIG. 5  is a perspective view illustrating a parallelogram linkage comprising one of the struts. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]     Referring first to  FIG. 1 , the invention is illustrated in the context of a typical office or “task” chair generally denoted  20 . It is, however, to be understood that the invention may equally well be applied to the backs of other types of chair including so-called sit/stand chairs of the type disclosed in U.S. Pat. No. 6,752,459 (Deisig).  
         [0016]     With continued reference to  FIG. 1 , chair  20  includes a base  22  (only part of which is shown) which supports a seat  24  of the chair. The chair also includes a back  26  that is supported from the seat by a pair of L-shaped arm structures  28 . These structures include respective struts  30  which are coupled to the seat back as will be described later, and a pair of forwardly extending arms  32  that are secured to support structure  24   a  of the seat. Though not shown in detail, a conventional tilt mechanism is provided between the base  22  and the seat  24  so that the seat  24  and back  26  can tilt as a unit with respect to the base. The base itself is also conventional and has therefore not been shown in detail. As shown, the base includes a column  22   a  that supports the seat for adjustment in the vertical direction. A conventional “spider”-type base (not shown) is provided at the bottom of column  22 .  
         [0017]     Referring now more particularly to  FIGS. 2 and 3 , the back  26  includes a backrest  34  for contact by the back of a person sitting on the seat  24 . The backrest has a contour which follows a generally S-shaped curve as seen in vertical section. That shape continues outwardly to the sides of the back. The backrest may also be curved laterally to partially wrap around the torso of a person seated on the chair. The backrest includes a lower region  34   a  which is forwardly curved in vertical section for contact by the lumbar area of the person, and an upper region  34   b  for contact by the upper thorax area of the person. The backrest is flexible so that the contour can be varied under the control of the struts  30  ( FIG. 1 ) as will be described in more detail later.  
         [0018]     In this particular embodiment, the backrest  34  is a polypropylene moulding, the normal “as moulded” shape of which is seen in  FIG. 2 . The moulding is flexible as indicated previously and has spring-like characteristics so that it always tends to return to the normal shape shown in  FIG. 2 . The moulding has a solid back wall  36  which carries a resilient moulded cushion structure  38  for contact with the back of a person using the chair. A cover  40  is provided over the moulding  34  to provide an appropriate aesthetically pleasing external appearance. In the embodiment illustrated in  FIG. 4 , the struts  30  are shown external and therefore visible at the exterior of the chair. In an alternative embodiment, the struts could be incorporated within the backrest or enclosed within the cover  40 .  
         [0019]     Reverting to  FIG. 2 , the two struts  30  of the chair back are identical and a representative one is shown in  FIG. 2 . Each strut spans substantially the entire height of the back including the lumbar region  34   a  and the thorax region  34   b . The strut has upper and lower ends  30   a  and  30   b  respectively, which are coupled to the backrest  34 . The struts are adapted to vary the contour of the profile of the backrest in response to rearward pressure exerted generally in the thorax region  34   b  by a person leaning back in the chair. The struts confine the backrest to change its contour from the configuration shown in  FIG. 2  to the configuration shown in  FIG. 3 . In  FIG. 2 , the front face of the backrest is indicated in dotted lines at  34 ′ to show the contour of the backrest in the position of  FIG. 3 .  
         [0020]     As noted previously in referring to  FIG. 1 , the two struts  30  form part of respective arm structures  28  that connect to the seat. Each arm structure includes a fixed upright portion  42  which extends upwardly behind the backrest  34  of the seat, approximately to the position of the lumbar region  34   a . The upright portion  42  of one of the struts is shown in detail in  FIGS. 2 and 3  and in exploded perspective in  FIG. 4 . The strut  30  extends upwardly from the upper end of this portion  42 . Each strut incorporates a parallelogram linkage comprising first and second links  44 ,  46  that are pivoted to arm portion  44  at respective pivot points  44   a  and  46   a . The links extend upwardly behind the backrest to a fitting  48  adjacent the upper margin of the backrest  34 , where the links are pivoted to the fitting by corresponding upper pivot axes  44   b  and  46   b  respectively. Fitting  48  is rigidly secured to the backrest  34  by a pair of bolts  50  threaded into a plate  52  at the front face of backrest wall  36 . Similarly, the upper portion  42  of the arm structure  28  is bolted to a lower region of the backrest by a pair of bolts  54  that are threaded into a plate  56  at the front face of wall  36 .  
         [0021]      FIG. 5  shows the two links  44  and  46  in isolation. Link  44  is an inner link (closest to the back) and comprises a flat plate with angled portions at respective ends. Link  46  is an outer link and is wider than link  44  and provided with a slot  58  that receives link  44 , so that the two links effectively “nest” one within the other. The respective pivot points for link  44  are represented in  FIG. 5  by plain holes  44   a  and  44   b  through link  44 , while the corresponding pivot points for link  46  are provided by passageways through respective upper and lower bosses  60  and  62  formed as part of link  46 .  
         [0022]      FIG. 4  shows in detail the components of the struts and arm structures in exploded positions. It will be seen that these components include a shroud  64  which, in the assembled chair (see  FIG. 1 ) covers and encloses the links  44 ,  46 , the upper fitting  48  and arm upper portion  42 . The shroud is appropriately finished to provide a decorative appearance.  
         [0023]     Reverting to  FIG. 2 , it will be seen that the parallelogram linkage provided by the two links  44  and  46  and the respective pivot points for those links are configured so that, when a person seated on a chair leans back and applies pressure to the backrest generally in the region of the thorax area  34   b , the links  44 ,  46  move with respect to one another to change the profile of the backrest so that the contour of the lumbar region  34   a  increases in curvature while the upper portion of the backrest in the thorax region  34   b  remains flat and moves rearwardly while remaining generally parallel to its normal rest position, instead of simply inching rearwardly as it otherwise would. This supports the upper thorax, shoulder and neck regions of the person against strains that would otherwise occur as the person attempts to, for example, continue looking at a computer screen while leaning back in the chair.  
         [0024]     Applicant does not intend to be bound by theory. At the same time, it is believed that the invention is based on medically sound principles of emphasing kyphosis in the thorax area of the person to compensate for lordosis in the lumbar area. Lordosis is defined as forward curvature of the spine and kyphosis is rearward curvature, both following the slightly S-shaped curvature of a healthy spine. In a healthy spine, the portion above about the sixth thorax vertebra tends to tilt forward. This tilt is accentuated or supported by the backrest contour that is achieved in accordance with the present invention.  
         [0025]     The precise changes in contour that occur of course depend on the relative lengths of the two links  44 ,  46  and on the relative positions of their pivot points. For example, in the illustrated embodiment, shortening the inner link would have the effect of tending to cause some forward curvature of thorax region  34   b  of the backrest, tending to increase support in that area.  
         [0026]     It will of course be appreciated that the preceding description relates to a particular preferred embodiment of the invention and that a number of modifications are possible, some of which have been indicated previously, while others will be apparent to a person skilled in the art. Obviously, the particular design and configuration of the parallelogram links may change. The manner in which the struts are coupled to the seat of the chair may also change. In the illustrated embodiment, each strut  30  is coupled to the seat support structure  24   a  of the chair seat by an arm  32  that extends forwardly from the lower end of the strut. The strut itself is then coupled to upper and lower regions of the seat back as shown in  FIGS. 2 and 3 . However, this arrangement may vary. As a minimum, the lower end of the strut should be coupled to the seat, either directly, for example, as shown, or via the lower region of the seat back where the back is coupled relatively rigidly to the seat. It is of course important that the lower end of the strut be held firmly with respect to the seat to provide a fixed point against which a person on the seat can push in leaning back.