Patent Publication Number: US-2018049551-A1

Title: Chair adjustment means

Description:
FIELD OF THE INVENTION 
     The present invention relates to mechanisms for adjusting the position of components of a chair to suit a user, including (but not exclusively) an office chair. 
     BACKGROUND TO THE INVENTION 
     The office chair has been a ubiquitous piece of equipment in the workplace for decades. With the increased use of computers by workers greater attention has been paid in the art to providing of well-designed ergonomic office chairs. 
     Computer use has been linked to several types of injuries known as upper extremity repetitive stress injuries, cumulative trauma disorders, or work related musculoskeletal disorders. These disorders may be associated with the upper extremities (the arms, forearms, wrists, hands, and fingers) as well as the neck, back and lower extremities. Common cumulative trauma disorders associated with computer input devices include carpal tunnel syndrome, tendonitis, and tenosynovitis affecting the hands, wrists, and forearms as well as neck tension syndrome, low back pain, and lower extremity pain. 
     Adjustable chairs have been successfully used in the workplace for many years to ameliorate some of the occupational health and safety issues associated with computer use. Customization of a chair to a particular worker, or to a particular task has been instrumental in reducing the occurrence of injuries and disorders associated with sedentary work. 
     The office chair has taken many forms in recent times with a wide array of workstation designs available for users. A typical office chair is disposed on a base, a seat pan, a backrest, and possibly arm rests. Modifications to these basic elements include recumbent, sit-stand, and kneeling designs. 
     The seat pan is provided in a variety of sizes, shapes, and materials. Seat pans may be hard or soft, adjustable or fixed in position. Seat Height (the relative height from the floor to the seat pan) is typically between 15″ to 20″. Seat height may be adjustable or fixed. 
     The backrest supports the user from behind. Backrest sizes range dramatically from “low-back” (˜10″) to “high-back” (˜30″) chairs. The backrest can be upright (90 degrees to the seat pan) to sloping back (120 degrees) to reduce loads on the lumbar spine. The backrest may be substantially flat or curved with a built-in lumbar support or curve. It may be hard or soft, adjustable, or fixed. 
     The chair base provides support for the entire body weight. A typical office chair has a 5-post base for safety and stability. 
     It will be noted from the above that many components of a chair are adjustable. Adjustability is an important feature that may be designed into the chair to alter positions, size, and height of the seat pan, and backrest. 
     Seat height should be adjustable to accommodate the variability of leg lengths. Most ergonomically designed chairs adjust between 16″ and 25″ vertically. When properly seated, the thigh should be parallel to the floor. The seat height is the first adjustment to be accomplished in fitting the chair to the user. The proper height adjustment establishes the placement of the remainder of office and computer equipment for the overall workstation layout. Some users prefer the seat low with the thigh slanting backward and downward to the hips. This is preferable to adjusting the seat too high, which can cause increased pressure behind the knees and impair circulation to the lower extremities. In the event the seat is adjusted too high for a user to place feet squarely on the floor a footrest may be appropriate. 
     Seat pan tilt angle (the relative position of the seat pan compared to level) may be adjusted to provide a positive slope, negative slope, or level. Most users prefer a level seat but others prefer a positive or negative slope for comfort or special needs. The ergonomically designed chair accommodates users with adjustability. 
     The backrest-seat pan angle should be adjustable for the individual user preference. The angle between the seat pan and backrest should be adjusted when the user is seated with thighs parallel to the floor and legs properly supported vertically. This angle permits the user to sit slightly forward, straight up, or recline back depending on the type of work performed, support needed, and comfort desired. 
     Ergonomic armrests are an optional feature of office chairs. Armrests aid in supporting upper extremity weight and thus help maintain comfort, endurance, as well as normal circulation by decreasing static load to muscles contracted to lift and hold limb position during computing. 
     Armrest users report enhanced performance including less fatigue, increased comfort, and better endurance with sustained computing. Armrests should be placed at least 18.5″ apart and made of soft or padded material. An ergonomically designed armrest should be adjustable vertically and not impair circulation due to direct pressure to contact areas but distribute that load over broad areas comfortably. Armrests should adjust between 2″ and 4″ vertically to accommodate user&#39;s preference. 
     It will be clear from the above that adjustability of various chair parameters is highly desired in the area of ergonomics. Chair adjustments should be simple, intuitive, and accomplished while sitting in the chair. 
     Prior art mechanisms for adjusting chair parameters can be difficult to for a user to operate. Prior art chairs capable of adjust to more than one chair parameter typically include multiple levers. It can take some time for a user to be become familiar with a particular chair. Sometimes the user must twist or bend their body to visually identify and reach a particular lever. This can make a user less likely to finely adjust a chair, leading to more injuries or disorders. 
     Chairs having multiple levers also a add complexity and cost to the manufacturing process. 
     There have been attempts in the prior art to provide chairs having a single adjustment lever for two chair parameters, however such contrivances can be difficult for a user to operate, or add complexity and cost to the manufacturing process. 
     It is an aspect of the present invention to overcome of alleviate a problem of the prior art by providing improved means for adjusting a chair, or simply an alternative to prior art means. It is a further aspect to provide a mechanism capable of independently adjusting three or more chair parameters. 
     Many office chairs provide a backrest. Typically, the backrest is attached to and supported by a rigid upright post which extends downwardly toward the seat pan. The upright member extends beyond the rear edge of the seat pan and engages with the main chassis of the chair, which is typically attached to the underside of the seat pan. 
     The presence of a backrest causes inefficiencies in shipping, given the considerable volume taken up by a fully assembled seat pan and backrest. To provide for more efficient stacking, an office chair may not be completely assembled for shipping. In particular, the backrest (with upright member) is generally not attached to the chair chassis. Greater product packing density can be achieved where the backrest is not engaged with the chassis, and is instead laid flat against the seat pan. 
     Upon delivery, the chair backrest is attached to the seat pan. Often, in the course of reassembling the chair the upright member of the backrest is inserted into a channel in the chair chassis. 
     It is a further problem in the art that after attachment, the backrest often feels unsecure to a user. A reason may be that the upright member is able to move laterally within the channel into which it has been inserted during assembly. Thus, when the user leans against the backrest, some undesired movement in the backrest is perceived by the user. Moreover, this movement can lead to a rattling noise, this being a further cause for consumer dissatisfaction. While the backrest may be perfectly secure in reality, it is not uncommon for a consumer to return a chair to the place or purchase (or to at least seek repair) based upon the perception that the backrest is insecure. These returns cause an economic penalty for the chair retailer and/or manufacturer. Alternatively, the consumer may simply put up with the perceived defect and desist from buying the same brand of chair in the future. 
     The prior art has attempted to address this problem by sizing the external dimensions of the upright member to provide a very close fit, thereby minimising lateral movement of the upright member. While this approach can provide for a snug fit of the upright member within the channel, assembly can be difficult given the need to apply some force in pushing the upright member into the channel. This is especially the case where a purchaser of the chair does not have sufficient strength to force the upright member into the channel. A further disadvantage in this approach is that coatings (such as paint) may be removed during insertion of the upright member, or metallic surfaces may be scratched. 
     Other attempts to limit movement of the backrest include the use of a fastener which extends through the channel and the upright member. While the fastener may prevent the upright member from moving longitudinally, it does little to prevent lateral movement when the user leans against the backrest. Even if lateral movement is prevented to some extent, fasteners tend to loosen over time leading to the problem returning. The use of fasteners also requires some mechanical ability and dexterity on the part of the person assembling the chair. Furthermore, additional hardware is required (such as a nut, bolt and washer) which adds to the overall cost of the chair. 
     It is an aspect of the present invention to overcome or alleviate a problem of the prior art to provide means for securely attaching a backrest to a chair which is an improvement on prior art means. Alternatively, the present invention may provide an alternative to prior art means. 
     The discussion of documents, acts, materials, devices, articles and the like is included in this specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each provisional claim of this application. 
     SUMMARY OF THE INVENTION 
     In a first aspect, but not necessarily the broadest aspect, the present invention provides an apparatus allowing a user to adjust a chair, the apparatus comprising:
         two or more chair parameter adjustment mechanisms, each chair parameter adjustment mechanism configured to allow adjustment of a separate chair parameter,   two or more cams, each cam configured to actuate one of the two or more chair parameter adjustment mechanisms, and   a mechanical coupling between the two or more cams,
 
wherein the apparatus is configured such that each of the two or more chair parameter adjustment mechanisms is independently actuatable by the two or more cams.
       

     In one embodiment of the apparatus, the mechanical coupling is a connecting member. 
     In one embodiment of the apparatus, each of the two or more cams extends substantially outwardly from the longitudinal axis of the connecting member. 
     In one embodiment of the apparatus, a first cam extends outwardly from the longitudinal axis of the connecting member at an angle to a second cam, the angle measured by reference to a longitudinal axial view of the connecting rod. 
     In one embodiment of the apparatus, the angle is at least about 45 degrees, or at least about 90 degrees, or at least about 145 degrees, or about 180 degrees. 
     In one embodiment of the apparatus, the angle is about 90 degrees or about 180 degrees. 
     In one embodiment of the apparatus, the two or more cams and the mechanical coupling are unitarily formed. 
     In one embodiment of the apparatus, the two or more cams and the mechanical coupling are formed from a single length of metal. 
     In one embodiment of the apparatus, the two or more cams are formed from a deformation of the surface of the single length of metal, or a deformation of the single length of metal. 
     In one embodiment, the apparatus comprises an actuating member, the actuating member being mechanically coupled to:
         one of the two or more cams,   the mechanical coupling, or   any other structure which is mechanically coupled directly or indirectly to one of the two or more cams or the mechanical coupling.       

     In one embodiment of the apparatus, the actuating member is configured so as to be movable independent of the two or more cams, the actuating member being configured such that movement thereof causes actuation of a third chair adjustment parameter independent to the first and second chair adjustment parameters. 
     In one embodiment of the apparatus, the actuating member comprises a rotatable component configured to be gripped and rotated by a user, wherein rotation of the rotatable component allows adjustment of a third chair adjustment parameter independent to the first and second chair adjustment parameters. 
     In one particularly advantageous embodiment, the apparatus comprises a third cam and a third chair adjustment mechanism configured to allow adjustment of a third chair adjustment parameter, wherein the rotation of the rotatable component causes actuation of the third cam which in turn actuates the third chair adjustment mechanism. 
     In one embodiment of the apparatus, one, both or all of the chair parameter adjustment mechanisms is/are configured to engage and disengage with a chair part involved in chair parameter adjustment. 
     In one embodiment of the apparatus, the engagement is frictional engagement. 
     In one embodiment of the apparatus, at least one the two or more chair adjustment mechanisms is a clutch. 
     In one embodiment of the apparatus, the clutch is a plate-type clutch. 
     In one embodiment of the apparatus, the independent chair parameters are selected from two or more of: backrest-seat pan angle, backrest height, backrest lateral position, seat pan tilt angle, seat pan height, seat pan lateral position, seat pan swivel lock, wheel lock and armrest height. 
     In one embodiment the apparatus consists of a single actuating member. 
     In a further aspect the present invention provides a chair comprising the chair adjustment apparatus as described herein. 
     In one embodiment of the chair, the chair comprises a chair backrest attachment system comprising: a backrest upright member, a channel configured to receive the backrest upright member, and a resilient member, wherein, in use, when the backrest upright member is inserted into the channel the resilient member bears directly or indirectly against the backrest upright member such that a movement of the backrest upright member within the channel is cushioned by the resilient means. 
     In one embodiment of the chair, the resilient means is configured to, when assembled, bear against a region of the backrest upright member lying between the extremities of the channel. 
     In one embodiment of the chair, the channel is configured to allow the resilient means to contact a portion of the backrest upright member that is within the channel. 
     In one embodiment of the chair, the backrest upright member comprises means for engaging with the resilient means. 
     In one embodiment of the chair, the means for engaging with the resilient means is an aperture, a depression, a protuberance, or a deformation in or of the backrest upright member. 
     In one embodiment of the chair, the resilient means is configured to engage with the aperture, a depression, a protuberance, or a deformation in or of the backrest upright member. 
     In one embodiment of the chair, the resilient means is required for a function other than bearing against the backrest upright member. 
     In one embodiment of the chair, the function other than bearing against the backrest upright member relies on the resilience of the resilient member. 
     In one embodiment of the chair, the function other than bearing against the backrest upright member is selected from the group consisting of: a set pan tilt adjustment function, a seat pan-backrest angle adjustment function, a backrest height adjustment function, an armrest adjustment function, a seat pan height adjustment function, a backrest lateral position adjustment function, and a seat pan lateral position adjustment function. 
     In one embodiment of the chair, the function other than bearing against the backrest upright member is a seat pan tilt adjustment function. 
     In one embodiment of the chair, the resilient means is a spring. 
     In one embodiment of the chair, the spring is a torsion spring. 
     In one embodiment of the chair, the resilient member bears indirectly against the backrest upright member, one or more intermediate component(s) is/are disposed between the resilient member and the backrest upright member. 
     In one embodiment of the chair, one of the one or more intermediate component(s) is a component of a backrest height adjustment mechanism. 
     In one embodiment of the chair, one of the one or more intermediate component(s) of is a first gear part configured to mesh with a second gear part of the backrest upright component. 
     In one embodiment of the chair, the backrest height adjustment mechanism comprises a rack-and-pinion system, whereby the rack component is provided on the upright member and the pinion component is a user-driven toothed wheel. 
     In one embodiment of the chair, the toothed wheel is lockable. 
     In one embodiment of the chair, the channel comprises a stop member configured to, during insertion of the backrest upright member into the channel, prevent the terminus of backrest upright member from being inserted beyond the terminus of the channel. 
     In one embodiment of the chair, the channel comprises a resilient member depressing means. 
     In one embodiment of the chair, the resilient member depressing means is disposed within the channel and configured to depress the resilient means so as to maintain the resilient means away from the interior space of the channel such that, upon assembly, would be occupied by the backrest upright member. 
     In one embodiment of the chair, the resilient member depressing means has a cross sectional profile that is identical, substantially identical or similar to that of the portion of the backrest upright member which, upon assembly, is disposed within the channel. 
     In one embodiment of the chair, the resilient member depressing means is configured to allow it be urged past the stop member of the channel upon insertion of the backrest upright member into the channel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of a perspective view of a preferred apparatus of the invention, showing both clutches in the engaged position. 
         FIG. 2  is a diagram of a lateral view of the embodiment of  FIG. 1 , showing greater details of the third cam actuation. 
         FIG. 3A  is a diagram of a further perspective view of the preferred apparatus shown in  FIG. 1 . 
         FIG. 3B  is a diagram of the embodiment of  FIG. 2A  showing sleeves used to support the metal bar carrying the cams. 
         FIGS. 4A, 4B and 4C  are plan diagrams of the preferred apparatus shown in  FIG. 1 , the apparatus in three distinct operational states. 
         FIG. 5  is a perspective drawing of a preferred attachment system of the present invention. This drawing shows the backrest upright member and channel in assembled form. 
         FIG. 6  is a perspective drawing of a preferred attachment system of the present invention as shown in  FIG. 5 . This drawing shows the backrest upright member and channel in unassembled form. 
         FIG. 7  is a perspective drawing of a preferred attachment system of the present invention as shown in  FIG. 6 . This drawing shows the backrest upright member being inserted into the channel. 
         FIG. 8  is a perspective drawing of a preferred attachment system of the present invention as shown in  FIG. 7 , with the backrest upright member being further inserted into the channel. 
         FIG. 9  is a perspective drawing of a preferred attachment system of the present invention as shown in  FIG. 8 , with the backrest upright member being further inserted into the channel. 
         FIG. 10  is a perspective drawing of the preferred attachment system shown in  FIGS. 5 to 9 , in assembled form, and as attached to a chair chassis. 
         FIG. 11A  is a cross-section of a preferred attachment system of the present invention, in unassembled form, and showing the position of a resilient member depressing means. 
         FIG. 11B  is a lateral view of the drawing of  FIG. 11A , showing the level at which the cross-section of  FIG. 11A  is taken. 
         FIG. 12  is a diagrammatic lateral view of a preferred attachment system whereby a torsion spring bears indirectly on the backrest upright member, via a toothed wheel involved in adjusting the backrest height. 
         FIG. 13  is a perspective drawing of a preferred mechanism for locking the height of the backrest in embodiments including a torsion spring bearing indirectly on the backrest upright member, via a toothed wheel involved in adjusting the backrest height. 
         FIG. 14  is a front lateral view of the locking mechanism shown in  FIG. 13 .  FIG. 14A  shows the locking mechanism disengaged, with  FIG. 14B  showing the locking mechanism engaged. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     After considering this description it will be apparent to one skilled in the art how the invention is implemented in various alternative embodiments and alternative applications. However, although various embodiments of the present invention will be described herein, it is understood that these embodiments are presented by way of example only, and not limitation. As such, this description of various alternative embodiments should not be construed to limit the scope or breadth of the present invention. Furthermore, statements of advantages or other aspects apply to specific exemplary embodiments, and not necessarily to all embodiments covered by the claims. 
     Throughout the description and the claims of this specification the word “comprise” and variations of the word, such as “comprising” and “comprises” is not intended to exclude other additives, components, integers or steps. 
     Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment, but may. 
     The present invention is predicated at least in part on Applicant&#39;s finding that a dual cam arrangement may be used to provide a chair adjustment apparatus whereby two chair parameters may be adjusted independently, with the independent adjustments being made by the user by way of a single actuating member. Accordingly, a user is enabled to adjust a chair using a single lever. For example, the user may first adjust the seat pan tilt angle to a desired angle and once that parameter is set subsequently adjust the backrest-seat pan angle. The set pan tilt angle setting is locked in the desired position while the user independently adjusts the backrest-seat pan angle. 
     The present invention is a significant advance over prior art adjustment contrivances which use two separate actuating members, with each member dedicated to adjusting a single chair parameter. It is found that users prefer the use of a single actuating member having alternate actuation means (for example two or more of: raising the member, lowering the member, pushing the member forwards, pulling member rearwards, or rotating the member). 
     Furthermore, production of the present apparatus is simplified and utilizes less materials leading to economic advantages. 
     Further advantage is gained over prior art contrivances that are configured to adjust two seat parameters with a single actuating member, with the user being forced to adjust both parameters contemporaneously. More precise customization of a chair to a user is allowed when the user adjusts each parameter in isolation, and sequentially. 
     The cams of the present invention may be any type deemed suitable by the skilled person having the benefit of the present specification. The cam is typically a rotary cam, being actuated by rotation about an axis of the cam. 
     The cam surface (i.e. the surface which contacts and actuates the chair parameter adjustment mechanism) may be of any geometry but will typically be curved, and configured to bear against a substantially planar surface of the chair parameter adjustment mechanism. For durability the cams may be fabricated from a metal, typically by either a casting or forging method. 
     As will be appreciated, lubrication between the cam surface and the actuation surface of the chair parameter adjustment mechanism will facilitate use and decrease wear on components of the apparatus. 
     The chair parameter adjustment mechanism may be any mechanism capable of being actuated by a cam. Typically, the adjustment mechanism comprises a resilient means (such as a spring) which is compressed when the cam bears against the actuation surface of the cam, and returns to the previous state when the cam surface ceases to bear on the actuation surface. 
     The chair parameter adjustment mechanism may operate on the basis of a frictional engagement or other reversible means for preventing the movement of a chair component involved in the adjustment of a chair parameter. One example of a chair parameter adjustment mechanism is a clutch mechanism. 
     It will be understood that a clutch mechanism in the context of an adjustable chair is not to be construed strictly to mean mechanisms used to transmit power from a driveshaft to a driven shaft (as used in automotive transmissions, for example). The term “clutch” is used in the art (and in the context of the present invention) to mean any contrivance capable of frictionally engaging with a moveable chair component to substantially prevent movement of that component. In this context, the clutch is able to alternately engage with a moveable chair component (to prevent movement, and therefore prevent adjustment of the chair parameter), and disengage (to allow movement, and therefore allow adjustment). 
     Thus, movement of a chair component is allowed when the user engages the clutch to permit the user to adjust a chair parameter. When the parameter is set at a desired position, the clutch is disengaged again by the user to cause locking of the chair parameter at the desired position. 
     For ease of use, the apparatus is configured such that, the cams only need depress the actuation bolt  14  or  16  a relatively short distance to disengage the clutch. In some embodiments the bolt only need be depressed 2 mm, 3 mm 4 mm or 5 mm. 
     While a clutch mechanism as discussed supra is generally preferred, the chair parameter adjustment mechanism may be any other means capable of alternately engaging and disengaging (whether or not by frictional forces) to alternately allow and prevent movement of the chair component involved in the adjustment a chair parameter. For example, the cam may bear against a sprung pin, the pin configured to be inserted into an aperture formed in chair component, insertion of the pin preventing movement of the component and removal of the pin allowing movement of the component. 
     Given the benefit of the present specification, the skilled person is capable of adapting the present invention to other types of chair parameter adjustment mechanism by routine means only. 
     The chair component, movement of which is allowed or prevented by the chair parameter adjustment mechanism may be any component identified by the skilled person given the benefit of the present specification. For example, the chair component may be a hinged component such as a backrest post capable of adjustment at an angle to the seat pan. The chair component may be a sliding component such as a seat pan support post capable of sliding within a sleeve and adjustable at a height with reference the floor. The chair component may be a pivoting component such as a seat pan capable of adjustment at an angle of tilt to the floor. The chair component may be a rotating component such as a wheel which may be adjusted from a rolling state to a locked state. 
     The present apparatus comprises a mechanical coupling between the two cams. The mechanical coupling may be any contrivance configured to allow each of the cams to independently actuate its associated chair parameter adjustment mechanism. In one embodiment, the mechanical coupling is a simple connecting member (such as a rod, bar or similar) which directly couples the cams such that movement of one cam directly moves the other. 
     For example, the mechanical coupling may connect two lobed cams, the lobed cams orientated at 180 degrees to each other. Thus, the first cam is rotationally positioned such that it actuates the first chair adjustment mechanism, and the second cam is (by virtue of the 180 degree offset) does not actuate the second chair adjustment mechanism (the second chair adjustment mechanism being substantially adjacent to the first chair adjustment mechanism). Upon rotation of the first cam 180 degrees, the connecting member rotates the second cam 180 degrees to actuate the second chair adjustment mechanism by virtue of the simple mechanical coupling between the cams. 
     It will be appreciated that the mechanical coupling may not be embodied in the form of a discrete structure. For example, two lobed cams may be disposed adjacent to each other at a required offset and mechanically coupled by use of an adhesive. In some embodiments (and as shown in the preferred embodiment of the Figures) the cams are unitarily formed with the mechanical coupling. 
     The chair adjustment apparatus may further comprise an actuating member, which is actuated by the user to make adjustments to the two or more chair adjustment parameters. Ultimately, the actuating member causes movement of the cams. The means by which that movement is caused may be due to a direct coupling to one of the cams. Alternatively, the movement of the cams may be due to an indirect coupling of a cam to the actuating member. For example the actuating member may be coupled to the mechanical coupling such that movement of the mechanical coupling in turn moves the cams. As another alternative, the actuating member may be coupled to an intermediate member which is in turn coupled to a cam of the apparatus. 
     The actuating member is typically an elongate member which extends toward or past the edge of the seat pan of the chair to which the apparatus is fitted. The user may grip the member directly, or more typically a handle attached to the actuation member. In one embodiment, the handle is configured so as to slide longitudinally along the actuation member so as to be alternately disposable within the perimeter of the chair pan or extended beyond the perimeter. 
     In the extended mode, the handle is easier to grasp and the further length can provide increased mechanical advantage in actuation of the chair parameter adjustment mechanisms. 
     While advantage is gained in the provision of means whereby two chair adjustment parameters are independently adjustable, further advantage is gained whereby a third seat adjustment parameter is individually adjustable. 
     Accordingly, in one embodiment, the actuating member is configured such that movement thereof causes actuation of a third chair adjustment parameter independent to the first and second chair adjustment parameters. For example, the actuating member may be configured to be pushed or pulled by a user along its longitudinal axis so as to move a third cam. In another embodiment the actuating member is rotatable about its longitudinal axis so as to move a third cam. 
     In one embodiment, the apparatus comprises a rotatable component configured to be gripped and rotated by a user, wherein rotation of the rotatable component causes actuation (optionally via a third cam) of a third chair adjustment parameter independent to the first and second chair adjustment parameters. The rotatable component may be a simple sleeve which surrounds the actuating member, and may optionally be extendable as disclosed supra. 
     It will be apparent that the present apparatus may be configured such that a single actuating member acts as a control mechanism for the independent adjustment of three or more seat adjustment parameters. For example, the first parameter may be adjusted by lifting the actuating member, the second by depressing the actuating member, and the third by rotating the member about its long axis. Thus, a user seeking to adjust a chair parameter is not forced to identify the correct actuating member amongst two or more available actuating members given that all adjustments are with a single member. 
     In addition to advantage for the user, advantage is gained from a manufacturing perspective given the need to supply and fit only a single actuation member. 
     Of course, an apparatus configured to provide for the adjustment of four, five, six or more chair parameters are contemplated. For example, a third clutch may be disposed between the first and second clutches, with the third clutch actuated by a fourth cam disposed between first and second cams. Each cam may be offset by 120 degrees to the adjacent cam such that independent actuation of the first, second and third clutches is provided for. Adjustment of a further chair parameter (making four in total) may be achieved by rotation of the chair adjustment means. 
     It will be appreciated that the chair parameter adjustment mechanism may be any mechanism operable by way of a cam, and includes any mechanism known to the skilled person having the benefit of this disclosure either at the filing date of this application or any date thereafter. 
     The chair parameter adjustment mechanism may be a part of chair, the chair further comprising a chair backrest attachment system comprising:
         a backrest upright member,   a channel configured to receive the backrest upright member, and   a resilient member   wherein in use, when the backrest upright member is inserted into the channel the resilient member bears directly or indirectly against the backrest upright member such that a movement of the backrest upright member within the channel is cushioned by the resilient means.       

     The presence of a member, while not necessarily preventing lateral movement of the upright member within the channel may nevertheless act to cushion movement. The term “cushion” is intended to include aspects such as soften, dampen, slow, restrain, diminish, absorb, quieten and the like. 
     The resilient member may be any apparent to the skilled person having the benefit of the present specification. More typically, the member is a spring including a helical spring, a flat spring, a machined spring, a compression spring, a gas spring, a leaf spring, or a flat spring. Torsion helical springs are preferred due their simplicity of construction, cost, durability and ease of incorporation into an office chair configuration. 
     Advantageously, in one embodiment the system requires only the insertion of the backrest upright member into a channel structure disposed elsewhere in the chair. Typically the channel is separately formed and extends beyond the rear edge of the seat pan, but is nevertheless attached to a frame or chassis of the chair. For example, many adjustable office chairs have a chassis that houses various adjustment means, clutches, levers and the like. The chassis is generally fabricated of metal plate and so provides a sound attachment point for the channel (and in turn the backrest upright member). 
     Insertion of the upright member into the channel automatically results in the resilient member exerting a force onto the member thereby resisting further lateral movement of the member. This represents an advance over prior art systems requiring the use of fasteners (such as screws, bolts, pins and the like) to firmly retain the upright member within the channel. 
     Many prior art chairs are devoid of any means for properly stabilizing the upright member, with the present invention providing similar ease of assembly but without the undesirable result of unrestrained movement of the upright member or the associated rattling sounds. 
     The resilient member may bear directly on the upright member, or in more complex arrangements bear indirectly thereon. The resilient means may bear on one or more intermediate components, which in turn transfer the force finally to the upright member. 
     In some embodiments, the resilient means bears against a component of a backrest height adjustment mechanism, thereby cushioning any movement between the components of the mechanism. 
     As another example of the resilient member bearing indirectly against the upright member, the resilient member may bear against the backrest itself thereby transferring the cushioning force to the upright member. It will be appreciated that in such embodiments, the backrest typically extends downwardly to the level of the seat pad, or even extends past the level of the seat pan. 
     In any event, the channel into which the upright member inserts generally does not completely surround the member, but instead is open in at least one region to allow the resilient member (or intermediate component where force is transferred indirectly) to bear against the upright member. Conveniently, the channel may be formed from a piece of sheet metal curved to provide a channel region, with panels adjacent the channel region configured to facilitate attached of the channel to a chair structure. 
     It is generally the case that a terminus of the resilient means bears against the upright member or intermediate structure. In preferred embodiments comprising a torsion spring the terminus is one of the two free ends of the spring. The terminus may only be a single wire of the spring, however preferably the wire is bent so as to present a U-shape or V-shape to contact the upright member or intermediate structure. 
     The upright member may be configured in any way to more securely engage with the resilient means or intermediate structure, as compared with the situation where no such configuration is provided. For example, the upright structure may comprise an aperture, a depression, a protuberance, or a deformation configured to engage with the resilient means of intermediate structure. Such engagement means may limit or prevent the longitudinal (i.e. upwards-and-downwards) movement, or axial rotational movement of the upright member within the channel. In some embodiment the engagements means limits or prevents the lateral movement of the upright member within the channel. 
     To further improve engagement between the resilient means and the upright member the resilient means may be especially configured. For example, where the upright member comprises a groove-shaped aperture a terminus of a resilient member may be substantially tongue-shaped and dimensioned so as to snugly fit within the aperture. Where the uptight member comprises a circular protuberance, the terminus of the resilient member may be substantially cylindrical so as to snugly fit over the protuberance. 
     Advantageously, the resilient means has two or more functions in the context of a chair, and includes a function other than bearing against the backrest upright member. As will be apparent to the skilled person, adjustable prior art office chairs comprise one or more mechanisms which rely on resilient means, such as a spring. For example, chair adjustment mechanisms often include a spring which urges a chair component (such as the seat pan, armrest, or backrest) toward a predetermined position, height or orientation. For example a seat pan tilt adjustment mechanism utilises a torsion spring to bring the set pan back to a level orientation where a controlling clutch is disengaged to allow for pivoting of the seat pan. 
     Use of a resilient member which is already a part of the chair construction provides cost savings to the manufacturer, and also simplifies construction. 
     To simplify assembly of the system components, the channel may comprise a stop member so as to prevent inadvertent excessive insertion of the terminal member edge through the lower edge of the channel. The stop member may be a discrete component, however conveniently the channel wall may be simply deformed at the lower edge for as to provide a barrier to passage of the upright member. 
     The present invention takes advantage of the ability of a resilient means to cushion the movement of the upright member within the channel. In order to better achieve the required cushioning, the resilient member is preferred to exert a significant force on the upright member. A problem arises in the use of powerful springs in so far as a user may find difficulty in depressing the spring out of the main channel space so as to make room for the upright member during insertion. This problem may be overcome or ameliorated by the incorporation of a resilient member depressing means within the channel, configured so as to maintain the spring in a compressed state. Compression of the spring and insertion of the depressing means is typically performed in the factory. 
     The depressing means is configured to be dislodged and replaced by the upright member during insertion of the member into the channel. In this way, the user is not required to compress the resilient member and instead simply inserts the upright member. The force applied to the depressing means by the resilient means is immediately transferred to the upright member upon dislodgement of the resilient means. 
     The depressing means may be shaped (cross-sectionally) the same or similar to that of the upright member, this ensuring a snug fit with the channel and a lesser chance of accidental dislodgement. However, it will be understood that the depressing means may be any shape or size so long as it is capable of maintaining the resilient means in a compressed state, yet is capable of dislodgement as required. 
     In embodiments of the channel having a stop member, it may be necessary to configure the depressing means to be capable of moving past the stop member so as to be expellable at the lower edge of the channel. Alternatively, the depressing means and upright member may be dimensioned so that both are retained within the channel after assembly. For example, where the length of the upright member is required to be 50 cm, an upright member of length 45 cm may be used along with a depressing means of 5 cm length. 
     It will be appreciated that in the description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. 
     Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those in the art. For example, in the following claims, any of the claimed embodiments can be used in any combination. 
     In the description provided herein, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description. 
     Thus, while there has been described what are believed to be the preferred embodiments of the invention, those skilled in the art will recognize that other and further modifications may be made thereto without departing from the spirit of the invention, and it is intended to claim all such changes and modifications as fall within the scope of the invention. 
     Although the invention has been described with reference to specific examples, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms. 
     The present invention will now be more fully described by reference to the following non-limiting preferred embodiments. 
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION 
     Turning to  FIG. 1  there is shown a preferred chair adjustment apparatus of the invention. This preferred embodiment is capable of adjusting three chair parameters: seat pan tilt, seat pan-backrest angle and seat pan height. 
     The apparatus comprises two clutches  10  and  12  adapted to alternatively engage and disengage with a movable chair component. In this embodiment the clutch  10  engages and disengages with a chair component involved in the adjustment of the seat pan angle by the user. Thus, when the clutch is disengaged the user is able to adjust the seat pan angle by rocking forward (or back) on the seat pan until a desired angle is achieved. Once the desired angle is achieved, the clutch is released such that the seat pan is locked at the desired angle. The clutch  12  performs a similar function, but for the backrest-seat pan angle. 
     The clutches  10  and  12  are of the dry multiple-plate type well known to the skilled person. Each clutch is operated by a simple spring mechanism whereby depression of the upper face of the clutch actuation bolt  14  or  16  against the resilient force of the spring  18  or  20  causes disengagement of the clutch from its associated chair component to allow movement of that component. When the depressive force acting on the actuation bolt  14  or  16  is removed, the spring  18  or  20  urges the actuation bolt away from the clutch body so that the clutches re-engages with its associated seat component to cause that component to lock in position. 
     The clutches  10  and  12  are engaged and disengaged by the cams  22  and  24 , each cam acting on the upper face of the clutch actuation bolt  14  or  16 . As will be noted, the cams  22  and  24  are formed by the deformation of a single length of steel bar  26 . The cams  22  and  24  are connected by the connecting portion  25 . 
     In  FIG. 1  the cams are shown in a position whereby neither cam is acting on a clutch actuation bolt  14  or  16 . Cam  22  is directly upwardly, and cam  24  (being disposed a 180 degrees to cam  22 ) is directly downwardly. With the cams so arranged, the seat pad and backrest are locked in position by action of the clutches  10  and  12 . 
     In order to disengage clutch  10 , the steel bar  26  is rotated counter-clockwise (when viewed from the right side of the page, as drawn) about its long axis by an angle of around 10 degrees. This rotation has the effect of urging the cam  22  against the upper surface of the clutch actuation bolt  14 , thereby disengaging the clutch  10 . It will be noted that contemporaneously the cam  24  is rotated away from the actuation bolt  16  of the clutch  12 , and accordingly clutch  12  remains engaged. In this way, the chair parameter associated with clutch  10  can be adjusted independently to the chair parameter associated with clutch  12 . Furthermore, the chair parameter associated with clutch  10  can be adjusted while the chair parameter associated with clutch  12  is fixed. 
     In this embodiment, the chair parameter associated with clutch  10  is the backrest-seat pan angle. The bolt  19  extending through the clutch is mechanically coupled to a pole (not shown) supporting the backrest (not shown). Where the bolt  19  is prevented from moving along a slot in the apparatus housing by engagement with the clutch  10 , the backrest pole is unable to pivot to alter the backrest-seat pan angle. Upon disengagement of the clutch  10 , the bolt  19  is permitted to move within the slot thereby allowing adjustment of the backrest-seat pan angle. 
     The counter-clockwise rotation of the bar  26  is achieved by the user lifting the bar extension  28 . The bar extension  28  is formed by a downward 90 degree bend in the bar  26 , and a further lateral 90 degree bed. The apparatus is typically installed on the inferior face of the seat pan, with the bar extension  28  extending toward an edge of the seat pan to facilitate manipulation by a user reaching downwardly. 
     In order to actuate clutch  12 , the steel bar is rotated clockwise about its long axis by 10 degrees. This rotation is achieved by the user pushing the bar extension  28  downwardly. This rotation has the effect of urging the cam  24  against the upper surface of the clutch actuation bolt  16 , thereby disengaging the clutch  12 . It will be noted that contemporaneously the cam  22  is rotated away from the actuation bolt  14  of the clutch  10 , and accordingly clutch  10  remains engaged. In this way, the chair parameter associated with clutch  12  can be adjusted independently to the chair parameter associated with clutch  10 . Furthermore, the chair parameter associated with clutch  12  can be adjusted while the chair parameter associated with clutch  10  is fixed. 
     In this embodiment, the chair parameter associated with clutch  12  is the seat pan angle. The pin  21  extending through the clutch is mechanically coupled to the seat pan (not shown). Where the pin  21  is prevented from movement by engagement with the clutch  12 , the seat pan is unable to pivot to alter seat pan angle. Upon disengagement of the clutch  12 , the pin  21  is permitted to move thereby allowing adjustment of the seat pan angle. 
     The preferred embodiment of  FIG. 1  is configured to allow for the adjustment of a third chair parameter. In this embodiment, the bar extension  28  is configured to actuate a third cam  30  upon rotation of a cylindrical cover  32  fitted over the bar extension. To the cylindrical cover is attached and elongate handle  34 , which is prevented from being removed from the cover  32  by a user by way of lugs  34  which engage with a ridge (not shown) on the interior face of the handle  34 . The cylindrical cover further engages with the handle  34  by way of a cam lug  36  by. The cylindrical cover  32  is permitted to rotate about its longitudinal axis, such that rotation of the handle  34  by a user in a clockwise direction causes the cam  30  to depress against the member  36  which in turn causes clockwise rotation of the bar  38  about its long axis to move the plate  40  in an arc and downwardly. 
     In the position shown in  FIG. 1 , the plate tip  42  locks into the slidable stem which supports the seat pan to prevent further movement of the seat pan in an upwards or downwards direction. When the cylindrical cover  32  is rotated clockwise the plate tip  42  can no longer effectively engage with the seat pan stem and so the height of the seat pan can be altered by the user. Typically, a gas spring (not shown) facilitates lifting and dampens the lowering of the seat pan. 
     It will be noted that this embodiment allows for the adjustment of three chair parameters by the use of a single member (being the bar extension  28 ), and the associated cylindrical cover  32 . Importantly, the three chair parameters are adjustable independently. 
     The actuation of the third cam  30  is shown more clearly in  FIG. 2 . Rotation of the sleeve  32  clockwise causes the curved cam surface to depress against the member  36  causing movement of the member  36  to the right (as indicated by the dashed arrow). Because of a pivot point about  38 , the plate  40  is caused to move to the left (as indicated by the dashed arrow) with the plate tip  42  downwardly. 
     The apparatus as shown in  FIG. 1  is housed in a frame (not shown), with the clutches  10  and  12  fastened to the housing to prevent undesired movement. The housing is in turn fastened to the underside of the seat pan. 
     The bar  26  is suspended at the correct distance from the clutches by the use of supporting sleeves extending from the housing. Reference is made to  FIG. 3A  showing the apparatus without the supporting sleeves to show the underlying structures, with  FIG. 3B  showing the sleeves  50 . The tabs  52  on the sleeves  50  are for mounting the sleeves onto a part of the apparatus housing (not shown). The lugs  54  formed in the bar  26  prevent slippage of the bar through the sleeves  50 , and therefore slippage of the cams  22  and  24  with reference to the clutch actuator bolts  14  and  16 . 
       FIG. 4  shows the apparatus in three distinct operating configurations. 
     The first configuration is shown at  FIG. 4A , which is the same as that shown in  FIG. 1  whereby the cams  22  and  24  are not urging against their respective clutch actuation surfaces  14  and  16 . In this first configuration, the bar extension  28  extends horizontally, and the clutches are not actuated. 
     The second configuration shown at  FIG. 4B  is achieved by the user lifting the extension bar  28  causing rotation of the cam  22  such that it urges against the clutch actuation surface  14  thereby disengaging clutch  10 . 
     The third configuration shown at  FIG. 4C  is achieved by the user lowering the extension bar  28  causing rotation of the cam  24  such that it urges against the clutch actuation surface  14  thereby disengaging clutch  12 . 
     Turning now to embodiments of the chair having the novel back backrest, reference is made to  FIG. 5 . shows components of the system in assembled form (i.e. with the backrest attached to the seat pan). The backrest upright member  10  is shown inserted into the channel  12 , this insertion having been effected by a purchaser of the chair. The external cross-sectional profile of the upright member  10  is similar to the internal cross-sectional profile of the channel  12 , this limiting the amount of lateral movement (or “play”) of the upright member  10  within the channel. 
     Nevertheless, to ease insertion the upright member  10  is dimensioned relative to the channel  12  so as to provide at least some play. It is this play (which may be minimal) that causes the undesirable movement and rattling in prior art attachment systems when a user leans against the backrest. 
     The presence and arrangement of the torsion spring  16  cushions any movement of the upright member  10  within the channel. It will be noted that the terminus  14  of the torsion spring  16  is disposed within the aperture  18  of the channel  12 . In this configuration the torsion spring  16  is in tension and so the terminus  14  bears against the rim of the aperture  18  so as to resist lateral movement of the upright member  10  within the channel  12 . 
     In some embodiments, the force of the torsion spring  16  against the upright member  10  is so great so as to substantially remove any play between the two components. It will be appreciated that in that situation the torsion spring may be difficult to compress during insertion of the upright member by the user. It will therefore be more typical for the torsion spring to still allow at least some play (and optionally allow all available play), however with a cushioning effect. Thus, when a user bears against the backrest, the torsion spring gradually absorbs the force thereby slowing the lateral movement of the upright member within the channel. This cushioning effect affords the user a feeling of security in the integrity of the backrest. Furthermore, the channel is prevented from hitting the channel at speed, thereby diminishing any rattling or other noise when the user reclines. 
     Similarly, when the user sits forward (thereby ceasing to bear on the backrest) the upright member is prevented from freely moving within the channel given that the torsion spring is constantly urging the member against the interior face of the channel. This constant urging of the upright member limits the amount of rattling and other noise which may lead the consumer to conclude that the chair is faulty. 
     In the preferred embodiment of  FIG. 5  the upright member  10  is prevented from movement along the longitudinal axis (i.e. in an up-and-down direction) by the locking of the torsion spring terminus in the aperture  18 . The torsion spring terminus  14  is dimensioned so as sit completely within the aperture  18 , with the torsion spring  16  providing sufficient force so as to prevent the terminus  14  from exiting the aperture. This feature provides an even greater feeling of security to the user, and further diminishes any noise resulting from the user reclining and sitting forward on the chair. 
     It will be noted from  FIG. 5  that the channel  12  is not completely closed, and is open in the region of the torsion spring terminus  14 . In this preferred embodiment, the channel  12  is open along the entire length, the opening flanked by opposing attachment panels  20 , each panel having a two bolt holes  22 . The attachment panels  20  are configured so as to attach to the main chair chassis disposed on the underside of the seat pan (not shown). Bolts (not shown) extend through the chassis and panels  20  to secure the channel  10  to the chassis. 
     The torsion spring  16  is a spring which is otherwise required in the chair. In this preferred embodiment, the torsion spring  16  is involved in the seat pan tilt angle adjustment mechanism (not shown). In that adjustment mechanism, the function of the spring is to return the seat pan to a predetermined angle when the user actuates a lever to unlock the seat pan and allow it to freely pivot. In prior art chairs, the terminus  14  of the spring bears against a part of the chassis, and performs no function other to maintain the tension of the spring. In the present invention, that part of the chassis is removed to allow the spring terminus  14  to bear against the upright member  10 . The upright member  10  acts to maintain tension on the spring such that the seat pan tilt adjustment mechanism still functions as per normal. 
     In this preferred embodiment, the wall of the channel  12  has a stop member  24  disposed at the lower end. The function of this member  24  is to prevent the upright member  10  from being inserted beyond the lower edge of the channel  10 , thereby ensuring that the spring terminus  14  is properly located in the upright member aperture  18 . 
     Turning now to  FIG. 6 , the upright member  10  and channel  12  are shown in a position ready for assembly. The backrest (not shown) is attached at a higher point on the upright member  10 . The seat pan (not shown) sits above the channel  12 , but is disposed such that the channel is accessible to the upright member  10 . The longitudinal axis of the upright member  10  is aligned with that of the channel  12 . The torsion spring terminus  14  is extended across the channel space, being in a substantially uncompressed state. 
       FIG. 7  shows the further insertion of the upright member  10  into the channel  12  to the point that the lower end of the member  10  contacts the torsion spring  16 . 
       FIG. 8  shows the further insertion of the upright member  10  into the channel  12  to the point that the lower end of the member  10  contacts the torsion spring terminus  14 . In this circumstance, the torsion spring is in a substantially compressed state 
       FIG. 9  shows the further insertion of the upright member  10  into the channel  12  to the point that the lower end of the member  10  is disposed on the edge of aperture  18 . Inserting the upright member  10  slightly further causes the torsion spring terminus  14  to securely locate in the aperture  18  (as shown in  FIG. 1 ). In this circumstance, the torsion spring  16  is in a substantially compressed state, and the chair is ready for use. Given that the torsion spring  16  is compressed by the upright member  10 , the spring is now able to function in its additional capacity as part of the seat pan tilt adjustment mechanism. 
       FIG. 10  shows the channel  12  attached to the chassis  26  of the chair. The lower face of the seat pan (not shown) sits on the surface  28  in a position as indicated by the dashed plane  30 . The seat pan attached to the chassis by way of screw fasteners (not shown) inserted through the grooved apertures  32 . This drawing more clearly shows the stop member  24  on the wall of the channel at the lower edge. 
       FIG. 11  shows the channel  12  in a preferred embodiment of the invention having disposed within a ring member  34  which acts to depress the terminus  14  of the torsion spring  16 . The cross-sectional diagram of  FIG. 11A  is taken through the line Y-Y as shown in  FIG. 11B . In this embodiment, the ring member  34  is inserted at the point of manufacture using a special tool to depress the spring terminus  14 . Upon insertion of the upright member into the channel by the user, the ring member  34  is dislodged an expelled through the lower end of the channel  12 , and discarded. It will be noted that the ring member  34  comprises a space to allow passage past the stop member  24  during expulsion. 
     Use of a ring member as described eases insertion of the upright member by the user. The user is not required to provide the substantial force needed to compress the torsion spring (this having been performed in the factory). Instead, the user only need push the ring member downwardly, this occurring automatically upon insertion of the upright member. The upright member then inserts into the space previously occupied by the ring member, and the torsion spring bears against the upright member. 
     It will be appreciated that the member need not bear directly against the upright member, with the present invention including embodiments whereby an intermediate structure is disposed between the resilient member and the upright member. An example of such an embodiment is shown in  FIG. 12  whereby a portion of the torsion spring  16  superior to the terminus  14  bears against the axle  40  of a toothed wheel  42 . The toothed wheel  42  is thereby urged against the upright member  10  thereby absorbing any free play in the upright member  10 . 
     In the preferred embodiment of  FIG. 12  the toothed wheel meshes with a series of grooves  44  in the opposing face of the upright member  10 . Thus, rotation of the toothed wheel causes the upright member  10  (and therefore the backrest, not shown) to move upwards and downwards to a position desired by the user. A hand crank (not shown) is used rotate the toothed wheel  42 . When the backrest is in a desired position, the toothed wheel  42  is locked by insertion of locking member  46 . 
     It will be noted that the longer arm of the torsion spring  16  exerts a force against the toothed wheel  42  thereby urging a meshed tooth laterally and toward the rear wall of the groove  44 . In this position, the spring  16  is in a substantially compressed state. Subsequent insertion of the vertical member forces the toothed wheel  42  forward (i.e. toward the front edge of the seat pan) thus transferring the compressive load from the rear wall of the groove  44  to the upright member  10 . Upon engagement the toothed wheel  42  is forced forward only by a minimal distance (typically about 2 mm) so as to facilitate home/office self-assembly. 
     Once the vertical member is engaged the full (or substantially full) compressive force of the torsion spring is transferred through the toothed wheel  42  and directly against the groove  44  of the upright member. 
     A further advantage of this arrangement is that the torsion spring  16  acts not only to cushion a movement of the upright member  10  within the channel  12 , but to also maintain the toothed wheel  42  and grooves  44  in close meshing association. Any play in the meshing association is cushioned by the torsion spring  16  bearing against the toothed wheel  42 , this providing an even greater feeling of security for a user leaning against the backrest. 
     While the rack and pinion arrangement shown in  FIG. 12  is preferred, it will be appreciated that many other arrangements achieving the same or similar mechanical result will have utility. For example, a roller pinion gear will be operable whereby the toothed wheel is replaced by a roller pinion, with the rack of the upright member remaining unchanged. 
     As a further alternative, a pulley system may be incorporated whereby the resilient means bears against a pulley disposed on the upright member. Where the resilient means bears against a pulley on the backrest, the cushioning force of the resilient means is nevertheless transferred to the upright member thereby cushioning any lateral movement of the upright member within the channel. 
     Turning now to  FIG. 13  there is shown an alternative toothed wheel locking mechanism to that of  FIG. 12 . The locking mechanism if  FIG. 12  directly contacts the toothed wheel, while that of  FIG. 13  is an indirect locking mechanism. 
     The locking mechanism of  FIG. 13  has an external adjustment wheel  50 , disposed lateral to one of the opposing attachment panels  20 . The adjustment wheel  50  is rotatably mounted on the same axis (not shown) as the toothed wheel (not shown). By the user rotating the wheel  50  clockwise, the toothed wheel is also rotated clockwise, this in turn raising the upright member and also the backrest. Lowering the backrest is achieved by counter-clockwise rotation of the wheel  50 . 
     The adjustment wheel  50  has integral locking members  54  disposed circumferentially about the inner face. Two adjacent locking members  54  define a space  56  therebetween into which a tab  56  may be accommodated. The tab  56  is formed by a deformation in the adjacent attachment panel  20 . In  FIG. 13 , the wheel  50  is shown in a locked position with rotation in both directions being prevented by the locking members colliding with the tab. Given that the toothed wheel is disposed on the same axis, rotation of the toothed wheel is also prevented thereby locking the backrest at the desired height. 
     The adjustment wheel  50  is movable along its axis of rotation to alternate from a locked state (whereby height adjustment of the backrest is prevented) to an unlocked state (whereby height adjustment of the backrest is allowed). Turning to  FIG. 14A , the locked state (of  FIG. 13 ) is shown in lateral view to more clearly show the wheel  50  pushed inwards towards the attachment plate  20 . In  FIG. 14B , the adjustment wheel  50  has been pulled outwardly by the user, such that the tab  52  no longer abuts the adjacent locking members  54 , and the user is free to rotate the wheel  50  to adjust the backrest height. When the desired backrest height is achieved, the user pushes the wheel toward the attachment panel  20  to return to the locked state shown in  FIG. 14A . 
     It will be appreciated that the adjustment wheel  50  may be spring-loaded, such that the spring is stretched when the wheel is pulled outwardly by a user. To return the wheel  50  the locked stated the user simply releases the wheel, causing it to recoil back toward the attachment panel  20 .