WO2024253601A1 - Multi pivot tilting seat support structure - Google Patents

Abstract

A seat support apparatus is provided. The seat support apparatus is used as a seat support mechanism, whereby a seat pan (D) is pivotally moved from a higher forward inclined seating position to a lower backward reclined seating position over a transverse pivotal axis with a fulcrum representing the knee joint (4.b). The apparatus comprises two transverse pivotally hinged pairs of levers, which form a parallelogram between a bearing base (2) and a seat pan support plate (6). The seat pan support plate (6) forms a parallelepiped with two unequal facing sides, in which a rear pivotal lever (4) is longer than a pivotal front lever (5), so that the rotation simulates the rotation of the thigh with the fulcrum in the knee joint (4.b) and thus evenly supports the length of the thigh through its entire movement. A third transverse mounted pivotal lever (7) is engaging by means of a helical surface gear (7.2) with the rear pivotal lever (4) in a point (4.4) to force a reverse synchronized movement of the backrest when tilting upward thus opening the angle between thigh and the upper body.

Description

MULTI PIVOT TILTING SEAT SUPPORT STRUCTURE

Cross-Reference To Related Applications

This application claims the benefit of the International Patent Application No. PCT/TH2018/000005, filed on February 07, 2018, which is a Continuation of International Patent Application No. PCT/TH2013/000033, filed on August 02, 2013, are incorporated herein by reference in their entirety.

Field of the Invention

The present invention relates to a multi pivotal horizontal rotating seat support structure or device that is commonly used in office chairs to support the weight of a person, in particular a seated person and especially to an adjustable support structure or device, that is adjustable to alter the seating or reclining position of a subject resting on the structure particularly from an inclined forward position to a rearward reclined position. The background for the invention will therefore be described in the next section of this specification with specific reference to an office chair.

Background to the Invention

There are imminent situations, where there is a need to provide an adjustable seating support structure. One of the best known, but not exclusive examples of this need, is in office and task chairs, where the posture of a seated person needs to be altered, in order to administer various tasks executed in a seated position. It also needs to be altered in terms of height, build and proportions of the seated person. Yet again, the person’s seating or reclining position may need to be altered, to provide a higher seating position in tandem with a height adjustable table or desk. Examples shown in Pat. US20120299350, Pat. US20090140567, Pat. JP2019013718A.

Support structures for use in offices and other situations, which allow altering the seat height, seat inclination and reclining position of a subject are known, (Pat. US7028580A, Pat. WO 01/8719 Al, Pat. US09356998B1, Pat. US11620043B1, Pat. US8944507). However, many of the known forms of support structures suffer from some disadvantages, either by not acknowledging the latest status of biomechanical science or focus only on some limited parts of the ergonomic implications of the seated person. The current embodiment of the invention is primarily, but not exclusively, focused on an office and task chair application. In the present invention the office chair comprises eight portions:

(A) A mobile base assembly portion (foot base)

(B) A pneumatic height adjustable swivel column (gas spring)

(C) An adjustable support structure portion (mechanism)

(D) A seat support portion (seat pan)

(E) An arm support portion (armrest)

(F) A spine support portion (backbone)

(G) A Backrest support portion (backrest)

(H) A head support portion (headrest)

Portion (A) supports the upper portions (B - H), can be equipped with glides or swiveling wheels for mobility while seated. Portion (B) contains a mechanical or pneumatic device to allow swiveling and a height adjustment of the upper portions (C - H). Portions (C) and (D) form the heart and center of this invention and enable an articulated movement of a person from an inclined and forward leaning seated position to a lower backward reclined position, which can be secured infinitely by the use of a mechanical, pneumatic or electric actuated means.

Portion (E) is attached to the height adjustable backrest (G) to maintain a constant equal and proportional correct height for the lower arm of a seated person.

Portion (F) is configured to support the portions G, E & H and is transverse pivotally hinged to portion (C) to allow for a reverse synchronized alteration of the backrest angle with the imaginative pivotal fulcrum in the hip joint.

Many known forms of support structure, upon which a user may be supported in a seated position, demonstrate limited knowledge of the current state of anthropological or biomechanical data or purposely choose to focus only on parts of it. For example, we see continuously chairs, which suffer from the following deficiencies: despite current knowledge and scientific feet, the majority of manufacturers still follow an outdated rule of sitting on a horizontal surface upright at fixed 90-degree angles between leg, thigh and upper body (torso). Scientific evidence (Mandal, Keagan) shows, that the angle between the vertical axis of the pelvis and thighs at the hip joint can only bend 60 degrees; the remaining 30-degree bending is affected by commensurate rotation of the pelvis axis back-wards, which changes the natural curve of the spine (lordosis) into an opposite curve (kyphosis), which causes asymmetric pressure on the inter-vertebrae discs, which are squeezed outwards. Although the spine can be forcefully pushed back into its natural curve with an outspoken lumbar support, this will eventually cause discomfort and back pain, because the muscles of the lower back, the buttock muscles and lower muscles of the thigh are subjected to prolonged static load, which can result in acidosis in these muscles and pain symptoms. The lack of movement will eventually result in un-sufficient synovial fluid, which serves to lubricate the lower vertebrae and the intervertebral discs.

In some support structures all the individual surfaces, that make up the ergonomic platform, must be secured into position in sequences, often with re-adjustments required, making the transition from one support position (e.g., a reclining position) to another (e.g., an upright seating position) cumbersome and time consuming.

The majority of products currently in the market do not match with the current requirement and regulations in many countries of height adjustable tables, which demand a flexible workplace solution to offer a table height for either working while sitting down or standing up without a chair entirely.

Many products currently in the market also do not offer alternative applications of their use for example in industrial, medical, educational or other fields that require an ergonomic seat support structure.

A need therefore exists for an adjustable support structure, which addresses one or more of the above problems associated with the prior art.

The present invention aims to provide a support structure, which recognizes and supports the current state of anthropological and biomechanical facts.

Furthermore, the current invention aims to allow and to encourage a ‘seated’ person into shifting the seating position upward and forward in a convenient an easy understandable and a self-explanatory way.

Summary of the Invention

The present invention generally provides a support structure for supporting a person in a seated position, the support structure comprising: a linear for- and upward or back- and downward moving apparatus to carry the weight of a person seated upon the support structure; a frame attached to the support structure to support the upper body portion, when leaning back (backrest), that supports angle variations of the seated person forward or backward through a reverse synchronized movement; In the present embodiment the support structure is incorporated into a chair, in particular a task or office chair, on which the subject may recline backward or ascent forward in a linear single movement, whereby the transverse pivotal axis of the motion is the knee joint as the reference fulcrum, meaning, while the upper thigh is evenly supported and in permanent contact with the seat pan surface in its entire length, the legs remain resting square on the floor. The compulsory dynamic movement allows the seated person to maintain throughout the motion a torso-to-thigh angle larger than 90 degrees. Incline and recline of the seat pan is achieved and controlled by two pairs of transverse connected pivotally hinged levers directly under the seat pan, which force the seat pan to move on a compulsory hinged parallelogram with uneven legs, also called parallelepiped, to describe an imaginative curved path with the fulcrum in the knee joint.

The fact, that throughout the entire motion from a backward reclined position to a high forward inclined position, a torso-to-thigh angle of 100 degrees or more can be maintained, the need to alter the angle ratio between seat and backrest being less relevant, since the primary function of the backrest is to support the upper body or torso, when it tips backward over its balance point into a reclining position. As a desire or need may occur to further open the body angle closer to the zero posture (described further below), a backrest to seat angle adjustment is incorporated through a compulsory reverse synchronized movement adjacent with the seat pan.

To be able to elevate a person into a higher forward leaning position, the seat pan and the parallelepiped levers need to be supported by means of a force to lift a weight. There are several options for this force available, such as a pneumatic cylinder or gas spring, a transverse torsion bar or an electric actuator.

When installed with a lifting force in the form of a coil spring or a transverse torsion bar, the adjustable support structure would be a floating device, meaning, that with a corresponding adjustment of the spring to the weight of the seated person, the adjustable support structure would allow balancing the seated body and by changing the center of gravity in the upper body forward to move into a forward elevated posture or, by leaning backward, into a relaxed backward reclining posture in a compulsory singular ‘floating’ movement without obstruction or effort.

Furthermore, while a forward inclining posture modulates the upper torso weight forward and away from the uplifting force and thus requires less force, the opposite movement backwards toward a reclining posture requires more uplifting force as the upper torso weight falls back and increases with further recline. Variations in the weight of different persons require a re-positioning of the characteristics of the uplifting force, which increases when reclining and lessens with the forward incline. This can be achieved through a progressive coil spring or by changing the pre-tension of a transverse torsion bar. It can also be achieved through a combined arrangement of a gas spring in tandem with a coil spring.

Finally, it may also be desirable, to lock the seat pan at certain angles for more comfort. A person may be unfamiliar with the floating movement and feel insecure, or a higher precision in movement control is required for a task at hand. Thus, the said adjustable support structure comprises a means for securing (locking) in the form of a lockable gas spring.

The ultimate solution for the uplifting movement is an electric actuator, which replaces all the other solutions and allows for a precise and controlled modulation of the desired seat movement.

In a preferred embodiment of the invention, the adjustable support structure would have only one releasable locking/detent means for the entire structure of the seat rest support and the arrest of the seat pan (D) movement in any inclination. A second means of lever or other device would be provided for the height adjustment and detention of the means of the uplifting force in portion (B) of the current embodiment.

The limitations of the terminal positions of the backward and forward movement of the seat pan will be controlled by limiting the movement of the weight lifting device through a travel limit extension of the locking device employed.

The present embodiment is designed to accommodate the most common seating situations in the current office environment. It also encourages a solution for the importance, to employ height adjustable tables in the office working environment aimed to solve the back pain problems, by switching effortlessly between a low sitting position and an elevated high up and articulate forward leaning working position with infinite positions in between.

In addition, the construction and movement of the present embodiment also offers to be applicable in many other situations, where a person needs an elevated seating support for various tasks, which traditionally can only be achieved with different specific chairs.

To complement the description which is being made and with the aim of aiding to better understand the features of the invention according to a preferred exemplary embodiment thereof, a set of drawings is attached as an integral part of said description, in which the following is shown with an illustrative and non-limiting character. Brief Description of the Drawings

A preferred embodiment of the Invention will be in the following described (by way of schematic example), with reference to the accompanying drawings, in which:

Fig. 1 depicts a schematic side view of the described embodiment of the described office chair construction in accordance with the present invention;

Fig. 2 depicts an isometric view of the embodiment of the said office chair:

Fig. 3 depicts the schematic side view of the anatomic premise which the current embodiment is based on and referring to;

Fig. 4 depicts a schematic side view of the common situation of seat structures offered to the marked at present, showing the angles of a generally backward tilted position and a second acute forward tilted position the current embodiment claims to achieve;

Fig. 5 depicts an isometric exploded view of the components of the entirety of the adjustable support structure portion (C);

Fig. 6 depicts the hinge arrangement in the apparatus (C) between the transverse pivotal backrest support lever and the transverse pivotal rear lever of the parallelepiped, that controls the reverse synchronized movement of the backrest in relation to the seat pan (D) by means of a dual engaging herringbone gear;

Fig. 7 depicts the fundamental schematic function of the adaptive structure of the parallelepiped used in the current invention;

Fig. 8 depicts the side view of the assembly (C) of the exploded support structure as in Fig. 7 and its members referring to the assumptions of Fig. 4 in the known horizontal position;

Fig. 9 depicts a side view assembly of the support structure and its members and proportions as shown in Fig. 10, but raised to the acute forward tilted position and describes the alignment with the fulcrum reference point of the knee joint;

Fig. 10 depicts a side view assembly of the support structure and its members and proportions as shown in Fig 10, but lowered to a reclined rearward position and with a continuous fulcrum reference point in the knee joint; Fig. 11 depicts the means of adjustment of the backrest support lever F or backbone, by which the height of the backrest and the seat depth are adjustable in relation to the seat pan (D);

Fig. 12 depicts the flexible foam insert arrangement of the seat (D) to form an articulated contour through different densities in the foam;

Fig. 13 depicts the arrangement of separating two different densities in the flexible seat foam and its insertion and fixing;

Fig. 14 depicts a side view of a human body seated upon the contoured seat foam insert in the elevated forward tilting position;

Fig. 15 depicts a side view of the preferred embodiment of the apparatus as an assembly of portions in a lower reclined position;

Fig. 16 depicts a side view of the preferred embodiment of the apparatus as an assembly of portions in a traditional ninety degree angle seat position;

Fig. 17 depicts a side view of the preferred embodiment of the apparatus as an assembly of portions in an acute forward tilted position;

Detailed description of the preferred embodiments of the Invention

It is to be understood, that the specific devices and processes illustrated in the attached drawings and described in the following specification are exemplary embodiments of the inventive concepts defined in the appended claims. Hence, specific physical characteristics relating to the embodiments disclosed herein are not to be considered as unnecessarily limiting, unless the claims expressly state otherwise.

Fig. 1 generally depicts an office chair and schematically represents a possible embodiment of the invention. As will be evident from viewing Fig. 1 and 2, the seating assembly generally comprises eight elements: a five-prong base assembly (demoted A), a support column assembly (demoted B), a support structure in the form of a mechanism (demoted C), a seat pan (demoted D), a support structure in the form of an armrest (demoted E), a support structure in the form of a backbone (demoted F), a backrest support structure (demoted G), a support structure in the form of a headrest (demoted H), The base A is structured as a classic 5-prong rolling leg frame which acts as a support mount for structure B, which comprises a standpipe with a nylon bush insert, which carries a telescopic gas cylinder, which can be adjusted in height in order to adapt to the specific seat height of each user for such a seat support structure.

Mounted upon structure A & B is the mechanism D and seat pan C, which form the heart of this current invention and support comprise a plurality of portions, which will be described in detail below.

Fig. 3 depicts the biomechanical background and explanation for the function of the current invention and describe part of the human skeleton, in particular the arrangement of the bone structure and movement around the hip joint. Fig. 3 shows the relative angles between the pelvis and thigh bone when sitting on a horizontal seating surface and when tilted up and forward by 18 degrees or more over the horizontal. It is by now a scientific established fact, that the maximum angle of lateral movement between hip bone and pelvis is 60 degrees, while the remaining 30 degrees take place in the lumbar part of the spine directly above the pelvis, the result of which is a conversion of the natural spine curve (lordosis) into the opposite curve (kyphosis).

Fig. 4 depicts firstly the seating configuration and the corresponding torso-to-thigh angles of a traditional seating support structure in a traditional horizontal position and illustrates the effect of the limitation of seat support structures most commonly present in the market, whereas 4.a represents the pivotal location of the hip joint, the knee joint in 4.b and the pivotal point of the foot in joint 4.c, which remains stationary through the whole movement and depicts secondly one of the claims and the aim of the current invention to modify the seating support structure in such a way, that a torso-to-thigh angle of 115 degrees or more can be maintained throughout the entire elevation movement from the utmost lower back recline to the upper most forward incline. This should allow the curve of the spine in the lumbar region to be maintained as close as possible to its natural curve, which requires a forward tilting angle of the seat pan in alignment with the thigh corresponding to the knee joint; it also describes the angles, the current invention claims to achieve through the multi- pivotal levered elevation of the seat pan, whereas during the entire movement in the forward inclined position a lower-to-upper thigh angle of 115 to 120 degrees and in the lower backward reclined position a torso-to-thigh angle of at least 110 to 115 degrees would be sustainable. Fig. 5 depicts an exploded isometric view of all components comprising the apparatus assembly of portion (C) in Fig. 1. It depicts a transverse mounted pivotal lever 7 which engages with the rear transverse mounted pivotal lever 4 of the parallelepiped by means of a herringbone gear incorporated on the surface of both the backrest support lever 7 and the rear pivotal lever of the parallelepiped 4, which forces a synchronized compulsory reverse movement of the backrest support with an over-proportional change of angle relative to the seat pan (see Fig. 8).

It depicts the transverse mounted pivotal front lever 5 of the parallelepiped, which is fixed at the front to the base housing 2 and at the upper rear end to the top support plate 6 and forms in combination with the rear pivotal lever 4 the parallelepiped.

It depicts the position of the release lever lO.a on the right side and the release bowden cable 8.3 for the release adjustment of the gas spring for the height adjustment in portion (B) in Fig. 1 and the release bowden cable 3.4 connected to the release lever lO.b, which activates the release or locking of the support gas spring 3. It further depicts the arrangement of the uplifting force 3 in the form of a coil spring supported gas lift between the base 2 and the attachment to the rear leaver support 4.

Fig. 6 depicts an illustration of the interconnecting herring bone gear between the lateral pivotal backrest support lever 7 and the lateral pivotal rear lever of the parallelepiped 4.

Fig. 7 depicts the underlying concept of the articulation of the movement of the seat pan from lower reclining to higher forward tilting by employing a parallelepiped to simulate a rotation of the seat pan around the imaginary fulcrum of the knee joint.

Fig. 8 In this embodiment of the invention, the various portions of the support structure (C) are connected via hinge or pivot means, to define the support structure C in a horizontal position, whereas the uplifting force 3 is at its lower hinge point pivotally hinged to the bearing structure 2 in point 3.1 and at its upper end to the transverse pivotal rear lever 4 in pivot point 3.2. The upper forward pivotal hinged lever 5 is at its lower end hinged pivotal to the front of bearing base 2 in point 5.1 and at its upper end to the seat pan support plate 6 in point 5.2. The lower backward pivotal hinged lever 4 is at its lower point pivotal hinged to the bearing base structure 2 in point 4.1 and at its upper pivotal point to the seat pan support plate 6 in point 4.2.

Fig. 9 shows the support structure C moved into a desired second forward inclined position and how it enables the subject supported by the structure to be moved from a relative lower backward reclining position to an acute elevated forward seated position (and vice versa), or to any position along a continuum between those two extremities. As is illustrated in this depiction, the range of movement between the first and any desired second position is controlled and limited solely by the extend of movement permissible of portion 3.

Fig. 10 depicts the preferred embodiment of the apparatus assembly of portion C and described in Fig. 8 in its rearward reclined status.

Fig. 11 shows the connecting bracket between the adaptive support structure C and the backrest support structure F with oval slots to allow ergonomic alignment according to the proportions of a seated person both lateral as vertical, by means of a screw type locking handle.

Fig. 12 and 13 describe the arrangement and fixing of two separate portions of flexible foam, whereby the overall seat foam density is higher than the inserted soft foam pad at the rear of the seat to allow the formation of a cavity or a bowl, which retains a seated persons buttocks in said bowl, based on the anatomy of the human body as shown in Fig. 14.

Fig. 14 depicts a side view of the assembly with a human body placed on the inclined forward tilting seat pan D and the contour formed under the upper thigh with the protruding buttock at the rear.

Fig. 15 depicts a side view as in Fig. 14, with a person sitting on it in reclining posture.

Fig. 16 depicts a side view as in Fig. 14, with a person sitting on the horizontal seat.

Fig. 17 depicts a side view as in Fig.14 with the desired forward seat inclination.

Best Mode for Carrying Out the Invention

According to best mode, the present embodiment applies to a seating support structure in the form of an office chair, which allows a user to sit upright on a forward tilting elevated seat support, which tilts the pelvis forward, opens the body angle between the thigh and the upper body and allows the user to maintain a natural curved spine while sitting. Because of the elevated forward tilting seat, it provides an easy- sitting and easy-rising motion, while the feet remain squarely placed on the floor. This encourages more frequent motion and supports a more dynamic seating in particular, when combined with a height adjustable table, and aims to solve one of the most prominent medical conditions of lower back pain.

Claims

Claims

1. An ergonomic seating device to improve the posture of a seated person with a dual tilt mechanism, that allows and supports both a forward inclining and a rearward reclining seating posture comprising: a foundation base assembly A; a height adjustable seat support column assembly B; an adaptive seat support link C, positioned on the base assembly A and the seat support column B to pivot horizontally about a vertical axis related to base assembly A; an adaptive link C supporting a pivotal seat assembly D configured to support a seated persons upper leg; a dynamic moving seat support C, that follows a modulated vertical motion along an arc path, with the fulcrum at the knee joint, allowing the seat assembly to maintain consistent contact with the full length of a person's thigh; a seat assembly D with a concave recessed portion along a longitudinal midline of the seat support structure with a portion of the flexible seat support assembly configured to receive the rearward lower pelvic area wherein a recessed bowl portion has greater flexibility than the overall flexibility of the seat support structure; a bowl portion at the rear of the flexible seat support structure configured to arrest the buttock protrusion of the lower pelvis when inclining forward; a backrest support portion F & G, which comprises a shell, foam and a backbone structure, which allows both horizontal and vertical adjustment; an armrest portion E, which is mounted permanent to the backrest portion F & G and follows the vertical and horizontal adjustment of the backrest movement; a headrest portion H, which is mounted to the back rest portion E + G and is height adjustable in a slide, lockable by screw handle and vertical tilting by means of a frictional restricted hinge attached to the headrest shell.

2. An ergonomic seating device as in claim 1, wherein the seat support assembly C is configured to enable a user to dynamically adjust the posture from a higher forward inclined position to a lower rearward reclined position through a continuous linear movement, which is facilitated by transverse mounted pivotal levers, which are based on a parallelogram, as a simple quadrilateral with two unequal pairs of feeing sides, in its three-dimensional form, also called a parallelepiped, wherein said seat, when comprising said support structure, is operative to enable a person to move from a higher forward inclined position into a lower rearward reclined position in one single continuous movement and the movement is arrestable by means of a lockable gas spring controlled by a lever handle on the side of the structure.

3. A seat assembly D as in claim 1, having an upper surface in the form of a flexible seat support structure, describing a modulated vertical motion on a path of an arc, that has its fulcrum in the knee joint and thus enabling a mounted seat pan D to maintain permanent even contact with the entire length of the thigh of a person seated upon it.

4. An adjustable seat support assembly as in claim 2, wherein the assembly of the transverse mounted pivotal levers described in claim 2 facilitate, but do not limit to, a lower rearward tilting angle of the seat pan of minus 5 degrees below a horizontal longitudinal alignment of the seat assembly D.

5. An adjustable seat support assembly as in claim 2, wherein the assembly of the transverse mounted pivotal levers described in claim 2 facilitate, but do not limit to, a forward inclining angle of at least plus 18 degrees over the horizontal longitudinal alignment of the seat assembly D.

6. An adjustable seat support assembly C as in claim 2, which comprises an uplifting force in the form of a longitudinal mounted lockable gas spring, which at the lower end is fixed onto the adaptive seat support assembly base and at the upper end to the transverse mounted pivotal lever in a semi lying position within the seat support structure, which allows a seated person to move in response to the weight of the user bearing onto the seat assembly from a higher first forward position to a second lower rearward position, when the gas spring is unblocked, and secondly arrests the seat assembly between the first and second position when blocked by releasing the locking handle attached to the support assembly C.

7. An adjustable seat support assembly C as in claim 2, with a means to carry a backrest support frame or backbone F, which is mounted horizontally and vertically adjustable to a transverse mounted pivotal lever, which engages with the rear transverse mounted pivotal lever of the parallelepiped, with lateral pivotal hinge points attached to the support structure, by means of a herringbone gear incorporated on the surface of both the backrest support lever and the rear pivotal lever of the parallelepiped, that a synchronized compulsory reverse movement of the backrest support occurs with an over-proportional change of angle relative to the seat pan in such a way, that the lateral angle between the seat and back increases to 120 degrees when tilting forward and decreases to 110 degrees when reclining backward.

8. A backrest support frame or backbone as in claim 7, which moves on a translateral hinge lever 7 and simulates a modulated movement with a fulcrum in the hip joint to prevent the up-lifting of a shirt or clothing of a seated person; which furthermore is adjustable both lateral as vertical to allow ergonomic alignment according to the proportions of a seated person, by means of a screw type locking handle, the depth adjustment facilitated through a slot in the transverse mounted pivotal lever of the support assembly C and a vertical slide and slot at the bottom and the rear of the backbone structure.

9. An adjustable support assembly as in claim 2, which facilitates the mounting of a modulated seat assembly structure comprising: an outer shell made of recycled ABS or a thermoset compound with natural fibers; a composite flexible injection molded foam insert made of PU or natural latex, which allows a soft cushion to be inserted in the rear part of the seating surface, which forms a bowl to arrest the buttocks of a seated person in such a way, that when tilting forward on the seat, sliding down is prevented through the contour that is formed in the seat by the bearing of the seated person; a foam cover made of a fabric or a perforated natural latex membrane, which has anti-skid properties to further prevent sliding on the surface and retain a seated person on an optimum contour.

10. An adjustable support structure as in claim 7, which facilitates the mounting of a modulated backrest support structure comprising: an outer shell made of recycled ABS or a thermoset compound shell with natural fibers; a composite flexible molded foam insert made of PU or natural latex; a foam cover made of a fabric or a perforated natural latex membrane.

11. A backrest support frame F as in claim 7, which allows a height adjustable headrest H to be mounted on top of the backrest support structure by means of a sliding bracket, which slides inside the backbone and is locked by a screw type locking handle limited by a slot at the back and the headrest adjusting by means of a hinge to a desired angle by the user.