A CHAIR

Abstract: In a chair (21) with a back rest (34), the rear portion of the seat (10) has a recess (13) to receive the posterior and the upper portions of the thighs of a person, whereby the recess partially surrounds the posterior and the thighs, and comprises between 45 % and 90 % of the length of the seat. The recess is preferably deepest at the rear end and preferably adjustable. The rear portion of the seat comprises preferably seat straps (17-18) or the like, suspended between side members (23). The length of arc of the straps and the length of the seat are preferably adjustable. Preferably, the back rest comprises horizontally extending back rest straps suspended between vertical columns, and its curvature and angle are preferably adjustable. The chair has preferably an open area between the rear edge of the seat and the back rest, so that no part of the seat or back rest abuts on backwards facing portions of the person's posterior.

Description: A CHAIR The invention relates to a chair or another piece of sitting furniture for use with an essentially upright sitting position, including a seat of any kind to carry a sitting person in an essentially upright sitting position, which furniture comprises a seat and a supporting structure which is preferably adjustable in height. For the sake of simplicity, this piece of furniture is in the following and in the claims referred to as "a chair" without this notion being considered as limiting. The International Patent Application No. WO 83/3195 discloses a chair for the prevention of pressure sores (de- cubi tis nicer) of sitting patients, where the shape of the seat cushion is formed by a moulded seat padding with a lowered area for the user's posterior, and where the lowered area is surrounded by heightened ribs forward and to each side, but is open at the back. The front rib is deeper (longer) than the side ribs and extends preferably over about 40% of the length of the seat. This chair has the inconvenience that there is no possibility of adjustment to individual users. The chair must therefore be either adapted to the user's proportions during production whereby it will be extremely costly and can only be used for actual treatment of patients, or used without adjustment whereby most users cannot obtain a reasonable sitting comfort as there will still be local specific pressure concentrations between the seat of the chair and the user's thighs and posterior. US Patent No. 5,058,952 discloses a chair where the seat cushion is carried by a number of adjustable belts, each extending from the right side to the left side of the seat in various distances from the front edge of the seat . Apparently, the seat is intended to be adjusted automatically when the user sits down as the belts stretch to a certain tension until a certain point on the seat is lowered to a predetermined depth whereupon the belts are locked in the length which they have then reached. The seat is to this end well equipped with mechanisms and " electromechanical control devices. This chair has the inconvenience that the seat because of its nature and enhanced by the automatics will permit the user to take any sitting position, and thus also an inappropriate or downright bad or injurious sitting position. Furthermore, such a seat will be extremely costly. From the International Applications Nos . WO 99/63868 and WO 00/15080, chairs are known with seats, the rear third of which are lowered with a relatively sharp- edged transition to the rest of the seat and/or can be swung downwards and fixed in a desired angle. Thus, the transition and the edge, respectively, "seize" the user's ischia (tuber ischii ) which are downward protuberances on the pelvis resting against the seat and the ischia are thus prevented from being displaced forwards, backward rotation of the pelvis (the two ilia and sacrum; pelvis) being prevented. The chairs in these two international applications have the disadvantage that the form of the seat has relatively sharp edges or transitions. Such edges and transitions will always give local specific pressure concentrations on the user's thighs and posterior, and consequently a considerable discomfort or even pain. The International Patent Application No. WO 99/62379 discloses a chair where the seat upholstery is placed on a rigid, plane plate and -has an oval depression at the back of the seat whereby the rear edge of the seat cushion is about 3 cm lower than the front edge. Furthermore, the back rest is apparently designed with a cut-out at the bottom so that the upper portion of the pelvis is not supported from the rear by the back rest . The description is, however, summary and extremely obscure, and the description of the back rest is based entirely on the figures 4 and 5 which are contradictory. This chair has the disadvantage that the seat is substantially plane and, therefore, an appropriate support of the user's thighs and posterior can probably not be obtained. The described oval depression in the seat cushion seems in this connection completely insufficient. It is moreover a considerable inconvenience that because of the cut-out, the back rest cannot support the upper edge of the pelvis against fall-back. From the Internet-addresses http: //www. obusforme . com/products/product8.htm and http://www.dic.dk/Obusforme. html a chair is further known with back rest and seat of the mark Obusforme ® where the middle and rear portions of the seat are apparently a little lower than the front portion of the seat, seen from the side. Thus, the hip end of a sitting person's thighbone is allegedly on a level with or a little lower than the knee end of the thighbone. Finally, for use in racing cars, moulded car seats are known to be individually adjusted to the driver or even produced as a cast of the driver's body. Such seats are, however, adjusted to a lying position and are not at all suitable for an upright sitting position. The object of the invention is to provide a piece of furniture belonging to the initially stated group of kinds where the above disadvantages are eliminated, and where a sitting person can be supported in such a way that the angle formed by a person's pelvis in the sitting position in the rotary movement which the pelvis can make around the hip joints while the femurs are substantially kept stationary, and the course shown by the person's lower vertebras in the sitting position are and remain optimal in order to prevent discomfort and pain in the lower articulations of the spinal column, including the articulation to the pelvis (sacrum) and discomfort in the sitting position in general . It is in particular an object of the invention to provide such a piece of furniture where a sitting person's thigh and posterior are supported with as even a load distribution as at all possible in order to avoid the above mentioned specific pressure concentrations. In a chair according to the invention, the rear portion of the seat has a recess which is dimensioned to receive the posterior and the upper portions of the thighs of a sitting person, the recess at least on the person's right and left side partially surrounding the posterior and the upper portion of the thighs. This recess has several favourable effects. Firstly, it helps to prevent that the user's thighs and posterior are displaced forward on the seat as this would require that the stout part of the thighs and the posterior should thus be pushed upwards from the recess; this is counteracted by the gravitation in connection with the inclination of the front portion of the recess. It is advantageous to avoid that a sitting person slips forward on the seat. A chair with back rest will thus prevent in a particularly effective way the so-called "pelvis fall-back", i.e. that the upper edge of the pelvis falls backwards in the direction of the back rest, relative to the bottom edge of the pelvis (the ischia) . Since the back rest is stationary in relation to the seat of the chair, the pelvis-fall-back would of course require that the bottom edge of the pelvis is displaced forward on the seat . Pelvis fall-back is by far the most common sitting defect, and one of the most serious defects, as it causes a heavy strain on ligaments and articular disks in the lower joints in the spinal column because these are subjected to bending at the same time as they must carry the weight of the back in a static way. Secondly, the recess has the effect that the vertical reaction force from the seat against the user's weight is distributed efficiently over a larger part of the surface of the user's thighs and posterior, as besides being supported in the middle below the pelvis and in the middle under the femurs, these are also supported to the right and to the left of these areas, i.e. where the oblique "sides" of the recess abut on the outer sides of the user's buttocks and the outer sides of the upper portions of the thighs. In this way, an efficient relief of the user's ischia is effected. The ischia will when sitting on a plane or nearly plane surface be strained rather heavily resulting in soreness and tiredness in the tissue positioned around the ischia. It is preferred that the recess comprises between 45% and 90%, preferably between 55% and 80%, and particularly preferred about 66% of the length of the seat. Tests have shown that an advantageous adjustment is thus obtained to the average proportions of the user's thighs and posterior, as the recess will be dimensioned to contain the stout part of the user's thighs and the posterior. In this way, a better engagement of these body parts is obtained in the recess and thus an increased security against a forward slipping on the seat and a consequent pelvis fall-back. In a chair with the described features, the above defined objects are according to the invention met in that the depth of the recess is adjustable, preferably separately (i.e. locally) adjustable in various distances from the front edge of the seat . A possibility is thus attained for a particularly good adjustment of the seat and the recess to the user's proportions in order to obtain a particularly secure fixing of the posterior and thighs against slipping forward, and to obtain an optimal distribution of the vertical reaction forces of the seat over the entire width of the user's posterior and thighs. This will essentially reduce the maximal (local) value of the specific pressure on the user's posterior and thighs arising from the reaction forces . The recess is preferably formed in that the supporting structure of the recess comprises flexible elements which are suspended between side members extending essentially along the right and left edges, respectively, of the seat . Thus, the recess has in an advantageous manner the character of a cylinder or conical surface with its axis parallel to the user's femurs. If the dimensions of this surface are selected in an appropriate way, in accordance with the user's proportions, it is obtained that the seat will surround the user's posterior and thighs as far as possible up the sides such that the reaction forces from the seat acting on the posterior and thighs are distributed over as large a part of the surface of the posterior and thighs as possible. In chairs according to prior art, it has been explained above that the back rest is important to prevent the said pelvis fall-back. In the chair according to the invention, it has surprisingly turned out that this effect to a considerable extent - but apparently not completely - can be obtained without the use of a back rest . Tests have thus shown that testees which on an ordinary chair without back rest develop pelvis fall-back after a short moment, can sit on a chair according to the invention for up to 10 minutes without developing pelvis fall-back. This test result has not been understood. However, this favourable effect of the invention is presumably due to a particularly advantageous coordination between the shape of the seat and the sitting person's skeleton. One assumption is that the additional support which the chair according to the invention provides on the outward sides of the thighs, can make the femurs turn around their longitudinal axes, and that this influence can reflect on the pelvis such that it does not fall back so easily. A second assumption is that the course of the femurs where each femur is horizontal or where the hip end of the femur is a little lower than its knee end, in itself results in a reduced tendency of pelvis fall-back. A third assumption is that the pelvis position which the chair according to the invention makes the user occupy is identical to the pelvis position of a person in an ideal, completely balanced, standing position. In a thus balanced, standing position all the articulations of the body, and in particular the articulations of the spinal column, are in a neutral position whereby in principle all intervertebral discs (discus intervertebral - is) are loaded evenly over their entire area, all ligaments - in particular the anterior and posterior longitudinal ligaments (ligamenta) of the spinal column - are equally tight and all the muscles of the back are relaxed or nearly relaxed. Obviously, a sitting position where the position of the pelvis and the body corresponds to such a balanced, standing position must be ideal in order to prevent problems with back and pelvis by sedentary work. The above observations all strongly indicate that the chair according to the invention incites exactly such an ideal sitting position of the person sitting in the chair. In accordance with these surprising observations, it has further turned out that a person sitting on a chair according to the invention can maintain his advantageous sitting posture with far smaller support from a back rest than necessary in a chair with a conventional seat, and that he can remain seated for hours without discomfort. In a preferred embodiment, the side members extend obliquely relative to the horizontal such that the rear portions of the side members is higher than their front portions . It is further preferred that the side members extend with varying distance between them, such that the distance between the rear portions of the side members is larger than the distance between the front portions of the side members. These two features permit an improved encompassing of the user's posterior and the upper portions of his thighs, the sides of the recess becoming higher, and there will be space between the side members for the user to be "lowered further down" between them. The flexible elements comprise preferably seat straps, fabrics or similar material layers or one or more sheet-shaped elements. The said elements are all in principle inextensible and at the same time very flexible. When a person sits in the chair according to the invention, a very advantageous pressure equalisation is thus obtained between different areas of the user's downward surface, and unwanted specific pressure concentrations are avoided to the greatest possible extent . In a preferred embodiment, the desired adjustability is obtained in that the lengths of arc of the flexible elements measured from side member to side member can be adjusted, preferably adjusted locally in various distances from the front edge of the seat . Hereby is obtained firstly that merely one type of elements in the constructional structure of the seat (i.e. the lengths of the straps) must be adjusted when adapting the recess to a user (and not e.g. the inclination of the side members) , and secondly that the recess can be adjusted even better to the individual user's proportions and "physiognomy" . It is preferred that there are several flexible elements in different distances from the front edge of the seat, and that the lengths of arc of these elements can be adjusted individually. In this way, a particularly simple and easy adjustability of the shape of the recess is obtained, the user being immediately able to feel which flexible elements are to be slackened and which ones to be tightened in order to obtain a desired sitting position. In a particularly preferred embodiment, the supporting structure of the rear portion of the seat comprises seat straps, each comprising two portions provided with burr fasteners such as Velcro ® tape to obtain adjustability for the effective lengths of the seat straps by joining the burr fasteners in different positions. In this way, adjustment of the depth of the recess is enabled in a simple and secure way and without the use of tools. Each seat strap can e.g. comprise two strap parts each fastened to its side member of the seat, and each provided with cooperating Velcro ® tapes; thus the length of the pieces of the two strap pieces which overlap each other can be varied whereby the entire effective length of the strap is varied. Alternatively, each strap can be circumferential, i.e. disposed as a loop around both side members whereby the two ends of the strap overlap each other and are provided with cooperating Velcro ® tape, so that the effective length of the strap can be varied by changing the length of the overlap. Combinations of these two principles are also included in the invention. It is preferred according to the invention that the length of the seat measured from its front edge to the rear edge of the recess is adjustable. This provides a very advantageous possibility of adjustment of the seat to the length of the user's thighs. It is thus in particular preferred that the length of the seat is adjusted by adjusting the length of the recess. In this way, interventions in the fixed front part of the seat are avoided. It is preferred that the rear edge of the seat is essentially flush with the bottom of the recess, whereby any suggestion of an upward rear edge is avoided, and such that a supplementary security is obtained for the user' s posterior to be able to be pushed all the way back on the seat of the chair without hindrance. At the same time, any pressure against the user's coccyx is avoided which would be very unfortunate and injurious for a good sitting position. The chair according to the invention can preferably comprise a back rest which is meant to abut on the user's back opposite the upper edge of the pelvis. As explained above, the seat according to the invention makes it per se considerably easier for the user to maintain a favourable sitting posture without pelvis fallback, and it is therefore not absolutely necessary with a proper back rest. However, it is preferred according to the invention that the chair has at least one back rest which abuts on the user's back opposite the upper edge of the pelvis in order to secure efficiently against pelvis fallback. In another embodiment, the chair has a back rest with a supporting structure which essentially comprises horizontally extending back rest straps which are suspended between two essentially vertical columns generally extending along the right and left edges, respectively, of the back rest . This provides a simple structure with correspondingly simple and varying adjustment possibilities as explained above regarding the seat of the chair where straps were suspended between side members. It is preferred that the curvature of the back rest is adjustable by adjusting the effective lengths of the back rest straps . A desired possibility of adjustment to the user's proportions is thus also obtained, especially for "adjustment" of the optimal course of the user's spinal column. By an appropriate choice of the number of straps and variation of their length, the invention permits to adjust the curve in a horizontal section plane (i.e. the curve of the backside of the body as seen from above) independent of the curve in a vertical section plane (i.e. the curve of the spinal column as seen from the side) . In a preferred embodiment, the angle of the back rest is adjustable, preferably in that the angle of the columns relative to the vertical direction is adjustable in or near a vertical plane parallel to the vertical longitudinal plane of symmetry of the seat . In this way, a specific adjustment possibility is obtained in an advantageous way for the angle between the user's bottom vertebras and sacrum which is extremely important for an appropriate load on the bottom articular disks of the spinal column. It is preferred that the chair has an open zone between the rear edge of the seat and the lower edge of the back rest in order to secure that no part of the chair abuts on backwards facing portions of the person's posterior. Thus is assured that an upward rear edge or alike on the seat does not counteract that the posterior is pushed entirely back on the seat. At the same time, pressure is prevented against the user's coccyx which would be very unfortunate and injurious for a good sitting position. Finally, it is preferred that the back rest be displaceable in the longitudinal direction of the seat when the user is sitting in the chair, whereby the user can force the back rest forward against the upper portion of the pelvis once he has taken his sitting position. By this feature, a considerable relief is obtained for the user in order to take a good sitting position in the chair according to the invention. Now, when a pelvis fall-back is to be prevented efficiently, it is e.g. desirable that the surface of the seat has a high friction against the user's surface, and that at the same time, the user's posterior is pushed entirely back on the seat and "in below" the back rest as he sits down. The desired high friction can, however, notoriously make it more or less difficult to push oneself entirely back on the seat, Therefore, it will be easier to sit in the chair and obtain a correct sitting posture if the user can merely sit in the seat and then move the back rest forward until the upper edge of the pelvis is forced sufficiently far ahead. In the following, the invention will be explained further by means of embodiment examples and with reference to the drawing, on which Fig. 1 shows in a schematic and simplified way the course of a sitting person's leg and back bones, and the body contour of back, posterior and thighs, together with the seat of a chair according to the invention, seen from the side; Fig. 2 shows a chair according to the invention with the seat cushion removed, Fig. 3 shows the chair in Fig. 2 seen from the side, Fig. 4 shows the chair in Fig. 2 seen "from the front, Fig. 5 shows the chair in Fig. 2 seen from above, Fig. 6 shows schematically a seat and a back rest of an alternative embodiment of the chair according to the invention, with the seat cushion removed, Fig. 7 shows schematically a seat and a back rest of another alternative embodiment of the chair according to the invention, with the seat cushion removed, Fig. 8 shows schematically a person sitting in a chair according to the invention with the seat and the back rest in Fig. 6 and with the seat cushion in place, Fig. 9 shows schematically a vertical cross section in the seat of the chair and the pelvis part of a person who sits in the chair in Fig. 8, seen from behind, the seat cushion having been removed again. In all figures, the same reference signs are used for corresponding elements. Fig. 1 shows schematically a sitting person seen from the side. The person's foot is symbolised by 1, the tibia and fibula by 2, the femur by 3, the pelvis by 4 and the spinal column by 5. The person's body contour is shown as the curve 6. In Fig. 1, the person is positioned in a particularly favourable sitting position, the pelvis 4 being localised such in the direction of rotation 7 - the rotating movement which the pelvis can perform about the two hip joints while the femurs 3 are kept stationary - that the pelvis 4 and the lumbar vertebrae 8 occupy approximately the same position or angle relative to each other as when the person stands up in perfect balance and approximately at rest. It is a generally recognised experience that this angle is particularly favourable in order to prevent back injury and back pain. It is, however, a problem in nearly all known chairs and all known sitting furniture that the pelvis has a pronounced tendency to "fall back", i.e. to rotate in the direction of rotation 7, clockwise in Fig. 1, the sitting person's thighs slipping forward on the seat 10 in the direction 9 and/or the upper edge of the person's pelvis 4 being displaced rearwards, inward against a back rest or into the upholstery of a back rest. Hereby, an unfavourable angle is developed between the pelvis 4 and the lumbar vertebrae 8, resulting in an unfortunate and usually downright injurious effect on the articular disks in the spinal column 5, and as a rule also on the ligaments and in marked cases also on the musculature of the back. In this connection, the literature states (cf. e.g. A.C. Mandal : Det Siddende Menneske - Homo Sedens (The Sitting Man - Homo Sedens) , Dafnia Publications, Klampenborg, Denmark, 1987, ISBN 87-982017-3-5) that a person's lumbar sway (retroflexion of the lumbar vertebrae 8, cf. Fig. 1) has a natural tendency to be straightened when the person is sitting down. An angle of 90° between thighs and torso is thus observed to be obtained mostly in that the hip joints are rotated 60° and the lumbar sway is straightened by 30° and thus almost disappearing. This straightening of the lumbar sway is generally considered to be very injurious. It is therefore desirable that a chair can force a person - or* help him - to counteract this straightening of the lumbar sway. The person in Fig. 1 is sitting on a seat 10 of a chair according to the invention. Seen from the side, this seat is divided into three portions: a front portion A which is essentially horizontal; a middle portion B which tilts to the rear; and a rear portion C which is also essentially horizontal. Tests have shown that this seat in a surprising way and surprisingly efficiently prevents the pelvis 4 from falling back as described. This effect can be supported by a well adapted back rest (not shown in Fig. 1) . The effect is supposed to be explained in that the inclined portion B of the seat because of its oblique angle together with the gravitation perform such a sufficiently increased resistance against the underside 1 of the thighs that in practice the said slipping does not take place in the direction 9. As the lower edge of the pelvis 4 cannot slip forward in the direction 9 and the upper edge of the pelvis 4 is preferably supported from behind by a back rest, the pelvis 4 is prevented in an efficient way from falling back and the desired lumbar sway can thus be maintained in a secure manner. In Figs 2-5, an embodiment is shown for a chair 21 according to the invention with a seat 10 and back rest 34 where the seat cushion is removed for the sake of clarity. This cushion is preferably merely a cushion of a uniform thickness and with uniform elastic properties, of the same size as the seat and placed upon this. One preferred embodiment of the seat cushion consists of an approximately 30 mm thick plate of polyether foam, covered with a cloth which is not smooth. In the embodiment in Figs 2-5, the front third of the seat 10 is constituted by an upholstery 12 with an essentially horizontal upper surface 14 and a rounded front edge 15. The seat is in particular characterised in that the rear two thirds of the seat are constituted by a recess 13. In the embodiment in Figs 2-5, this recess is provided in that the rear portion of the seat has an opening, across which two straps 17-18 are suspended with a certain slack, the rear portion of the sitting surface of the seat, formed by the straps 17-18, being lowered relative to the upholstery 12. The straps 17-18 are suspended between two branches or side members 22-23 of a circumferential frame 28 which also carries the upholstery 12. The frame 28 is carried in an adjustable manner by a bar 19 which is positioned on an underframe 20 of a type known per se . The underframe 20 is besides not as such of importance for the characteristic mode of operation of the chair 21 according to the invention, except that it should in a manner known per se be adjustable in the height, cf . below. By the fact that the rear portion of the seat 10 is formed by straps 17-18 suspended between branches or side members 22-23 of a frame 28, it is obtained that the seat has a curve in a vertical cross plane, e.g. as seen from the front in Fig. 4. Here it becomes clear that the straps 17-18 both extend along upwards concave curves. This curve has surprisingly turned out to contribute partly to an efficient retaining of the user's thighs and posterior on the seat from slipping forward in the direction 9 (Fig. 1) , and partly to a favourable reduction in the discomfort of the continuous load provided by the sitting position on the underside of the posterior and the backside of the thighs of the sitting person. This is further illustrated in Fig. 9 which shows a vertical section in the seat 10 of the chair 21 with a sitting person, seen from behind. The rear portion of the seat 10 with the recess 13 is in Fig. 9 illustrated by the strap 18 which extends between the two branches or side members 22-23 of the frame 28 of the seat. As the load from the sitting person's body 24 is mostly carried by the strap 18, the strap will obviously follow the contour of the body, as shown in the figure. The seat cushion is omitted in the figure for the sake of clarity. In general, a sitting person's posterior will rest the hardest against the base opposite the ischia 25-26 and under the femurs 3. However, as both the strap and the body contour are resilient (the strap is very flexible, but inextensible, and the body is very elastic and to some extent plastic) , a balance will arise between the normal forces 29-30 of the curve followed by the strap 18. In places where the body 24 rests against the strap 18 with a larger normal force 29, the strap will bend outwards, the normal force 29 then being reduced, and in places where the body rests against the strap with a smaller normal force 29, the strap 18 will be tightened until- the normal force 29 and the corresponding reaction force 30 are increased proportionally. With transverse straps of preferably variable length, a favourable distribution of the load forces is thus obtained, and the normal forces 29-30 in local load maxima, e.g. under the ischia 25-26 are reduced as much as possible. As appears from Fig. 9, solely by the depth of the recess 13, a further distribution of the load on the person' s posterior and thighs is obtained in that the body rests against the seat over a considerably larger width 31 than on a conventional seat which is essentially plane, seen in a section corresponding to Fig. 9. In Fig. 4 is seen that the distance between the branches or side members 22 and 23 is larger at the rear end of the seat 10 than at its front end. This contributes to the fact that the user's posterior is enclosed by the seat appropriately high up and thus is supported on a larger area . In figs. 3-4 is shown a favourable setting of the recess 13 of the seat 10. The lengths of the two straps 17-18 are adjusted so that the strap 18 hangs further down than the strap 17 which in its turn hangs further down than the upper side 14 of the upholstery 12. The contour of the centre line of the seat 10 (in the vertical symmetry plane of the chair) will thus extend according to the curve 27 in Fig. 3. It is obvious that the curve extends essentially horizontally in the portion A, then obliquely in the portion B and finally almost horizontally in the portion C (cf . also Fig. 1) . Fig. 3 further shows schematically a mechanism according to a preferred embodiment of the invention to bring the back rest 34 in engagement with the user's back after he has taken a seat in the chair 21. An essentially horizontally extending rail element 41 is fastened behind the seat 10 of the chair 21, preferably rearmost on the frame 28, on which rail element a carriage or a slide 42 can be displaced forwards and backwards in the direction 43, and fixed in a desired position. The direction 43 is preferably horizontal and parallel to the vertical symmetry plane of the chair. The back rest is fastened to the slide 42 and thus follows it forwards and backwards, being displaced parallel to the direction 43 which is indicated by the arrows 43 in Fig. 3. The back rest 34 is displaced and fixed by an operating handle 45 which is arranged such that it can conveniently be operated by the user while he sits in the chair. The operating handle must not necessarily be arranged in the way shown in Fig. 3; Fig. 3 serves exclusively as an example illustration of the principle of operation of the mechanism. The operating handle 45 functions according to the invention preferably in the way that it can be displaced between to extreme positions, i.e. a first position where the back rest 34 is locked in a front position, and a second position where the back rest is displaced backwards by a certain distance; the length of this distance can preferably be adjustable. When the user sits down in the chair while the operating handle 45 is in its second position and the back rest is thus displaced backwards, he can now sit with an arbitrary posture, i.e. without having to exert himself to push the posterior further far back on the seat. When sitting, he can then move the operating handle 45 to its first position, whereby the back rest 34 is displaced parallel forward and locked in its forward position. The back rest cushion 37 thus affects the upper portion of the user's pelvis 4 such that this is guided forwards in a rotating movement 7 (Fig. 1) and occupies the above described ideal position. When using this mechanism according to the invention, the user can be relieved from having to be careful about placing his posterior further far back on the seat or guiding his posterior "inward below" the back rest cushion 37. It is within the limits of the invention to design this mechanism such that it is actuated by the user's weight when he sits in the chair instead of the mechanism being actuated by control of the operating handle. In Fig. 6, an advantageous embodiment of the chair according to the invention is illustrated. Fig. 6 shows merely the seat 10 and back rest 34 of the chair. In this embodiment, the recess 13 of the seat 10 is formed by three straps 16-18 which on the same way as described above extend between branches 22-23 of a frame 28. In the embodiment shown here, the ends 32-33 of the straps are guided through the branches 22-23. According to the invention, the lengths of the straps are preferably individually adjustable, e.g. in that the ends of the straps can be clamped in the branches 22- 23 by appropriate screw or clamp mechanisms (not shown in the figure) . The clamping can advantageously be made where the ends 32-33 of the straps are guided through the branches 22-23. As an alternative, the adjustability of the lengths 16-18 of the straps can according to the invention be provided in that each strap is placed as a closed loop around the branches 22-23 of the frame 28, the two ends of the strap being fastened to each other by means of burr fasteners such as Velcro ® tape. The effective length of each strap will thus in a simple way be adjustable without use of tools. The length of the straps can besides be made adjustable in any suitable way without thus contesting the validity of the favourable effect of the invention. By making the lengths of the straps adjustable according to the invention, it is obtained that the recess 13 of the seat 10 can be adjusted individually to the person who is to sit in the chair. It is thus possible to obtain an optimal retaining of the person's posterior and thighs against forward slipping, and an optimal distribution of the load on the posterior and thighs stemming from the weight of the body 24 over as large an area as possible. The chair in Fig. 6 is intended to be provided with an underframe carrying the seat 10 and the back rest 34 by supporting the circular frame 28 of the seat in an appropriate way, e.g. in a manner corresponding to the one shown in Fig. 2. Fig. 7 shows another preferred embodiment of the invention with the same seat as in Fig. 6 but with another back rest 34. The back rest 34 in Fig. 7 comprises a back rest frame 53 having two essentially vertically extending columns 54-55, between which a number of essentially horizontally extending back rest straps 48-50 are suspended. The back rest frame 53 is in the shown embodiment carried by two adjustable joints 46-47 which are fastened to the frame 28 of the seat 10, by means of which joints the angle of the back rest frame 53 with the frame 28 of the seat can be adjusted. The ends 51-52 of the straps 48-50 are in Fig. 7 guided through the columns 54-55 and clamped in these in a manner not shown in detail . The lengths of the straps 48-50 are thus preferably individually adjustable in a way corresponding to that of the above seat straps 16-18. This adjustability can also be provided in any suitable way without falling outside the scope of the invention. By the fact that the lengths of the back rest straps 48-50 are thus according to the invention made adjustable, it is obtained that the form of the back rest 34 can be adjusted to the individual person who is to sit in the chair. An optimal support of the person's back and a desired course of the person's spinal column are thus made possible. Now, as illustrated in fig. 7, the effective curvature of the back rest 34 can be adjusted separately in two planes . By an appropriate choice of the number of back rest straps 48-50 and variation of their effective lengths, the curve 56 in a horizontal section plane (i.e. the curve of the backside of the body seen from above) and the curve 57 in a vertical section plane (i.e. the curve of the spinal column seen from the side) can be adjusted independently of each other. The curve in the horizontal section plane, which curve is illustrated by the dotted line 56 in Fig. 7, is a result of the back rest straps 48-50 being tightened more or less. The more the back rest straps are tightened, the more straight the line 56 will be, and the more plane the back rest will be (seen from above) . The curve in the vertical section plane (the symmetry plane of the chair 21) , which curve is illustrated by the dotted line 57 in Fig. 7, is a result of the back rest straps 48-50 being tightened or slackened differently. The larger the difference of the slack of the back rest straps is made, the more curved the line 57 will be and the more curved the back rest 34 will become (seen from the side) . It is generally preferred that the back rest 34 is saddle-shaped such that the line 56 is forward concave and such that the line 57 is backward concave in its entire length. A desired lumbar sway can thus be set for the user of the chair, and at the same time his back can receive support in the entire width of the back; the latter contributes considerably to reducing the effective specific pressure on the back. Fig. 8 shows schematically a person 35 sitting in a chair 21 according to the invention with the seat 10 and back rest 34 as shown in Fig. 6. The person 35 occupies a sitting position which is very characteristic for the chair according to the invention. A closer analysis of the sitting position in Fig. 8 reveals that the lumbar sway is maintained in the sitting position, and that the positions of the femurs are close to horizontal. It is correspondingly assumed that the angle in the hip joints is approximately 90° different from the angle of the joints in a standing position. The lower 37 of the two cushions 36-37 of the back rest 34 is in Fig. 8 set to a height corresponding to the most curved part 38 of the lumbar sway (Fig. 1) and forces and helps, respectively, the person to maintain this sway. The upper back rest cushion 36 is adjusted to support the back just below the scapulae, providing the person 35 with some support, but at the same time maintaining a desired movability to perform backwards stretching exercises in the sitting position. It is obvious in Fig. 8 that a person's pelvic region is forced by the back rest cushion 37 and the oblique portion B of the seat 10 to take an essentially right-angled position, whereby the angled position is essentially merely obtained by a rotation of 90° in the hip joints. As earlier described, the effective securing of the person's thighs and posterior in the recess 13 contributes to this. According to the invention, the upper back rest cushion 36 can be omitted without deteriorating the favourable effect of the invention. On a chair with a back rest which only has the lower back rest cushion 37, this is preferably arranged at a height where the middle of the cushion 37 is at height with the upper edge of the pelvis 4, cf. Fig. 1. Whether to use one or two cushions in the back rest 34 or a high back rest in one piece, depends among other also of the sitting person's tendency to take an injurious sitting position such as pelvis fall-back. The length of the seat 10 is according to the invention adjustable in order to obtain an optimal matching to the user's body proportions and in order to provide an optimal support of the user's thighs to avoid local load maxima. In a preferred embodiment, this matching is obtained in that an upholstery 12 forming the front edge of the seat 10 can be displaced in relation to the frame 28 in the direction 44, cf . Figs 6-7. In practice, it is thus the length of the recess 13 which is changed when the total length of the seat 10 is changed by displacement of the upholstery 12. According to the invention, it is preferred that the underframe 20 of the chair is adjustable in the height, e.g. by means of an adjustment device 39 known per se (Figs 3-4) . This adjustability is essential in order to permit the user's feet to transfer an appropriate part of the user's weight to the floor at the same time as the lower part of the user's thighs can transfer an appropriate part of the user's weight to the front part . 14 of the seat 10. It is thus preferred that the user's heels rest in the floor and thereby essentially carry the weight of the tibia (calf) , however, without being subject to so much load that the front part of the seat 10 is relieved completely or almost completely from the weight of the lower portions of the thighs. In this way, it is obtained that the ability of the seat 10 in the chair according to the invention to even the load from the user's body - and thus avoid local load maxima - can be fully utilised. In Fig. 8, the seat cushion 40 can just be seen in position under the user's thighs: the seat cushion 40 is e.g. and preferably a thin, rectangular cushion of a uniform thickness and with a uniform compressibility over the entire area of the seat. Finally, it appears clearly from Fig. 8 that the side members 22-23, of which the left side member 23 is visible in Fig. 8, extend obliquely relative to the horizontal direction, the rear end of the side member 23 lying higher than the front edge.

Claims: C L I M S 1. A chair (21) with a seat (10) and a preferably height-adjustable supporting structure (20) , where the rear portion of the seat (10) has a recess (13) which is dimensioned to receive the posterior and the upper portions of the thighs of a sitting person (35) , where the recess at least on the person's right and left side partially surrounds the posterior and the upper portions of the thighs, where the recess comprises between 55% and 80%, and preferably about 66% of the length of the seat (10) , and where the depth of the recess (13) is largest at the rear, c h a r a c t e r i s e d in that the depth of the recess (13) is adjustable. 2. A chair according to claim 1, c h a r a c t e r i s e d in that the depth of the recess (13) is locally adjustable in various distances from the front edge of the seat . 3. A chair according to claim 1 or claim 2, c h a r a c t e r i s e d in that the supporting structure of the recess comprises flexible elements which are suspended between side members (22-23) extending essentially along the right and left edges of the seat, respectively. 4. A chair according to any of the preceding claims, c h a r a c t e r i s e d in that the side members (23) extend obliquely relative to the horizontal direction such that the rear portions of the side members are more elevated than their front portions . 5. A chair according to any of the preceding claims, c h a r a c t e r i s e d in that the side members (22-23) extend with varying mutual distance such that the distance between the rear portions of the side members is larger than the distance between the front portions of the side members. 6. A chair according to any of the claims 3-5, c h a r a c t e r i s e d in that the flexible elements comprise seat straps (16-18) , fabrics or similar material layers or one or more sheet-shaped elements. 7. A chair according to any of the claims 3-6, c h a r a c t e r i s e d in that the lengths of arc of the flexible elements (16-18) measured from side member (22) to side member (23) are adjustable. 8. A chair according to claim 7, c h a r a c t e r i s e d in that the lengths of arc are locally adjustable in various distances from the front edge of the seat. 9. A chair according to claim 7 or to claim 8, c h a r a c t e r i s e d in that there are several flexible elements (16-18) in different distances from the front edge of the seat, and that the lengths of arc of the elements can be adjusted individually. 10. A chair according to any of the preceding claims, c h a r a c t e r i s e d in that the supporting structure of the rear portion of the seat comprises seat straps (16-18) , each comprising two portions provided with burr fasteners to obtain adjustability of the effective lengths of the seat straps by joining the burr fasteners in different positions. 11. A chair according to any of the preceding claims, c h a r a c t e r i s e d in that the length of the seat (10) measured from its front edge to the rear edge of the recess (13) is adjustable. 12. A chair according to claim 11, c h a r a c t e r i s e d in that the length of the seat is adjusted by adjusting the length of the recess (13) . 13. A chair according to any of the preceding claims, c h a r a c t e r i s e d in that the rear end of the seat is essentially at height with the bottom of the recess (13) . 14. A chair according to any of the preceding claims, c h a r a c t e r i s e d in further comprising a back rest (34) which is adapted to abut on the user's back opposite the upper edge of the pelvis (4) . 15. A chair according to any of the preceding claims, c h a r a c t e r i s e d in further comprising a back rest (34) with a supporting structure which essentially comprises horizontally extending back rest straps (48- 50) which are suspended between two essentially vertical columns (54-55) essentially extending along the right and left edges of the back rest, respectively. 16. A chair according to claim 15, c h a r a c t e r i s e d in that the curvature (56-57) of the back rest is adjustable by adjustment of the effective lengths of the back rest straps (48-50) . 17. A chair according to claim 15 or to claim 16, c h a r a c t e r i s e d in that the angle of the back rest (34) is adjustable, preferably in that the angle of the columns (54-55) with vertical is adjustable (46-47) in or near a vertical plane parallel to the vertical longitudinal plane of symmetry of the seat . 18. A chair according to any of the claims 14-17, c h a r a c t e r i s e d in having an open area between the rear edge of the seat (10) and the lower edge of the back rest (34) in order that no part of the chair abuts on backwards facing portions of the person's posterior. 19. A chair according to any of the claims 14-18, c h a r a c t e r i s e d in that the back rest (34) can be displaced (43) in the longitudinal direction of the seat when the user is sitting in the chair, whereby the user can force the back rest forward against the upper portion of the pelvis after having taken his sitting position.

Inventors: HANSEN BRIAN AABY, HANSEN, BRIAN, AABY
Applicants: HANSEN BRIAN AABY, HANSEN, BRIAN, AABY
Application year: 2001
Main classification: A47C7
IPC codes: A47C7