Patent Publication Number: US-7594700-B2

Title: Contoured seating structure

Description:
RELATION TO OTHER APPLICATIONS 
   This application is a continuation of U.S. patent application Ser. No. 10/772,935, filed Feb. 5, 2004 now U.S.Pat. No. 6,966,604, which is a of continuation of U.S. patent application Ser. No. 10/256,671, filed Sep. 27, 2002 now U.S.Pat. No. 6,722,741, which is a continuation of U.S. patent application Ser. No. 09/859,694, filed May 17, 2001 now U.S.Pat. No. 6,588,842 which is a continuation of U.S. patent application Ser. No. 08/347,475, filed Nov. 17, 1995 now U.S.Pat. No. 6,386,634 which was the National Stage of International Application No. PCT/US93/05731, filed Jun. 15, 1993, which is a continuation-in-part of U.S. patent application Ser. No. 07/898,907, filed Jun. 15, 1992 now abandoned, the entire disclosures of which are incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   The present invention relates generally to office chairs, and more particularly, to a height-adjustable, tiltable office chair which supports the body of a user in ergonomically desirable positions for performing various tasks. 
   Office chairs are typically configured to allow tilting of the seat and backrest as a unit or tilting of the backrest relative to the seat. In chairs having a backrest pivotally attached to a seat in a conventional manner, the movement of the backrest relative to the seat can create shear forces acting on the legs and back of the user. These shear forces tend to cause an uncomfortable pulling of the user&#39;s clothing. In an attempt to compensate for these shear forces, some office chairs include a backrest which pivots while the seat tilts, such as those disclosed in U.S. Pat. No. 2,859,801 (Moore) and U.S. Pat. No. 4,429,917 (Diffrient). To provide a chair which naturally conforms to the pivotal movement of both the legs and trunk of a user between tilt positions, it is desirable to provide a chair having a seat and backrest which pivot generally about the axis of the hip joints of the user. 
   To further ensure comfortable tilting between tilt positions and enhance the comfort of a user while in a given tilt position, it is desirable to provide a chair having a tilt mechanism with an effective pivot point about the ankles of the user. Such an ankle tilt feature decreases the effort required to tilt the chair, reduces the pressure of a forward edge of the seat acting on the underside of a user&#39;s leg, and allows the feet of the user to remain flat on a floor. 
   Although some tilting chairs have incorporated such an ankle tilt feature, none have comprehensively addressed the overall body posture and relative positioning of body parts for ensuring comfort and minimizing fatigue regardless how intensely a user works. In most office environments, a worker performs several tasks such as writing at a desk, dictating, using the telephone, or typing at a video display terminal (VDT). Not only do such tasks vary in the inherent intensity of the work being performed, but an individual may also desire to increase or decrease the work intensity of a given task. As a result, the optimal position of the body for ensuring comfort and minimizing fatigue also varies. Thus, it is desirable to provide a chair which automatically supports the body parts of a user in ergonomically optimal positions for performing intensive, moderate, or relaxing modes of work. It is also desirable to provide adjustable armrests for positioning the arms of a user in optimal locations for various tasks regardless of the size and shape of a user. 
   A related disadvantage of conventional office chairs is the configuration and material of the seat and/or backrest. Such seats typically include single or multi-density foam padding with a covering such as cloth, leather or the like. This type of seating provides a deformable cushion which conforms to the user&#39;s buttocks. However, a deformable cushion does not provide a self-adjusting support which varies according to the position of the user and the tilt position of the seat. Such seating also tends to provide insufficient aeration since it acts as another layer of clothing. In chairs incorporating flexible membranes, the membranes are typically attached directly to the frame of a seat. Often the membrane is attached to the frame by wrapping edge portions of the membrane around spaced apart rods which define the frame. The membrane of such a seat is difficult to repair and/or replace since the chair would typically have to be disassembled to allow such maintenance. In addition, the structural requirements of such an attachment limits the shape and size of the frame and the membrane. 
   Typically, the seats of office chairs are supported by a single stage telescoping column which provides for vertical adjustment of the seat. These columns include a gas spring mounted in a telescoping tube which is slidable within a base tube. In accordance with guidelines set by the American National Standards Institute (A.N.S.I.) and Business and Institutional Furniture Manufacturer&#39;s Association (B.I.F.M.A.), conventional office chairs in the United States are typically adjustable from a seat height of about 16.0 inches from a floor to about 20.5 inches from a floor. Nevertheless, it is desirable to exceed this range of height adjustment to account for very small or large users and to accommodate the international population in general. 
   Typically, it is difficult to exceed this range of height adjustment with seats which tilt about the knees or ankles of a user. To offset the moments acting on single stage support columns, pneumatic manufacturers typically set a minimum overlapping distance of 2.95 inches (75 mm) between the tubes. Because such “ankle tilt” and “knee tilt” chairs have relatively large tilt housings, it is difficult to provide a lower minimum and higher maximum seat height while maintaining the required overlapping distance between the tubes. These types of tilting chairs also impart a greater moment on the tubes since the pivot axis is offset from the support column. It is therefore desirable to provide a vertically adjustable support column having a greater overlapping distance to permit a greater stroke which decreases the minimum height and increases the maximum height of a chair seat. 
   SUMMARY OF THE INVENTION 
   Briefly stated, the invention is directed to an office chair having a seat, a back and a pair of armrests which support the body of a user in ergonomically desirable positions for performing various tasks. 
   In one aspect of the invention, a linkage assembly is adapted to allow the seat and back to tilt downwardly and rearwardly and to allow pivotal movement of the seat about a pivot axis in substantial alignment with the hip joints of a user to inhibit shear forces from pulling the clothing on the body of a user. 
   In another aspect of the invention, the linkage assembly is adapted to allow the seat and back to tilt downwardly and rearwardly such that the seat pivots about an effective pivot point at substantially the ankle of a user having feet resting on a floor. 
   In yet another aspect of the invention, a vertically adjustable column includes an outer guide tube, an intermediate telescoping tube slidably positioned within the outer guide tube, and an inner telescoping tube slidably positioned within the intermediate tube. The outer guide tube is mounted to a support stand and the inner tube has an upper portion thereof connected to a seat support member. A gas spring is provided and includes a cylinder mounted within the inner tube, a piston rod extending outwardly from the cylinder and having an end connected to a bottom portion of the outer guide member, and a control pin assembly mounted to an upper portion of the cylinder for operable engagement with an actuator member. The piston rod is extensible between a collapsed position in which the cylinder and inner tube are substantially within the intermediate tube and the intermediate tube is substantially within the outer guide tube, and a raised position in which a portion of the cylinder and inner tube extend outwardly from the intermediate tube and a portion of the intermediate tube extends outwardly from the outer guide tube. 
   In another aspect of the invention, the seat and back of the chair include a frame member having an central opening therethrough and a receptacle formed around the perimeter thereof. A membrane of elastic material covers the central opening and is adapted to be attached to a carrier member which fits inside the receptacle. 
   In yet another aspect of the invention, a pair of armrests are adapted to be mounted to the chair to allow pivotal movement thereof in a plane substantially perpendicular to the general plane of the back to accommodate the angle at which the forearms of a user are positioned. 
   In a preferred embodiment of the invention, the linkage assembly includes a pair of first inks having an end pivotally attached to the lateral portions of the seat to define a pivot axis in substantial alignment with the hip joints of a user. A pair of second links have a first section rigidly connected to the first links and the back and a second section angled downwardly from the first section. The second section terminates in an end pivotally mounted to a forward portion of the base member. A restraining link has one end pivotally attached to a rear portion of the seat and another end pivotally attached to the base member to limit tilting of the seat. The first links, second links, and restraining links are configured to create an effective pivot point of the seat at substantially the ankles of a user having feet resting on a floor. Thus, the seat pivots about the hip axis of a user such that the angle therebetween increases as the back and seat tilt rearwardly. 
   The chair preferably includes a bowed section to support the lumbar region of a back of a user. The greater rearward tilting of the back relative to the seat automatically lowers the bowed section to accommodate the changing position of the lumbar region of the user. 
   Preferably, the chair also includes a height adjustable, flexible strap member positioned horizontally across the back at approximately the lumbar region of the back of a user. The strap member is also laterally adjustable to provide a desired tension for supporting the back of the user. The armrests are preferably adapted to tilt with the back such that the angle therebetween remains constant during tilting. Preferably, the armrests are height adjustable relative to the back frame by a pawl and rack mechanism. 
   In addition, the membrane of elastic material is preferably pre-stretched prior to insertion of the carrier member in the receptacle to accommodate for the contour of the body of a user. The carrier member is preferably of one-piece construction and is adapted to be press-fit in the receptacle, and the edge portion of the membrane is preferably molded with the carrier member. The carrier member can also be configured to snap-fit into the frame member receptacle, and the edge portion of the membrane can be welded to the carrier member. 
   The embodiments of the present invention provide significant advantages over other tiltable office chairs. For example, in the most preferred embodiment, the seat and back naturally conform to the body of a user during tilting of the chair to thereby reduce shear forces acting on the thighs and trunk of the user and minimize pressure acting on the underside of the user&#39;s thighs at the knees. This chair also automatically supports the body of the user in ergonomically desirable positions for performing tasks of varying intensity. The user&#39;s weight is distributed evenly and shifts of the center of gravity are compensated for to allow the user to maintain equilibrium and good balance. Thus, the user&#39;s body is positioned to reduce fatigue, allow proper alignment for performing various tasks, achieve maximum alertness or relaxation, and ensure overall comfort. Moreover, the range of vertical adjustment of the chair is increased to allow a lower minimum height and higher maximum height than conventional office chairs. 
   The present invention, together with further objects and advantages, will be best understood by reference to the following detailed description taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a perspective view of a preferred embodiment of a chair having a backrest, a seat, and a pair of armrests. 
       FIG. 2  is a front view, of the chair. 
       FIG. 3  is a right side view of the chair. 
       FIG. 4  is a left side view of the chair. 
       FIG. 5  is a rear view of the chair. 
       FIG. 6  is a top view of the chair. 
       FIG. 7  is a bottom view of the chair. 
       FIG. 8  is a side view of the chair showing the seat and backrest in a forward tilt position. 
       FIG. 9  is a side view of the chair showing the seat and backrest in a reclined tilt position. 
       FIG. 10  is a side view of the chair showing the seat and backrest in preferred forward, middle, and reclined tilt positions. 
       FIG. 11A  is a fragmentary side view of the chair in a raised position showing a preferred embodiment of a column assembly in cross-section. 
       FIG. 11B  is a fragmentary side view of the chair in a lowered position showing the column assembly in  FIG. 11A  in a collapsed position. 
       FIG. 12A  is a fragmentary side view of the chair in a raised position showing an alternative embodiment of the column assembly in cross-section. 
       FIG. 12B  is a fragmentary side view of the chair in a lowered position showing the column assembly in  FIG. 12A  in a collapsed position. 
       FIG. 13  is a cross-sectional view of yet another embodiment of the column assembly shown in  FIGS. 11A and 11B . 
       FIG. 14  is a top view of the chair showing the armrests in various pivoted positions indicated by broken lines. 
       FIG. 15  is a fragmentary perspective view of the chair showing an exploded view of an armrest assembly, a backrest frame and a linkage assembly. 
       FIGS. 16 and 17  are exploded top views of an alternative embodiment of an armrest assembly. 
       FIG. 18  is a cross-sectional view of the armrest assembly shown in  FIGS. 16 and 17 . 
       FIG. 19  is a fragmentary rear view of an alternative embodiment of the backrest and a brace member. 
       FIG. 20  is a cross-sectional view of the backrest and brace member shown in  FIG. 19 . 
       FIG. 21  is a front view of a tilt control mechanism with various portions removed for clarity. 
       FIG. 22  is a cross-sectional view of the tilt control mechanism taken along the line  22 - 22  in  FIG. 21 . 
       FIG. 23  is a top view of the tilt control mechanism shown in  FIGS. 21 and 22  with various portions removed for clarity. 
       FIGS. 24 and 25  are side views of the tilt control mechanism showing a rearward tilt limiter mechanism. 
       FIGS. 26 and 27  are side views of the tilt control mechanism showing a forward tilt limiter mechanism. 
       FIGS. 28 and 29  are cross-sectional views of mechanisms for actuating the tilt control mechanisms. 
       FIG. 30  is an exploded perspective view of the seat. 
       FIG. 31  is a perspective view of the seat shown in  FIG. 30 . 
       FIG. 32  is a cross-sectional view of the seat taken along the line  32 - 32  in  FIG. 31 . 
       FIG. 33  is a cross-sectional view of the seat shown in  FIG. 31 . 
       FIG. 34  is a cross-sectional view of the seat taken along the line  34 - 34  in  FIG. 32 . 
       FIG. 35  is a cross-sectional view of the seat shown in  FIG. 31 . 
       FIG. 36  is an exploded fragmentary view of a seat membrane. 
       FIGS. 37 and 38  are cross-sectional views of the membrane taken along the lines  37 - 37  and  38 - 38  in  FIG. 36 . 
       FIG. 39  is a top view of upper and lower loom members and clamp members clamped to the seat membrane. 
       FIG. 40  is a cross-sectional view of the loom members, clamp members, and membrane taken along the line  40 - 40  in  FIG. 39 . 
       FIG. 41  is a top view of a lower mold member. 
       FIG. 42  is a top view of the loom member and membrane loaded in the lower mold member. 
       FIG. 43  is a cross-sectional view of the loom member, membrane and mold members taken along the line  43 - 43  in  FIG. 42  and showing the mold members prior to closure. 
       FIG. 44  is a cross-sectional view of the mold members in  FIG. 43  shown closed against the loom member and membrane. 
       FIG. 45  is a cross-sectional view of the loom member, membrane and mold members taken along the line  45 - 45  in  FIG. 42  and showing the mold members prior to closure. 
       FIG. 46  is a cross-sectional view of the mold members in  FIG. 45  shown closed against the loom member and membrane. 
       FIG. 47  is an exploded cross-sectional view of the closed mold members in  FIGS. 44 and 46  showing a cavity and the membrane. 
       FIG. 48  is a cross-sectional view of the mold members and membrane taken along the line  48 - 48  in  FIG. 47 . 
       FIG. 49  is a perspective view of an alternative embodiment of the seat. 
       FIG. 50  is an exploded, cross-sectional view of the seat shown in  FIG. 49 . 
       FIG. 51  is a perspective view of another alternative embodiment of the seat. 
       FIG. 52  is a cross-sectional view of the seat shown in  FIG. 51 . 
       FIG. 53  is a perspective view of another alternative embodiment of the seat. 
       FIGS. 54-56  are cross-sectional views of the seat shown in  FIG. 53  taken along the lines  54 - 54 ,  55 - 55 , and  56 - 56  in  FIG. 26 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to the drawings,  FIGS. 1-7  show a preferred embodiment of a chair  30  in a middle tilt position. The chair  30  includes a seat  32 , a backrest  34 , and a pair of armrest assemblies  36 . The seat  32  and backrest  34  are connected to a tilt control housing  38  by a linkage assembly  40 . The tilt control housing  38  is mounted on a vertically adjustable, dual stage support column  42  which is secured to the center of a pedestal  44 . The pedestal  44  is movably supported on a floor  46  by a plurality of casters  48  or the like. 
   In a preferred embodiment of the invention, the linkage assembly  40  includes a pair of first links  50  pivotally attached to upwardly extending side portions  52  of a seat frame  33  at pivot points  54  to define a pivot axis at substantially the hip joints of a user. A pair of second links  56  each have a substantially straight first section  58  to which the first links  50  are fixedly attached and a second section  60  angled downwardly from the first section  58 . An upwardly extending rear end portion  62  of each first section  58  is connected to a frame  64  of the backrest  34 , and an inwardly extending front end portion  66  of each second section  60  is pivotally mounted to a forward portion of the tilt control housing  38 . Thus, the rigidly connected first links  50  and second links  56  form a forward link member and act as one bar of a four bar linkage which creates an effective pivot point  68  at substantially the ankles of the user having feet resting on the floor  46 . 
   As best shown in  FIGS. 8-10 , the seat  32  and backrest  34  both pivot about the hip pivot points  54  while simultaneously tilting rearwardly. To limit tilting of the seat  32 , linkage assembly  40  includes a pair of restraining links  70  which form a four bar linkage in conjunction with the first links  50  and second links  56 , the seat  32  and the tilt control housing  38 . The restraining links  70  have one end  72  pivotally attached to a front portion of the tilt control housing  38  rearwardly and below the attachment of the end portions  66  of the links  56  to the housing  38 . Another end  74  of the restraining links  70  is pivotally attached to a corresponding clevis  76  extending downwardly from a rear edge of the seat  32 . Thus, the seat  32  pivots about hip pivot point  54  since it is pivotally mounted to the first links  50 , and the backrest  34  pivots about the same pivot point  54  since the second links  56  are fixedly connected to the first links  50 . In addition, the seat  32  and backrest  34  simultaneously pivot about pivot point  66 , and the restraining links  70  cause the seat  32  to pivot about the effective pivot axis  68  at the ankles of a user. 
   One advantage of the foregoing chair configuration is the minimizing of shear forces acting on the clothing worn on the legs and torso of the user as the user tilts between various positions. Since the legs and torso of a user naturally pivot about the hip joints, and both the seat  32  and backrest  34  pivot about the same pivot axis  54 , the clothing of the user will not pull when changing tilt positions. Although such tilting of the seat  32  and backrest  34  is described in conjunction with a seat which pivots about the ankles of a user, the seat  32  and backrest  34  can be adapted to pivot about other axes. For example, the second links  56  can be configured such that a front end portion is pivotally mounted to the tilt housing assembly  38  directly above the support column  42  to provide a conventional “knuckle tilt”. The second links  56  can also be rigidly attached to the first links  50  and the fit links  50  can be pivotally mounted to the tilt control housing  38  at a desired location. 
   The comfort of a user while tilting between various tilt positions is also enhanced by the ankle tilt feature of the invention. Since the seat  32  tilts rearwardly about the ankles of a user, the user can tilt rearwardly with little effort without lifting the feet off the floor. The configuration of the linkage assembly  40  also allows tilting of the seat  32  such that a forward edge portion  78  moves rearwardly without rising a substantial amount to minimize the pressure on the underside of a user&#39;s thighs at the knees. 
   Another advantage of the invention is the positioning of the body of the user in ergonomically desirable postures regardless of the task being performed or the intensity at which a user works. To this end, the seat  32  and backrest  34  are tiltable between at least a forward, middle, and reclined tilt position corresponding to an intense, normal, and relaxed mode of work. 
   To increase attentiveness and minimize fatigue in a work intensive mode, it is desirable for the chair  30  to be tilted forwardly as shown in  FIGS. 8 and 10 . In this position, the seat  32  is tilted forwardly at an angle of approximately 6° from the floor  46  and the angle between the seat  32  and backrest  34  is approximately 95°. This tilt position supports the body of a user in a slightly leaning forward posture in which the user&#39;s feet are flat on the floor, the angle between the trunk and thighs is greater than 90°, the spinal centerline is approximately perpendicular to the floor  46 , and the head of the user is either perpendicular to the floor or slightly bent downward and forward. The forward tilt position tends to elevate blood pressure which enhances reticular formation of the eyes and causes a state of alertness desirable for performing work-intensive tasks. The open angle between the thighs and trunk of a user also enhances breathing to further enhance alertness. 
   To ensure comfort and minimize fatigue while maintaining the body in an attentive posture for normal modes of work, the chair  30  is tiltable to a middle position as illustrated in  FIGS. 1-7  and shown in broken lines in  FIG. 10 . In this position, the seat  32  is substantially horizontal and the angle between the backrest and seat is approximately 104°. Thus, the body of user is supported in a slightly leaned back position in which the user&#39;s feet are flat on the floor, the angle between the trunk and thighs is increased, and the head of the user is perpendicular to the floor. 
   In a reclined tilt position ( FIGS. 9 and 10 ), the seat  32  is tilted rearwardly at an angle approximately 11° from the floor  46  and the angle between the backrest and seat is approximately 108°. The body of a user is supported in a leaned back position in which the feet are flat on the floor and further away from the chair  30  to open up the angle between the calf and thighs of the user. The angle between the trunk and thighs of a user is increased and the head of the user remains perpendicular to the floor. The trunk of the user is also angled back to the point where a substantial weight shift occurs away from the buttocks to the spine, thereby relieving pressure and compression on the spinal discs. 
   To obtain the foregoing positions of the seat  32  relative to the backrest  34 , the first links  50 , second links  56 , and restraining links  70  are configured to allow the angle between the seat  32  and backrest  34  to increase as the seat and backrest are tilted rearwardly ( FIGS. 8-10 ). Preferably, the angle between the backrest  34  and the floor  46  increases at a greater rate than the angle between the seat  32  and the floor. To provide a desired resistance to rearward tilting of the seat  32  and backrest  34  about ankle pivot axis  68 , and further to limit the tilting of the first links  50  and backrest  34  about hip pivot points  54 , a tilt control mechanism such as a torsion or compression spring is positioned in the tilt control housing  38 . The second links  56  are biased forwardly and upwardly by the tilt control mechanism which is described in more detail below. In addition, an adjustable rearward tilt limiter mechanism is provided to vary the maximum rearward tilting of the chair  30  and a forward tilt limiter mechanism is provided to prevent forward tilting of the seat  32  past the generally horizontal middle position shown in  FIGS. 1-7 . The rearward and forward tilt limiter mechanisms will be described in more detail below. A tilt lock mechanism can also be provided to lock the chair  30  in the forward, middle, and reclined tilt positions. This can be accomplished by locking the second links  56  and backrest  34  in the desired tilt position which also prevents movement of the seat  32 . An example of this type of tilt mechanism is disclosed in U.S. Pat. No. 4,555,085 (Bauer et al.) and U.S. Pat. No. 4,099,775 (Mizelle). 
   The chair  30  is also height adjustable to position the body of a various size users in ergonomically desirable positions relative to a floor and/or worksurface. The dual stage, vertically adjustable support column  42  can be incorporated in any type of chair and is not limited to the chair  30  described herein. 
   In  FIGS. 11A and 11B , the support column  42  includes an outer guide tube  110  mounted to the pedestal  44  such that a bottom wall  112  thereof is spaced apart from the floor  46 . An intermediate telescoping tube  114  is slidably positioned within the outer guide tube  110 . The intermediate tube preferably  114  has an interior shoulder  116  and an exterior shoulder  118  at the middle of the tube to define a lower section  120  having a larger inner and outer diameter than an upper section  122 . The lower section  120  of the intermediate tube  114  slidably bears against the outer tube  110 , and when locked in a desired position, the overlapping area of the outer tube  110  and intermediate tube lower section  120  offsets any moments acting on the tubes to support a user sitting on the chair  30 . To limit the upward travel of the intermediate tube  114 , a retaining collar  124  is mounted to the top of the outer tube  110  and slidably receives the upper section  122  of the intermediate tube  114 . In a raised position, the exterior shoulder  118  of the intermediate tube  114  bears against the collar  124  of the outer tube  110 . 
   An inner telescoping tube  126  is slidably positioned within the intermediate tube  114  and has a top portion which is mounted to the tilt control housing  38 . The inner tube  126  slidably bears against the upper section  122  of the intermediate tube  114 , and when locked in a desired position, the overlapping area of the inner tube  126  and intermediate tube upper section  122  further offsets any moments acting on the tubes to support a user sitting on the chair  30 . In addition, the moment acting on the tubes is minimized because an upper edge  128  of the intermediate tube  114  is closer to the tilt housing  38  than conventional support columns, thus decreasing the moment arm acting on the tubes. To limit the upward travel of the inner tube  126 , a retaining collar  130  is mounted to a bottom edge of the inner tube  126  and slidably bears against the lower section  120  of the intermediate tube  114 . The retaining collar  130  also carries the intermediate tube  114  therewith when the inner tube  126  moves upwardly. 
   To adjust the vertical position of the chair, a conventional gas spring  132  including a pneumatic cylinder  134  is mounted within the inner tube  126 . A piston rod  136  extends outwardly from the cylinder  134  in an axial direction and has an end  138  connected to the bottom wall  112  of the outer guide tube  110 . A control pin  140  extends upwardly from a top wall of the cylinder  134  for operable engagement with a conventional actuator member (not shown). Preferably, the actuator member is activated by a control knob on the end of a cable (not shown) which is housed on the first link  50 . The piston rod  136  is extensible between a collapsed position ( FIG. 12 ) and a raised position ( FIG. 11 ). In the collapsed position, the cylinder  134  and inner tube  126  are substantially within the intermediate tube  114  and the intermediate tube  114  is substantially within the outer tube  110 . In the raised position, a portion of the cylinder  134  and inner tube  126  extends outwardly from the intermediate tube  114  and the upper section  122  of the intermediate tube  114  extends outwardly from the outer tube  110 . 
   Thus, the intermediate tube  114  provides an additional overlapping support area and decreases the moment arm which would otherwise act on the outer tube  110  to allow the tilt housing  38  and seat  32  to be raised to a greater height. The additional height obtainable by the tilt housing  38  and seat  32  because of the intermediate tube  114  also decreases the required height of the outer tube  110 . As a result, the chair  30  can be lowered to a lower as well as a higher position than conventional chairs. Preferably, the distance between the floor  46  and the bottom wall  112  of the outer tube  110  is approximately ½ inch and the height of the outer tube  110  is approximately 8½ inches to allow the tilt housing  38  to be lowered to a height of approximately 9 inches from the floor. Moreover, the stroke of the piston rod  136  is preferably about 7 inches to allow the tilt housing  38  to be raised to a height of approximately 16 inches from the floor. 
     FIGS. 12A-13  illustrate alternative embodiments of the support column  42 . Since these embodiments are similar to the previously described embodiment, similar parts appearing in  FIGS. 12A-13  are represented by the same reference numerals. Referring now to  FIGS. 12A and 12B , the intermediate tube  114 , preferably made of steel, is substantially cylindrical and radially spaced from the outer tube  110  and inner tube  126 . To guide and support the intermediate tube  114  within the outer tube  110 , a bushing  124  extends radially inward from a top portion of the outer tube  110 , and a lower bushing  111  extends radially outward from a bottom portion of the intermediate tube  114 . Thus, the bushing  124  slidably bears against an exterior surface  113  of the intermediate tube  114  and the lower bushing  111  slidably bears against an interior surface  115  of the outer tube  110  when the intermediate tube  114  moves axially within the outer tube  110 . When a user sits on the chair, the distance between the load-bearing bushings  111  and  124  defines a moment arm which acts to offset any moments acting on the intermediate tube  114 . To limit upward movement of the intermediate tube  114  within the outer tube  110 , a first spacer  117 , preferably in the form of an annular band, is positioned between the outer tube  110 , intermediate tube  114 , and bushings  111  and  124 . As illustrated in  FIG. 12B , the first spacer  117  preferably fits loosely between the tubes  110  and  114  so that it remains seated on the intermediate tube lower bushing  111  as the bushing moves downwardly with the intermediate tube  114 . 
   The inner telescoping tube  126  is likewise radially spaced from the intermediate tube  114  and is preferably cylindrical and made of steel. To support and guide the inner tube  126  within the intermediate tube  114 , a bushing  130  extends radially outward from a bottom portion of the inner tube  126 , and an upper bushing  119  extends radially inward from a top portion of the intermediate tube  114 . To guide the inner tube  126  within the intermediate tube  114 , the inner tube bushing  130  slidably bears against an interior surface  121  of the intermediate tube  114  and the intermediate tube upper bushing  119  slidably bears against an exterior surface  123  of the inner tube  126 . Thus, the distance between the load-bearing bushings  119  and  130  defines a moment arm which acts to offset any moments acting on the inner tube  126 . To limit the upward travel of the inner tube  126 , a second spacer  125 , preferably in the form of an annular band, is positioned between the inner tube  126 , intermediate tube  114 , and bushings  130  and  119 . As shown in  FIG. 12A , the maximum height of the chair is limited by the height of the first and second annular spacers  117  and  125  which bear against the bushings  111 ,  119 ,  124  and  130 . 
   Therefore, the overlapping distance between the tubes  110 ,  114  and  126 , or more particularly, the distance between the load-bearing bushings  111 ,  119 ,  124  and  130  provides more lateral support than conventional columns. As a result, the tilt housing  38  and seat  32  can be raised to a greater maximum height and lowered to a lower minimum height. Moreover, the embodiment of column  42  is less costly than conventional support columns which typically have a single telescoping tube slidably bearing against a sleeve which is mounted within a top portion of the outer tube. A relatively tight tolerance must be held between the sleeve and the telescoping tube in these columns to prevent any angular movement or swaying of the tube. Because the present invention utilizes a plurality of spaced apart, load bearing bushings positioned at the ends of the tubes, the tendency for the intermediate tube  114  and inner tube  126  to sway is reduced, thus obviating the need for such a tight tolerance. 
   Another aspect of the embodiment shown in  FIGS. 12A and 12B  is a frusto-conically shaped mounting member  127  attached to an upper portion of the outer tube  110 . The outer surface of the mounting member  127  has a relatively large taper and mates with a frusto-conically shaped cavity  129  formed in the pedestal or support stand  44  which also has a relatively large taper. Preferably, the cavity  129  is formed in a center portion  131  of the pedestal  44  and is defined by a hub  133  extending downwardly therefrom. The tapered mounting member  127  fits within an upper portion of the cavity  129 , and a lower portion of the outer tube  110  bears against an inner wall  135  of the cavity  129  at a lower portion thereof, which provides additional lateral support for the column  42 . Typically, the outer tubes of conventional support columns have a cylindrical top portion extending out of a pedestal and a slightly tapered bottom portion mounted directly to a similarly shaped cavity in the pedestal. If the bottom portion of the outer tube does not fit exactly within the cavity when assembled to the pedestal, the slight tapers may prevent the outer tube from dropping entirely within the cavity which raises the minimum height of the seat. Thus, the slightly tapered portion of conventional outer tubes typically require a tight tolerance in order to properly fit within a cavity in the pedestal. Such tight tolerances are difficult to maintain and costly. The mounting member  127  and cavity  129  obviates this problem by providing a larger taper which allows the outer tube  110  to drop entirely within the cavity  129 . In addition, the outer tube  110  does not require as tight a tolerance since the upper portion is not mounted directly to the pedestal  44 . 
   Another difficulty with conventional support columns is that the top portion of the outer tubes typically cannot be tapered since the stress acting on the bearing sleeves, which are typically mounted within the top portion of the outer tubes, tends to deform the sleeves. As discussed above, such deformation is unacceptable since a tight tolerance must be held between the sleeves and the outer tubes. As a result, the cylindrical top portions of the outer tubes often extend out of the pedestal which further reduces the minimum height of the seat. Because the column  42  does not require such tight tolerances between its components, and since a plurality of load-bearing members are movably spaced from the top portion of the outer tube  110 , the frusto-conical mounting member  127  can be mounted to the top portion of the outer tube  110  without fear of deformation which could bind the tubes. As a result, the top portion of the outer tube  110  can be positioned within the cavity  129  to further reduce the minimum height of the seat  32 . 
     FIG. 13  illustrates yet another embodiment of the support column  42  which includes a spring  142  positioned within a bottom portion of the outer guide tube  110 . The intermediate tube  114  does not have an interior shoulder for engagement with a retaining bushing to raise the intermediate tube with the inner tube  126 . Rather, the spring  142  engages a bottom edge  144  of the intermediate tube  114  to bias the intermediate tube upwardly. 
   Another aspect of the invention is the height adjustable, pivotal armrests  36 . As best shown in  FIG. 14 , the armrests  36  are pivotal about axes adjacent side edge portions of the back. The axes are positioned for approximate alignment with the elbows of a user when the user&#39;s forearms are resting on the armrests  36  to accommodate the angle at which the forearms are positioned. The armrests  36  are mounted to the backrest frame  64  to ensure proper alignment with the forearms of a user in any tilt position. 
   Referring now to  FIG. 15 , the armrest  36  includes a base plate  150  on which a pad is mounted and a support arm  152  having a substantially horizontal top plate  154 . The base plate  150  has a threaded hole  156  therein and the top plate  154  has a hole  158  therein for receiving a threaded fastener  160  which allows the base plate  150  to pivot relative to the support arm  152 . A detent member  162  extends upwardly from the top plate  154  for operable engagement with an irregular surface on the underside of the base plate  150  (not shown). The detent member  162  is biased in a direction toward the irregular surface on the base plate  150  by a spring or the like to lock the base plate  150  in a desired position. The armrests  36  are preferably pivotal approximately 20° inward and 10° outward from a vertical plane substantially normal to the general plane of the backrest  34 . Thus, the armrests  36  are pivotal to a desired angle to ensure sufficient contact with the forearms of a user to accommodate various size users and to ensure proper alignment with various work devices such as narrow keyboards or the like. 
   Again referring to  FIG. 15 , the support arm  152  includes a cavity  164  defined by spaced apart side walls  166 , an end wall  167  having a radius, and a top wall  168 . A pawl  170  is positioned in the cavity  164  for pivotal movement about a pin  172  which extends between the side walls  166  of the support arm  152 . An actuation button  174  extends downwardly from an upper end of the pawl  170  for pivoting a lower end portion  175  of the pawl  170  into and out of engagement with a plurality of teeth  176  extending outwardly from the side member  90  of backrest frame  64 . The end portion  175  of the pawl  170  is normally biased toward the teeth  176  by a spring or the like. The upwardly extending rear end portion  62  of the first link  56  is rigidly mounted to the side member  90  of backrest frame  64  by conventional fasteners (not shown) and is preferably configured as a sleeve member having a vertical slot  178  therein for slidably receiving the pawl  170 . A guide member  180  is positioned between the sleeve member  62  and the teeth  176  of the backrest frame side members  90 . The guide member  180  has a rounded surface  182  which slidably bears against the sleeve member  62  and a slot  184  therein substantially the same size and shape as the end portion  175  of the pawl  170  for receiving the end portion  175 . The guide member  180  also has a plurality of threaded holes  186  therein and the end wall  167  of the support arm  152  has threaded holes  188  therein for receiving conventional fasteners (not shown) which slidably hold the support arm  152  flush against the sleeve member  62 . 
   In operation, the actuation button  174  is depressed to pivot the pawl  170  about pin  172  to disengage the end portion  186  from the teeth  176  on the backrest frame side member  90 . The support arm  152  is slidably moved to a desired height, and the actuation button  174  is released to engage the end portion  186  of the pawl  170  with the teeth  176  and lock the support arm  152  in the new position. 
     FIGS. 16-18  illustrate an alternative embodiment of the armrests  36 . The top plate  154  of the support arm  152  has an index bushing  190  extending upwardly from a forward portion thereof and a pivot bushing  192  extending upwardly from a rear portion thereof. The base plate  150  has a hole  194  in a rear portion thereof for receiving the pivot bushing  192  and a curvilinear slot  196  therein which is positioned forwardly of the hole  194  for receiving the index bushing  190 . The base plate  150  is pivotable about the pivot bushing  192  and the curvilinear slot  196  has a constant radius about the pivot bushing  192 . To provide intermittent stops for the base plate  150 , an intermediate plate  198  is positioned between the base plate  150  and the top plate  154  of the support arm  152 . The intermediate plate  198  is attached to an underside of the base plate  150  and has a hole therein for receiving the pivot bushing  192 . The intermediate plate  198  also has an indexed slot  200  therein which underlies the slot  196  in the base plate  150  and receives the index bushing  190 . The indexed slot  200  has a centerline with the same constant radius as the slot  196 . Preferably, the slot  200  is defined by an edge  201  approximating three circular portions  202 ,  204 , and  206  having a slightly larger diameter than the index bushing  190 . The circular edge portions  202 ,  204  and  206  are configured to retain the index bushing  190  yet allow it to pass from one circular portion to the next upon the application of a lateral force on the intermediate plate  198 . Thus, the base plate  150  can be pivoted between three locked positions defined by the circular edge portions  202 ,  204  and  206  of the intermediate plate  198 . Preferably, the index bushing  190  and circular edge portions  202 ,  204  and  206  are oriented to lock the base plate  150  in a first position perpendicular to the general plane of the backrest  34 , a second position 20° inward from the first position, and a third position 10° outward from the first position as shown in  FIG. 14 . To provide other angular positions of the armrests  36 , the indexed slot  200  can be provided with a desired number of circular edge portions at desired angles relative to the pivot bushing  192 . In addition, the base plates  150  and cushions have a bowed portion  208  which provide a comfortable support for the user&#39;s forearms when the chair  30  is in any tilt position. 
   Other features of the chair  30  are provided which conform to the body of a user between tilt positions and assist in supporting the body in ergonomically desirable positions. For example, the seat  32  includes a self-adjusting elastic membrane  210  and the backrest  34  includes a similar membrane  212  for comfortably supporting the user in any tilt position. The membranes  210  and  212  and the manner in which they are attached to the seat and backrest frames  33  and  64  will be described in more detail below. 
   To support the lumbar region of a user&#39;s back, the frame  64  of the backrest  34  includes a bowed section  214  and the membrane  212  includes a corresponding bowed section  216 . Since the angle between the backrest  34  and the floor  46  increases at a greater rate than the angle between the seat  32  and the floor as the chair is tilted rearwardly, the bowed sections  214  and  216  of the backrest  34  automatically move downwardly, preferably a distance of about 1.5 inches between the forward tilt and the reclined positions, to insure proper positioning of the lumbar support in any tilt position. 
   To further adjust the positioning of the lumbar support, an adjustable brace member  218  is attached to side members  220  of the backrest frame  64 . The brace member  218  is positioned horizontally between the side members  220  and behind the backrest membrane  212 .  FIG. 15  shows one embodiment of a brace member  218  which is made of a flexible material such as woven nylon or the like. The brace member  218  includes hook members  222  received by a vertical slot  224  formed in the side members  220  of the backrest frame  64 . To adjust the height of the brace member  218 , a conventional fastener such as VELCRO® or the like (not shown) is disengaged to decrease the tension in the brace and allow movement of the hook members  222  within the slots  224 . To reconnect the brace member  218  in a desired location and/or laterally adjust the brace member to obtain a desired tension, the fastener is tightened a desired amount. 
     FIGS. 19 and 20  illustrate another embodiment of the brace member  218 . In this embodiment, the backrest frame  64  is angled relative to the membrane  212  so that a rear inner edge  226  of the frame  64  is spaced apart from the membrane  212 . The brace member  218  is preferably oval shaped and made of a relatively soft yet semi-rigid material such as rubber or the like. As shown in  FIG. 20 , the brace member  218  has an inner surface  228  which bears against the backrest membrane  212 . The brace member  218  is rotatably attached to the inner edge  226  of the backrest frame  64  to allow adjustment of the angle of the brace member  218 . This angular adjustment stretches the membrane  212  and provides the desired location and amount of support for the lumbar region of a user&#39;s back. For example, the brace member  218  can be rotated from a standard position shown in solid lines in  FIG. 20  to a different position shown in broken lines in  FIG. 20 . Preferably, an upper edge portion  230  and a lower edge portion  232  of the brace member  218  each have a radius which provides a comfortable surface area supporting the user&#39;s back when the brace member  218  is rotated to an angle relative to the membrane  212 . 
   The brace member  218  is rotatably attached to the frame  64  by a pair of swivel connectors  234 ,  236  which are mounted to the ends of the brace member  218 . The structure of the connectors  234  and  236  is substantially identical and will be described particularly only with reference to the connector  234 . The connector  234  has a plurality of vertically aligned hook members  238  extending perpendicularly outward from a plate  240  for grasping the frame edge  226 . A pivot rod  242  extends perpendicularly inward from the plate  240  and is received by an insert  244  in a ball and socket type arrangement. The insert  244  is mounted within a cavity  246  in the brace member  218  and has a plurality of annular ribs  248  which are received by corresponding annular grooves in the cavity to prevent axial displacement of the insert  244 . The pivot rod  242  is preferably mounted within the insert  244  with sufficient frictional engagement to require manipulation of the brace member  218  in order to pivot the brace member. 
   Thus, an adjustable brace member  218  is provided which supports the lumbar region of a user&#39;s back a desired amount at a desired location. The connectors  234  and  236  can also be adapted for releasable attachment to the frame edge  226  to allow vertical adjustment of the brace member  218 . 
     FIGS. 21-27  illustrate the tilt control mechanism of the present invention. As described above, the inwardly extending front end portions  66  of links  56  are pivotally attached to a forward portion of the tilt control housing  38 . The ends  72  of the restraining links  70  are pivotally attached to the forward portion of the housing  38  rearwardly and below the attachment of the end portions  66  of links  56 . Preferably, the end portions  66  of links  56  are rigidly attached to a hexagonal axle  250  which extends transversely through the housing  38  and is rotatably attached to spaced apart side walls  251 ,  252  of the housing  38  via a pair of bushings  254 . Likewise, the ends  72  of the restraining links  70  are rigidly attached to a transversely extending bar  256  which is rotatably attached to the side walls  251 ,  252  of the housing  38 . To provide a restoring torque against the rearward tilting of the seat  32 , an elastomeric torsion spring  258  is mounted to the hexagonal axle  250 . The spring  258  is mounted for twisting movement about the axle  250  to resist rotation of the axle  250  when a user sits on the seat  32 . A torsion spring of this type is manufactured by the B. F. Goodrich Company and is designated as TORSILATIC® spring. As shown in  FIGS. 21-23 , a bushing  260  having a hexagonal core is fixedly mounted to the hexagonal axle  250 , and a molded sleeve  262  of a rubber-like elastomeric material is fixedly mounted to the bushing  260 . An outer metal sleeve  264  is fixedly attached to the elastomeric sleeve  262 , and an arm  266  extends radially outward from the outer sleeve  264 . To provide a restoring torque against rotation of the axle  250 , the arm  266  is adapted to be fixed to the housing  38 . Thus, the rotation of the axle  250  resulting from a user sitting on the seat  32  causes the elastomeric sleeve  262  to twist which exerts a restoring torque against the axle  250 . 
   The initial restoring torque exerted by the spring  258  against the rotation of axle  250  can be adjusted by changing the position of the outer sleeve arm  266 . To provide easy adjustment, the outer sleeve arm  266  has spaced apart side portions  268  defining a recess  270  in the end of the arm  266 . The side portions  268  operably engage a transversely oriented block member  272  which is threadably attached to a screw  274 . The screw  274  is mounted to a bottom wall  276  of the housing  38  and extends upwardly through the recess  270  in the arm  266 . The axis of the screw  274  is positioned generally tangentially to the outer sleeve  264 , and a bevel gear  278  is attached to a top portion  280  of the screw  274 . A bevel gear  282  meshes with the bevel gear  278  and has an axis which intersects the axis of the bevel gear  278 . The bevel gear  282  is attached to the end of a shaft  284  which is rotatably mounted to the side wall  251  of housing  38 . The shaft  284  extends horizontally outward from the side wall  251  of the housing and has a handle  286  thereon to provide easy access for a user. In operation, the shaft  284  and gear  282  are rotated a desired amount to rotate the gear  278  which in turn rotates the screw  274 . Rotation of the screw  274  causes the block member  272  to move linearly along the axis of the screw  274 , which moves the outer sleeve arm  266  of to the desired radial location. Preferably, the gear ratio of bevel gears  278  and  282  is such that a minimal amount of effort is required to move the arm  266 . Thus, the outer sleeve arm  266  is easily moved a desired amount to vary the initial restoring torque of the spring  258  and thereby control the rate at which the seat  32  and back  34  tilts rearwardly when a user sits on the seat  32 . 
   An adjustable rearward tilt limiter mechanism  290  is also provided to vary the maximum rearward tilting of the seat  32  and backrest  34 . As best shown in  FIG. 24 , a cam member  292  and gear  294  are mounted to a rod  296  which is rotatably mounted to the side wall  252  of housing  38 . The cam member  292  preferably has a plurality of concave surfaces  298  formed in an outer edge  300  thereof. An arm  302  is fixedly mounted to the axle  250  and has a convex follower member  304  attached to an end thereof. The arm  302  extends rearwardly from the axle  250  such that the follower member  304  is in operable engagement with one of the concave surfaces  298  of the cam member  292  when a user sits on the seat  32 . As viewed in  FIGS. 24 and 25 , the maximum clockwise rotation of the axle  250  and therefore the maximum rearward tilt position of the seat  32  and backrest  34  is determined by the position of the cam member  292 . To adjust the position of the cam member  292 , a pie-shaped member  306  is rotatably attached to the side wall  252  of the housing  38 . The member  306  has a plurality of teeth  308  on a circular edge portion thereof which mesh with the gear  294 . A spring  310  is attached to the pie-shaped member  306  and the side wall  252  of the housing  38  to bias rotation of the member  306  in a clockwise direction. A cable  312  is attached to the member  306  opposite the spring  310  and guided within a guide member  315   d  which is attached to the side wall  252  of the housing  38 . In operation, the cable  312  is moved axially a desired amount to rotate the pie-shaped member  306 , which in turn meshes with the gear  294  to rotate the cam member  292  to a desired position. When the chair is tilted rearwardly, one of the concave surfaces  298  will act as a stop for the follower member  304  to limit the rearward tilting of the seat  32  and chair  34 . As shown in dotted lines in  FIG. 25 , the cam member  292  and arm  302  can be rotated to lock the seat  32  and backrest  34  in a forward tilt position. 
   In addition, a forward tilt limiter mechanism  313  is provided to prevent forward tilting of the seat  32  past the generally horizontal middle position shown in  FIGS. 1-7 . As best shown in  FIGS. 26-27 , a pivot member  314  is mounted to a rod  316  which is rotatably mounted to the side wall  251  of housing  38 . The pivot member  314  has forward tilt abutment surface  318  and a standard tilt abutment surface  320 . An arm  322  is fixedly mounted to the axle  250  and has a load bearing member  324  attached to an end thereof. The arm  322  extends rearwardly from the axle  250  such that the load bearing member  324  is operably engageable with either the forward tilt abutment surface  318  or the standard tilt abutment surface  320 . As viewed in  FIGS. 26 and 27 , the maximum clockwise rotation of the axle  250  and therefore the maximum forward tilt position of the seat  32  and backrest  34  is determined by the position of the pivot member  314 . To actuate the pivot member  314  between the standard and forward tilt positions, a cable  326  is attached to the pivot member  314 . The cable member  326  is guided within a guide member  328  which is attached to the side wall  251  of the housing  38 . In addition, a spring  330  is attached to the side wall  251  of the housing  38  and to the pivot member  314  opposite the cable  326  to bias rotation of the pivot member  314  in a counterclockwise direction as viewed in  FIGS. 26 and 27 . In operation, the cable  326  is moved axially a desired amount to rotate the pivot member  314  so that the load bearing member  324  is operably engageable with the standard tilt abutment surface  320  as shown in  FIG. 26  or with the forward tilt abutment surface  318  as shown in  FIG. 27 . When the chair is unoccupied or when a user leans forward, the pivot member  314  will act as a stop for the load bearing member  324  to limit the forward tilting of the seat  32  and chair  34 . 
   Preferably, the cables  312  and  326  are adapted to be actuated from a location near the seat frame  32  so that a user does not have to lean over to adjust the tilt adjustment mechanisms  290  and  313 . As shown in  FIG. 28 , a handle  332  can be configured for pivotal attachment within a cavity  334  formed in one of the second links  50 , the seat frame  33 , or other convenient location. The cable  312  or  326  can thus be actuated by merely pivoting the handle  332  a desired amount. Alternatively, a guide member  336  and slot  338  can be provided to allow slidable movement of a handle  340  to actuate the cable  312  or  326  as shown in  FIG. 29 . 
   Another feature of the chair  30  which assists in comfortably supporting a user in ergonomically desirable positions is the configuration of the seat  32 . As best shown in  FIGS. 30-35 , the frame  33  of seat  32  supports the elastic membrane  210  across a central opening  352 . Preferably, the frame  33  is a single molded piece of glass-filled thermoplastic polyester and the membrane  210  includes a plurality of interwoven fibers as discussed in more detail below. To provide a rim which conforms to the body of a user, the side portions  52  and a rear portion  354  of frame  33  curve upwardly. To minimize pressure on the underside of a user&#39;s thighs near the knees, especially when the chair  30  is tilted rearwardly, a front portion  356  of the frame  33  curves downwardly. The membrane  210  has a similar downwardly curving portion  357  which overlies the front portion  356  of the frame  33 . To further reduce the pressure on the legs of a user, a cushion  358  made of polyurethane foam or similar material fits in a recess  360  formed in the front portion  356  of the frame. 
   Preferably, the entire periphery of the membrane  210  is attached to a one-piece carrier member  362  which is removably received by a continuous channel  364  in the seat frame  33 . The channel  364  is formed in a top surface  366  of the seat frame  33  and extends around the entire perimeter of the frame  33 . Although the carrier member  362  is securely held by the frame  33  within the channel  364 , a strip  367  is provided on the underside of the carrier member  362  and an interlocking strip  369  is provided on a bottom surface of the channel  364  to further secure the carrier member  362  to the frame  33 . The interlocking strips  367  and  369  can be hook and loop type fasteners such as VELCRO® and can be configured as separate tabs spaced around the periphery of the carrier member  362  and channel  364 . The carrier member  362  is preferably made of a pliable yet semi-rigid thermoplastic polyester material such as polybutylene terephthalate (PIB), polystyrene or glass-filled polypropylene. The membrane  210  is preferably in-molded with the carrier member  362  as described in more detail below. Thus, the carrier member  362  is deformable yet has sufficient rigidity to maintain the desired contour of the membrane  210  when inserted in the channel  364 . To this end, the carrier member  362  is formed with the same contour as the channel  364  including a downwardly extending front portion similar to the curvature of the front portion  356  of the frame  33 . 
   In addition, a top surface  368  of the carrier member  362  is configured to follow the contour of the top surface  366  of the frame at any location around the perimeter. Thus, the top surface  368  of the carrier member  362  has a variable slope which generally corresponds with the downwardly extending front portion  356 , the upwardly extending side portions  52 , and the upwardly extending rear portion  354  of the frame  33 . A smooth transition from the carrier member  362  to the frame  33  is therefore provided, which is especially desirable in the front portion of the seat where the legs of a user rest. To provide a smooth transition from the membrane  210  to a front portion  370  of the carrier member  362 , the periphery of the membrane  210  is attached to an upper inner corner  372  of the carrier member  362  at generally the same angle as the top surface  368  of the carrier member  362 . The remaining portion of the membrane  210  is shown attached to the carrier member  362  at a different angle than the corresponding top surface  368  of the carrier member  362 . However, it will be understood that the method for attaching the membrane  210  to the carrier member  362  allows any desired “entry-angle” between the periphery of the membrane  210  and the carrier member  362 . 
   As shown in  FIGS. 36-38 , the membrane  210  is preferably made of a plurality of elastomeric monofilaments  374  interlaced with a plurality of strands  376  of fibrous yarn typically used in textile upholstery weaving. The elastomeric monofilaments  374  are extruded from a block copolymer of polyetramethylene terephthalate polyester and polytetramethylene ether. Preferably, this material is Hytrel® which is produced by the E. I. DuPont DeNemours Company and has a durometer of 55 on the D-scale, or more specifically, Hytrel® grade 5544 or 5556. The monofilaments  374  are extruded by standard industry techniques which are well known to those skilled in the art. During the extrusion process, the monofilaments  374  are annealed while under tension to orient the polyester molecules in one direction while leaving the poly ether molecules unaffected. This increases both the tensile strength and the modulus of elasticity of the monofilaments  374 . 
   Preferably, the block copolymer is extruded into 2350 denier monofilaments having the following properties: 
   EX-120 (Eytrel 5556) Lot X-2174 Properties Summary 5556 
   
     
       
         
             
             
             
             
             
             
          
             
                 
                 
             
             
                 
               Diameter (mils) 
               Load @ 5% 
               Load @ 10% 
               Load @ 
                 
             
          
         
         
             
             
             
             
             
             
             
          
             
               Set No. 
               Min. 
               Max. 
               Elongation (g) 
               Elongation (g) 
               Break (g) 
               Elongation @ Break (%) 
             
             
                 
             
          
         
         
             
             
             
             
             
             
             
          
             
                1 
               16.5 
               24.9 
               215 
               455 
               4903 
               117 
             
             
                 
               17.2 
               26.2 
               225 
               477 
               4803 
               113 
             
             
                 
               16.6 
               24.9 
               210 
               457 
               5330 
               129 
             
             
                 
               16.0 
               24.2 
               227 
               480 
               4980 
               122 
             
             
                 
               16.0 
               24.2 
               213 
               461 
               5058 
               122 
             
             
               10 
               15.9 
               24.1 
               239 
               481 
               4967 
               125 
             
             
                 
               16.6 
               25.1 
               221 
               455 
               5067 
               122 
             
             
                 
               16.5 
               25.2 
               200 
               428 
               4944 
               124 
             
             
                 
               16.1 
               24.3 
               211 
               441 
               4921 
               124 
             
             
                 
               16.0 
               24.3 
               220 
               450 
               5121 
               128 
             
             
               20 
               16.6 
               25.1 
               244 
               486 
               5389 
               127 
             
             
                 
               16.6 
               25.4 
               248 
               489 
               4958 
               123 
             
             
                 
               17.5 
               26.5 
               233 
               472 
               4958 
               116 
             
             
                 
               16.5 
               25.0 
               229 
               465 
               4999 
               126 
             
             
                 
               15.8 
               23.9 
               225 
               455 
               4429 
               102 
             
             
               37 
               15.8 
               24.0 
               235 
               489 
               4835 
               123 
             
             
                 
               15.9 
               24.1 
               246 
               515 
               4890 
               127 
             
             
                 
               16.3 
               24.4 
               234 
               513 
               5266 
               131 
             
             
                 
               16.4 
               25.1 
               193 
               464 
               4930 
               122 
             
             
                 
               16.4 
               24.8 
               234 
               513 
               5198 
               129 
             
             
               Average 
               16.36 
               26.79 
               225.10 
               472.30 
               4997.30 
               122.60 
             
             
               Hi 
               17.50 
               26.50 
               248.00 
               515.00 
               5389.00 
               131.00 
             
             
               Low 
               15.80 
               23.90 
               193.00 
               428.00 
               4429.00 
               102.00 
             
             
                 
             
          
         
       
     
   
   The elastomeric monofilaments  374  are the primary load-carrying members of the membrane  210  and preferably run laterally in the warp direction between the side portions  52  of the seat  32  to comfortably support a user. The monofilaments  374  conform to the shape of a user&#39;s buttocks and also conform to the natural movement of the body when the chair  30  is in any tilt position. Preferably, the monofilaments  374  are prestretched between 6% and 9% elongation to maintain the desired contour of the membrane  210  prior to imparting a load on the membrane  210 . In addition, the prestretching produces the optimum conforming characteristics of the monofilaments  374 . A plurality of elastomeric monofilaments can also run longitudinally in the weft direction between the rear portion  354  and the front portion  356  of the seat  32  to provide further support which may add to the comfort of the seat  32 . If elastomeric monofilaments are provided in both the lateral and longitudinal directions of the seat  32 , the monofilaments in the lateral direction can be pretensioned a desired amount and the monofilaments in the longitudinal direction can be pretensioned a different amount to produce the desired pressure distribution of the seat  32 . 
   To provide greater comfort to a user, the cross-sections of the elastic monofilaments  374  preferably have a width to height ratio in the range of 1.5/1 to 2/1. This provides greater comfort because the increased width of the monofilaments provides a greater surface area for supporting a user which distributes the forces acting on the user. Thus, the user feels less pressure from the individual monofilaments  374  as opposed to round monofilaments which are more like concentrated pressure points. In addition, the greater width of the monofilaments  374  creates a more opaque appearance of the membrane  210  which is attractive and may lessen the perception that the user is sitting on a net rather than a conventional cushion. In addition, the cross-section of the monofilaments  374  are preferably elliptical as shown in  FIGS. 37 and 38  to provide a less abrasive support. The monofilaments can be configured with various other cross-sectional shapes which are less abrasive than a conventional round monofilament. To extrude the monofilaments  374  into the desired elliptical shape, the dies through which the block of copolymer material is drawn can have a octogonal cross-section. Preferably, the elliptical monofilaments  374  have a width of approximately 0.02479 inches and a height or thickness of approximately 0.01636 inches. With these dimensions, the membrane  210  has about 24-26 monofilaments per inch in the lateral direction. 
   Referring again to  FIGS. 36-38 , the fiber strands  376  run longitudinally in the weft direction of the seat  32  and are preferably arranged in groups of three. Each strand  376  preferably includes adjacent multifilament bundles  376 A and  376 B of spun, textured, or twisted 1500 denier Nylon or polyester yarn. To provide additional support in the longitudinal direction of the seat  32 , an elastic monofilament  378  such as spandex is incorporated into each strand  376  by spinning, air jet texturing or covering the monofilament  378 . The monofilaments  378  are preferably Lycra® monofilaments sold by the E. I. DuPont DeNemours Company, although other materials such as Hytrel® can be used to provide the desired support. The monofilaments  378  can be secured to the strands  376  in any suitable manner such as wrapping the fibers of bundles  376 A or  376 B around the monofilaments  378 . In addition, a desired number of monofilaments  378  can be provided. The strands  376  are preferably prestretched between 3% and 5% elongation in order to maintain the desired contour of the membrane  210  with no load imparted on the membrane  210 . In addition, the strands  376  are secondary load bearing members of the seat  32  and the prestretching produces the optimum conforming characteristics of the strands  376  when a user sits on the membrane  210 . Preferably, the density of the strands  376  is approximately 7-10 strands per inch. 
   As shown in  FIG. 36 , the strands  376  are interlaced with the elastomeric monofilaments  374  in an attractive, tightly woven pattern which facilitates aeration and provides a smooth seating surface. The strands  376  are held in groups of three by pairs of the elastomeric monofilaments  374  which cross over between each group of strands. For example, monofilaments  374 A and  374 B are shown in  FIG. 36  crossing over between a group  380  and a group  382  of strands  376 . To maintain the spacing between each strand  376  in a group, the monofilaments  374  weave alternately above and below adjacent strands in the group. The plurality of strands  376  provide a relatively large surface area of nonabrasive fabric which distributes the forces acting on a user to avoid a “grid-mark” type feel resulting from the concentration of pressure. In addition, the weave pattern provides sufficient aeration through the openings between the monofilaments  374  and the strands  376  to allow evaporation of perspiration and facilitate air circulation to minimize heat buildup. The longitudinal orientation and the grouping of the strands  376  also provide an attractive seat with a longitudinal design and a relatively opaque appearance. 
   The method for forming the carrier member  362  and attaching the membrane  210  thereto will now be described with reference to  FIGS. 39-48 . As shown in  FIGS. 39 and 40 , a loom  384  having an upper member  386  and a lower member  388  is provided to capture and hold the membrane  210  in a stretched condition. The lower member  388  of loom  384  is initially placed in a stretching machine (not shown). The membrane  210  with the previously described weave pattern is then placed over the lower loom member  388 , and clamp members  390  of the stretching machine clamp the edges of the membrane  210  and stretch it a predetermined amount in both the lateral and longitudinal directions. The upper loom member  386  is then clamped against the lower loom member  388  to hold the membrane  210  in the stretched condition. The clamp members  390  of the stretching machine release the membrane  210 , and excess edge portions  392  of the membrane  210  outside the loom are trimmed a desired amount. 
   The loom  384  has a semi-rectangular shape with a central opening larger than the area defined by the carrier member  374 . The loom  384  also has a downwardly extending front portion  394  which is similar to the contour of the downwardly extending front portion  356  of the frame  33 . The upper and lower loom members  386  and  388  each include a frusto-conically shaped cover member  396  made of plastic such as an epoxy, urethane, or other suitable soft material which is molded over a bent steel tube  398 . The steel tubes  398  are provided to give the loom members  386  and  388  structural support and the plastic members  396  are provided protect a steel molding tool from chipping or other damage. The upper loom member  386  has a pair of ridges  400  extending downwardly therefrom and the lower loom member  388  has a pair of matching grooves  402  formed therein to secure the membrane  210  therebetween. A plurality of clamp devices (not shown) are also provided to hold the loom members together and maintain the membrane  210  in the stretched condition. 
   As best shown in  FIGS. 41-46 , a molding tool  404  includes an upper mold member  406  and a lower mold member  408 . The upper and lower mold members  406  and  408  have corresponding recesses  410  and  412  which are configured to receive the upper and lower loom members  386  and  388 . The mold members  406  and  408  also have upper and lower recesses  414  and  416  which form a cavity  418  when the mold members  406  and  408  are closed together. The cavity  418  has the desired shape and contour of the carrier member  362  and a substantial portion of the cavity  418  is the same shape and contour of the channel a 14  in the seat frame  33 . 
   After the clamp members  390  of the stretching machine are removed and the excess edge portions of the membrane  210  are trimmed, the assembly of the loom  384  and stretched membrane  210  is removed for placement in the lower mold member  408 . The lower loom member  388  is then placed in the recess  412  in the lower mold member  406  such that the membrane  210  is shaped over a curved inner male portion  420  of the lower mold member  408  as shown in  FIG. 43 . Thus, a periphery  422  of the membrane  210  is positioned at the desired angle over the recess  412  in the lower mold  408  and the prestretched membrane  210  attains the desired contour prior to closing the upper mold  406  against the lower mold  408 . The upper mold  406  is then closed against the lower mold  408  without further stretching the membrane  210  or changing the position of the periphery  422  of the membrane  210  over the recess  412 . The loom recess  410  in the upper mold  406  receives the upper loom member  386 , and a female inner curved portion  424  of the upper mold  406  which is the same contour as the male inner curved portion  420  of the lower male mold  408  bears against the membrane  210 . A plastic resin is then injected into the cavity  418  to secure the periphery  422  of the membrane  210 . The upper and lower mold members  406  and  408  are then pulled apart, the carrier member  362  and membrane  210  assembly are removed, excess edge portions outside the carrier member  362  are trimmed, and the interlocking strip  367  is adhesively bonded to the underside of the carrier frame  362 . 
     FIGS. 47 and 48  illustrate the upper and lower mold members  406  in a closed or “shut-off” position during which the resin is injected into the cavity  418 . Because the membrane  210  has a thickness, the mold members  406  and  408  cannot be completely clamped against each other. The weaving of the monofilaments  374  and strands  376  creates a variable thickness membrane  210 , and the closest the mold members  406  and  408  can be clamped together is determined by the thickest portions of the membrane  210 . As shown in  FIGS. 47 and 48 , the thickest portions of the membrane  210  are in the regions where the elastomeric monofilaments  374  cross-over between the groups of strands  376 . When the mold members  406  and  408  are closed against the monofilaments  374  in the cross-over region, the strands  376  and the single monofilaments  374  which are not overlapping have less thickness, thus creating a gap  426  between the mold members  406  and  408 . It is desirable to minimize the gap  426  to inhibit the resin from leaking out of the cavity  418 . Because the overall thickness of the overlapping elliptical monofilaments  374  is less than the thickness of overlapping round monofilaments, the mold members  406  and  408  can be clamped closer together to minimize the gap  426  and decrease the amount of leakage. In addition, the reduction in overall thickness tends creates less stress when the mold members  406  and  408  are clamped against the membrane  210  which tends to cause less damage to the monofilaments  374  and strands  376 . 
     FIGS. 49-56  illustrate alternative embodiments of the seat  32 . Since these embodiments are similar to the previously described embodiment, similar parts appearing in  FIGS. 49-56  are represented by the same reference numerals. In  FIGS. 49-56 , the seat frame  33  includes a plurality of spaced apart slots  430  formed therein which extend through a bottom wall  432  of the channel a 14 . The carrier member  362  has a plurality of spaced apart tabs  434  extending downwardly from therefrom. The tabs  434  have a hook portion  436  extending outwardly from a lower end thereof and the slots  430  are of sufficient size to receive the tabs  434 . The carrier member  362  is secured to the seat frame  33  by inserting the carrier member into the channel a 14  and snapping the tabs  434  into the slots  430  such that the hook portions  436  extend through the slots  430  and engage an underside  438  of the frame  33 . This embodiment for attaching the carrier member  374  to the frame  33  can be used with any type of seating utilizing a carrier frame regardless of the method used to attach the seating surface to the carrier member. 
     FIGS. 49-56  also illustrate alternative embodiments for attaching the membrane  210  to the carrier frame  362 . For example, the periphery  422  of the membrane  210  can be molded with an insert  440  which is press fit inside a cavity  442  formed in a top surface of the carrier member  362  ( FIGS. 49-50 ). The periphery  422  of the membrane  210  can also be vibration or sonic welded to a bottom surface  444  of the carrier member  362  as shown in  FIGS. 51-56 , and a plurality of teeth members  446  can extend downwardly from the bottom surface  444  to further secure the membrane  210  thereto as shown in  FIG. 52 . 
   Preferably, the backrest  34  is constructed with the same materials and in the same manner as the seat  32 , although the desired amount of prestretching of the elastomeric monofilaments and strands may vary to reflect the different support required for the back of a user. 
   Pre-assembly of the seat and backrest membranes to the carrier members facilitates maintenance since the membrane/carrier member assemblies can be easily removed for repair and/or replacement. This configuration also provides greater manufacturing flexibility. Since the carrier members do not have the structural requirements of a seat or backrest frame, a desired material can be used for a desired type of attachment method. For example, the material of the carrier member can be chosen based on whether the membrane is to be insert molded therewith or welded thereto. A semi-rigid material can be used to facilitate the insert molding process and pre-stretching of the membrane, and a material having a low melting point can be used to facilitate welding of the membrane thereto. 
   The in-molding process for attaching the membranes to the carrier frames also provides significant advantages over other attachment methods. For example, the configuration of the mold cavities can be varied to provide the frames and carrier members with any type of contour which facilitates design flexibility. The top surfaces of the frames and carrier members can have the same slope to provide a smooth transition between the frames and the carrier members. The shape of the membranes and the angle at which the membranes are attached to the carrier members can also be easily adjusted. In addition, the in-molding process allows the frames to be thinner because a relatively small channel is all that is required for attachment of the carrier members to the frame. 
   The membranes also provide a flexible support which conforms to the natural movement of the body of a user when the chair is in any tilt position.  FIGS. 2-10  show the approximate position of the seat and backrest membranes when a user is sitting thereon. The membranes tend to minimize fatigue because they are responsive to micro-postural changes of a user which stimulates muscles in the trunk and allows spinal movement to hydrate the spinal discs. 
   In the forward tilt position ( FIGS. 8 and 10 ), the seat membrane  210  maintains a forward angle of the pelvis which insures a proper curvature of the spine. The elasticity of the seat membrane  210  allows the thighs of a user to slant forward while keeping the pelvis on a horizontal plane, thereby giving the user the sensation of not sliding out of the chair and reducing shear forces acting on the underside of the user&#39;s thighs. The seat membrane  210  also passively positions the lumbar region of a user&#39;s back against the bowed section  86  of the backrest membrane  82 . In the middle and reclined tilt positions shown in  FIGS. 2-7  and  9 , the elasticity of the seat membrane  210  automatically causes larger buttocks to wedge more deeply into the pocket between the seat  32  and backrest  34  to insure correct positioning of the user&#39;s lumbar region against the bowed section  86  of the backrest mat. 
   The porosity of the backrest and seat membranes allows air to flow through the membranes to aerate the skin of a user. Providing such aeration decreases uncomfortable heat buildup which would otherwise occur when a user sits for an extended period of time on conventional chair upholstery which acts like another layer of clothing. 
   Thus, the tilt motion of the chair  32  and the resiliency of the seat and backrest membranes passively stabilizes the pelvic-lumbar process to reduce muscle activity heretofore associated with leg crossing and slumping postures. In addition, the membranes accommodate for angular variations in the sacral plates of various user&#39;s spinal columns. 
   Thus, in accordance with the most preferred embodiment, a chair is provided which naturally conforms to the body of user during tilting of the chair to reduce shear forces acting on the thighs and trunk of the user and minimize pressure acting on the underside of the user&#39;s thighs at the knees. The chair automatically supports the body of the user in ergonomically desirable positions for performing tasks of varying intensity, and the range of vertical adjustment of the chair allows a lower minimum height and higher maximum height than conventional office chairs. Although the present invention has been described with reference to preferred embodiments, those skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. As such, it is intended that the foregoing detailed description be regarded as illustrative rather than limiting. It is the appended claims, including all equivalents thereof, which are intended to define the scope of the invention.