The present invention relates to a device for detecting the weight distribution of a load, in particular of a person, as well as a method for detecting the weight distribution of a load, in particular of a person with a chair and a chair, evidently for use in the device or method.

Optimal tensegrity is hard to achieve with the increased sedentary lifestyle and general lack of compensatory movements that extend the spine and stretches the back muscles, tendons and nerves consequently. In a completely healthy body the normal upright standing or upright sitting posture uses a minimum of muscular activity and thus expends only a minimum of energy.

Back problems, such as pain, deformation, slipped discs, kyphosis and scoliosis, are generally a result of longstanding postural and muscular imbalance, which starts mainly during the growth period. In the current day and age many people have jobs that involve sitting behind desks and a computer for a large portion of the day. An incorrect sitting posture on chairs behind desks and behind computers can result in back problems and a reduced overall flexibility of the back. A correct sitting posture involves the upright orientation of the back, neck and head of a person.

CN201452303 for instance describes a seat for use by workers in an office, with a measuring circuit at the bottom of the seat board and a sensor connected with the measuring circuit via a signal wire, in order to determine the body weight of a person with the seat.

The objective of the present invention therefore is to detect the weight distribution of a load.

The invention thereto proposes a device for detecting the weight distribution of a load, in particular of a person, comprising: a chair, comprising: a substantially upward facing convex seating surface; at least three mutually spaced legs, for supporting the seating surface on a surface, such as a floor; wherein each of the legs is provided with a first sensor, for detecting a force exerted on the leg; and a control unit, connectable to the sensors, for determining the weight distribution of a person over the legs based on the forces detected by the sensors. The exerted force is typically a compressing or downward directed force. A chair according to the invention could be a chair or a stool or any seating element supported by legs. The distribution of the legs may be symmetrical, but could also be asymmetrical. In an asymmetrical distribution the geometry of the distribution of the legs should be determined in order to be able to transform the measured forces into a weight distribution.

The advantage of using a chair is that the weight distribution of a person sitting on it can simply be determined by sitting on it. In order to determine the weight distribution of a person correctly, the influence of pressure and forces of the persons legs supporting the person should be disregarded. To do so, the chair comprises the convex seating surface, such that the person sitting on that surface is sitting balanced on their sitting bones (or tuber ischiadicum) on the chair. In particular the person is sitting substantially on the upper surface or curvature of the convex seating surface. On traditional chairs, the seating surface is concave or flat and widely cushioned, to accommodate the backside of a person. While such shape does accommodate the backside and may provide comfort for the person, it does not aid in achieving the correct, or active, sitting position of person on a chair. The connection between the chair and the control unit could for instance be wireless, wired, through a memory or through a transmission unit.

The device may be calibrated before a load is placed on the chair. In this initial stage all sensors can be set such that they register no load. When a load is placed on the chair, for instance when a person is sitting on the chair, the load is placed on the convex seating surface, and is distributed over the legs of the chair. The sensors in each leg register the force exerted on each leg, and the control unit, based on the measured forces in each leg, determines the weight, or load, distribution over the legs of the chair.

When someone is sitting balanced, on their sitting bones (or tuber ischiadicum) on the chair, the weight distribution of the person is a measure for the posture of the person. A person, sitting balanced and correctly on the middle of the chair, would have a substantially equal distribution of force exerted on the legs, which is measured by the sensors. By measuring this distribution, for instance periodically and every second or every half second, an continuous or semi-continuous indication of the posture of the person on the chair can be provided.

The convex seating surface results, when a person is sitting on this surface, that the load, typically exerted through the sitting bones (or tuber ischiadicum), is located in the middle of the seating surface, on the highest point of the convexly curved seating surface. This highest point of the surface can therefore be used as a mathematically determined point in space. When a person is sitting upright and balanced on their sitting bones, their load is equally balanced over the sitting bones, and the load is equally distributed over the convex seating surface.

Another advantage of the convex seating surface is that a person sitting on the chair can make a larger angle between the spine and the thighs compared to for instance a concave or a flat seating surface. The angle between the spine and the thigh which is the least burdensome for the back is an angle of about 135 degrees, as studied by Keegan JJ - Alterations of the lumbar curve related to posture and sitting from 1953. A convex seating surface, in which the spine is situated on top of the convex seating surface, allows for a greater angle, more towards 135 degrees, compared to a non-convex seating surface.

The load exerted on the convex seating surface is spread over the legs, in dependency on the geometry of the legs, and the sensors located in the legs can measure the loads. From the loads in each of the legs, and the mutual position of the legs, one can determine the centre of gravity, or centre of mass, of the load on the seating surface, at least in the plane of the surface supporting the legs of the chair. When this centre changes, it is an indication of a movement or redistribution of the load.

Movement of the person, or a load, on the chair typically results in a change of the load distribution of the sitting bones of the person, and thus in a change of the load distribution on the convex seating surface. This in turn leads to a changed load distribution on the legs, which can be measured by the sensors in the legs. The device may also be used to detect the postural deviations and functional spinal movement impairment of a person in a seated position. The load distribution over the legs of the chair for posture deviations and functional spinal movement impairment differ from normal load distributions, and can thus be determined by measurements of the forces on the sensors in the legs.

The seating surface can be substantially elongated, which makes it easy for a person to sit on it. The seating surface may be convex in a first direction, such as in width wise direction, and substantially flat in a second direction, perpendicular to the first direction, such as in longitudinal direction. The resulting pommel horse like shape promotes active sitting on the chair.

The curvature of seating surface in first direction may differ from the curvature of seating surface in second direction, perpendicular to the first direction, at least seen from a central point of the seating surface. This results in a double convex shape, convex in two directions. Such a shape promotes the correct and active seating on the seating surface. The muscles of a person are, with such shape, stimulated to balance the body on the chair, close to the balancing point on which the torso of the person is supported on the chair. The balancing point is typically the centre of a line connecting the tubercles of a person. Due to the convex, or even double convex, seating surface, the balancing of the sitting bones on the seating surfaces is focussed on a single point. This point can be used for mathematical calculation on the posture on the person sitting on the seating surface. The first sensors are for instance located at the bottom of a leg.

In the double convex shape, the curvature in the first and second direction preferably differ such that the curvature in one direction is aimed at balancing the sitting bones of the person, whereas the second direction, for instance

perpendicular to the first direction, is aimed at achieving a suitable angle between the spine and the thighs of the person. Both curvatures promote correct sitting posture of a person sitting on the chair.

The double convex shape allows that, when a person is sitting on this surface, that the load, typically exerted through the sitting bones (or tuber ischiadicum), is located in the middle of the seating surface, on the highest point of the convexly curved seating surface. This highest point of the surface can therefore be used as a mathematically determined point in space. When a person is sitting upright and balanced on their sitting bones, their load is equally balanced over the sitting bones, and the load is equally distributed over the double convex seating surface. The load exerted on this convex seating surface is then spread over the legs in dependency on the geometry of the legs, and the sensors located in the legs can measure the loads.

In a preferred embodiment of the invention the device is a chair with four legs, one on each of the corners of the elongated seating surface. This allows for a symmetrical distribution of forces over the legs, for instance when the highest point of the convex seating surface is located in the centre of the four legs.

Together with the double convex seating surface and four legs, the device represents a four legged arch shape in which force vectors of loads located on the seating surface are transferred to the smallest possible contact area. The area where a load of a person sitting on the chair is located is similar to the keystone of an architectural arch, such that the load is evenly distributed over the arch, or the convex seating surface.

The seating surface is for instance formed such that a line from the highest point of the seating surface towards an end of the seating surface in first direction is at an angle between 2 and 10 degrees with respect to the horizontal, in particular about 3-5 degrees, and/or wherein a line from the highest point of the seating surface towards an end of the seating surface in second direction is at an angle between 30 and 60 degrees with respect to the horizontal, in particular between 40 and 45 degrees. This particular shape promotes the correct and active seating on the seating surface. By using such shape the muscles of a person are stimulated to balance the body on the chair, close to the balancing point on which the torso of the person is supported on the chair. The first direction may for instance be the width of the seating surface, whereas the second direction may for instance be the depth of the seating surface.

The horizontal is for instance a plane parallel to the plane of the surface the chair is resting on. The specific convex shape of the seating surface stimulates the sitting on the chair with a correct posture, beneficial for spinal alignment. Sitting balanced on their sitting bones on the convex shaped surface brings the spine into a protective S-curve. The surface further promotes an active sitting posture, in which the upper body is balanced on the surface. To do so, the back muscles stay active and are trained during sitting. The balance position when a person is sitting on the curved surface is an instable balance, which causes the constant micro correction using the musculature.

The seating surface can be symmetrical. A symmetrical surface can be used in several ways, whether or not the chair is turned 180 degrees. This makes use of the device easier and clear.

The device may comprise a second sensor, locatable on the head of the person, for determining the position of the second sensor with respect to a vertical central axis of the chair. The position of the head of a person is one of the features that could be used to determine whether or not a person is sitting upright. When the head of the person is located straight above the sitting bones of the person, chances are that the person is sitting up straight. The head and the centre point between the sitting bones of the person will then be located on the same vertical central axis. To determine the position of the second sensor with respect to the vertical central axis the distance with respect to the axis could be determined. The second sensor may be used to determine the inclination of the head of a person with respect to the horizontal, to make sure the weight distribution of a person on the chair is not affected by the tilting of the head of the person or to determine the influence of the tilting of the head on the distribution of weight over the legs of the chair.

The vertical central axis may be a vertical axis, or vertical line, running through the centre of the seating surface. The centre of the seating surface may be determined by the centre of gravity of the chair, or the centre of gravity of a polygon set by the ends of the legs of the chair. The measurement relating to the second sensor may also performed relative to the vertical central axis, such that the precise location of this axis is less important than the movement of the second position with respect to this central axis. The second sensor may be formed by a head reflector, to be placed on or close to the temporal bone, and in particular the petrous part thereof (os petrosum). To determine the distance from the second sensor to a fixed point in space, a first laser located on the fixed point can send a first laser beam to reflect on the head reflector and determine the mutual distance between the first laser and the head reflector. The first laser is for instance a triangular laser.

The second sensor may comprise a gyroscope, to determine the inclination of the head of a person with respect to the horizontal. To displace the second sensor from the vertical central axis, one could move their complete back forward, or tilt their head. The inclination of the head can be used to provide the information on how the displacement with respect to the vertical central axis is achieved.

The vertical central axis can for instance be used with the device in order to track the movement of a person on the chair, in particular the head of the person using the second sensor, with respect to the vertical central axis. The extent to which a person can move with respect to the vertical central axis, while sitting balanced on its sitting bones, is a measure of the flexibility of the spine of a person.

The vertical central axis can also be used to determine whether or not a person is sitting straight and whether or not a person could have an abnormal spine. When the central vertical axis and the second sensor indicate that a person is sitting up straight and correctly, the weight distribution over the legs of the chair should have a certain predefined distribution. If, even though the person is sitting right, the weight distribution is off, for instance too much to the front, the person could have a spinal deformity or other unspecified internal weight shift, which should normally not be there. Such can be an indication to ask for medical assessment.

The position of the vertical central axis is for instance defined by the centre of gravity of a polygon defined by the ends of the legs or by the geometric centre of this polygon. The centre of gravity of the polygon defines the middle of the legs, and therefore a centre of the chair. Provided that the seating surface of the chair is shaped similarly to the polygon defined by the legs, (or at least the projection of the surface on the polygon) the centre gravity also coincides with the middle of the seating surface. The polygon as defined by the end of the legs is for instance symmetrical. The polygon defined by the legs is for instance a rectangle, defined by the ends of four legs. The centre of gravity can be defined by the intersection of the two diagonals of the rectangle. The seating surface, although convex, may also be rectangular, at least in top view, and could be dimensioned a bit larger compared to the polygon. If, in that case, the overlap of the seating surface is equal in all directions compared to the polygon, the centre of gravity of the polygon is located on the same vertical axis as the middle (or centre of gravity) of the seating surface.

The device may also comprise a locator, located underneath the middle of the seating surface, for determining the vertical central axis of the chair. The locator is thus located beneath the seating surface on the location of the middle of the sitting bones of a person when sitting on the chair. The locator is preferably located right underneath the highest point of the convex seating surface. The locator can be located at the centre of gravity of a polygon defined by the ends of the legs, when the legs are distributed evenly around the seating surface. The locator could be used determine the position of the vertical axis with respect to a fixed point in space.

The locator is for instance a reflector, such as a mirror. To determine the distance from a fixed point in space, a fixed second laser can send a second laser beam to reflect on the locating reflector and determine the mutual distance between the second laser and the locating reflector. Instead, the locator could be formed by a second laser. To determine the distance from a fixed point in space, a second laser beam can then be send from the second laser to a fixed locating reflector mirror and determine the mutual distance between the laser and the reflector. The second laser is for instance a triangular laser.

The locator may comprise a spacer of known length, wherein one end of the spacer forms the vertical central axis, and the other end forms a fixed point in space.

The locator may also comprise a transmitter and/or receiver, wherein based on for instance the time of flight between transmittal and receipt of a signal the mutual distance between the two is calculated. The device may comprise a second sensor, locatable on the head of the person, for determining the position and/or orientation of the second sensor with respect to a vertical central axis of the chair, but the device can also be arranged to determine the position of the second sensor with respect to the first sensors. The second sensor can comprise a transmitter and the first sensors may comprise receivers, wherein based on the time of flight of a transmitted signal to the first sensors the mutual distance between the first and second sensors is determined. The second sensor may also comprise another element suitable to determine its position in space, such as workspace systems also used in virtual reality environments, alignment tools for computer assisted surgery, or very accurate GPS and the like.

The second sensor may comprise a gyroscope to determine the inclination with respect to the horizontal and/or vertical. The inclination of the head of the person with respect to the horizontal and vertical can then be determined.

The second sensor may also comprise at least one accelerometer, to measure the acceleration of the second sensor, and thus the head of a person. In an

embodiment using multiple accelerometers they can be used to sense the orientation of the second sensor due to the direction of weight changes in the accelerometers. These accelerometers are for instance used in portable gaming devices to detect the orientation of a game controller to provide input.

To determine the position of the second sensor with respect to the first sensors, the distance and angle with respect to the first sensors could be determined. By determining the angle and distance of the second sensor with respect to the first sensors, given that the dimensions of the chair are known, one could determine the height of a person from sitting bones to nose bone.

The second sensor may be arranged to be located on the nose or behind the ears of the person. The second sensor may for instance be incorporated in head mounted gear, hereto the second sensor could be located on a type of glasses which can be put on by the person. The second sensor is then either located on the nose bone of the person, and/or the temporal bone, and in particular the petrous part thereof (os petrosum), of the person. Both anatomical feature can be easily interchanged between different people, as they are generally located at the same place of a person. The second sensor may also be part of ear buds, to be worn in and/or on the ears of the person, or the second sensor may be incorporated in a virtual reality (VR) mask.

The second sensor may also be formed by for instance a reflector, for reflecting a laser beam. The second sensor may also comprise another element suitable to determine its position in space, such as workspace systems also used in virtual reality environments, alignment tools for computer assisted surgery, or very accurate GPS and the like.

The device may be arranged to determine the forces exerted on the first sensors during the movement of a person on the chair. Such movement is preferably a predefined movement, which can be performed by multiple people in a standard way. The movement is for instance a movement in which the person on the chair moves forward and backwards and/or sideways, as far as they can without losing balance. During this movement, the forces exerted on the legs of the chair will be distributed over the legs differently. When the person is leaning forward, the forces on the front legs will increase, to the left will increase forces on the left legs and so on. The amplitude of the forces measured in this movement, adjusted for the total weight of the person, can be a measure for the flexibility of the spine of a person. To do so, the device is preferably arranged to determine the weight of a load placed on the chair, for instance the total body weight of a person sitting on the chair. This determination can be done by having the person sit on the chair without their feet touching the ground. The total body weight is then supported by the chair only.

The predefined movements may comprise at least one of the following, possibly in chosen sequences:

a. Sitting as straight as possible (head up, shoulders wide, lower back curved) and change to a slumping or sloughing or hanging position (shoulders down, flex hip backward).

b. Bend head and neck forward to a bookstand

c. Place one feet backward under measuring device and repeat bend head and neck forward to a bookstand

d. Bend head forward (with feet next to each other on ground) towards knees e. Bend backward with feet on the ground with spine, neck and head towards position where eyes are pointed vertically toward ceiling

f. Bend backward with spine , neck and head in one line toward most backward position possible

g. With feet kept stable on floor rotate to left with spine , neck and head as far as possible

h. With feet kept stable on floor rotate to right with spine, neck and head as far as possible

The device may also comprise a recorder, such as a video recorder, to record the movement of the person. The movement can for instance be stored on a memory. When the person, after having performed the first movement resulting in a measure for the flexibility of the spine, is given a set of exercises to improve flexibility and returns later, can then perform the same movement again, and compare the results to see if the exercises helped to improve flexibility. To make sure the same set of movements is performed, a video feedback of the earlier stored movement can be provided. The recorder preferably records the movement in connection with the measurements of the sensors.

The device may comprise a carrier structure, placable on the load, wherein the carrier structure comprises an array of indicators, such as lights or gyroscopes. The carrier structure is for instance placed on the back of a person, such that the array of indicators lines up with the spine, and in particular with different vertebra of the spine of the person, such that each vertebra is linked to at least one separate indicator.

The indicators are for instance placed on fixed distances relative to each other. In this way, the back or spine of a person can be divided into several measuring points, one for each indicator. By determining for instance the relative movement of the indicators, the movement of the back or spine of the person can be determined by measuring the mutual displacement of the indicators.

The indicators may emit light, and the device may further comprising a receiver, for receiving light emitted by the indicators. Using light emitting indicators has the advantage that next to the position of the lights, also the intensity of the emitted light could be detected. The intensity of the received light could be a measure for the distance between the light source and the receiver, such that distances of different lights with respect to the receiver can be determined.

The indicators may comprise gyroscopes, arranged to determine the mutual angles of the gyroscopes over the carrier structure. Gyroscopes are generally used to determine the angles of objects. In this case, when the carrier is aligned with the spine, and in particular with certain vertebra of the spine, the mutual orientation of the gyroscopes can be determined. The gyroscopes are for instance provided with Bluetooth or other wireless transmitters, to transmit their orientations to a central receiver.

The carrier structure is for instance a cloth, such as a T-shirt, which can easily be put on by a person without the need for special instructions or help.

The control unit may comprise a screen, to display the measured forces. The screen may thus provide direct feedback during the measurement of the exerted forces. The screen may also be used for video feedback of previous

measurements, or to show a series of movements the person on the chair should perform. The screen may also show a person sitting on the chair whether or not they sit with a correct posture.

The control unit may be arranged to convert the weight distribution and/or the position of the second sensor to a control signal, for instance to control a game. In a large number of games a game character is guided through a virtual world by moving the character left, right, forward and backward. The same movement can be performed on the chair, wherein the distribution of forces over the legs of the chair can be used to determine whether a person on the chair is moving forward, backward, to the left or to the right. By converting the distribution of weight, and therefore the distribution of forces, to a signal, the game character could be guided through the virtual world by moving on the chair.

The device may comprise a memory to save the mutual positions of the first and second sensors and/or to save the detected forces. Saving the data from measurement enables the processing of the measurements at a later stage, and enables to use of the measurements as reference data for further measurements. The memory preferably comprises a database with earlier measurements. In this memory the following input can be stored: an identifier to identify the person, their age, gender, body length and body weight, the performed movement pattern and/or medical condition.

The database can for instance be filled with reference data. The reference data may comprise the data of very flexible persons, for instance over various age groups, performing movements, the data of persons with variable flexibility, and the data of persons performing movement which persons are known to have various back problems. The database for instance comprises measurements of healthy persons, persons with scoliosis with a Cobb angle of at least 20 degrees, persons with thoracolumbar kyphosis and/or stiff flat spines. Determination of these back problems is done with external devices such as MRI scanners.

By comparing the measured data of a person on the chair with the measured data in the database, an indication of the flexibility of the spine of the person in the chair can be given, with respect to the reference data. The indication could for instance be that the flexibility is poor for a 20 year old. When the measured data compares well to the data measured of a person with known back problems, it is also possible to provide an indication of back problems, and advice the person on the chair to seek counsel of a specialist. The chair can thus provide an early diagnose for back related problems.

Every measurement on the chair can be added to the reference data, to expand the database. To do so, the age and body weight are entered into the control unit and saved in the memory.

The control unit may comprise a timer, to measure the time a load is placed on the chair. A timer may be beneficial when a movement on the chair is performed. By assigning timing data to the shifting weight distribution over time, a sense of the speed in which the movement is performed can be provided. Measuring the velocity and acceleration of the movements are another useful tool to characterize the individual flexibility (or stiffness) of the spine. The timer may also be used to time how long a person on the chair is sitting with a correct and/or incorrect weight distribution or posture.

The invention also relates to a method for detecting the weight distribution of a load, in particular of a person, in particular with a device according to the invention, comprising the steps of detecting the weight distribution of a person on a chair over the legs of the chair, and converting the detected weight distribution to a signal. The signal can thus be a measure indicating the weight distribution and/or posture of a person on the chair.

The method may also comprise the steps of filling a database with measurements; and comparing the detected weight distribution and/or the signal with the measurements from the database.

The method may also comprise the steps of moving the load on the chair; and detecting the weight distribution over the chair during the movement. This step could be having a person on the chair perform a particular movement.

The movement may be a predefined movement and wherein earlier measurements of the predefined movement are stored in the database.

The method may comprise the step of having the person on the chair sit straight. When sitting straight, the weight distribution over the legs of the chair should be fairly equal, or at least according to a predefined distribution, corresponding to a normal upright position. If a person is sitting straight and the weight distribution is not equal, or not according to the predefined distribution, the person may have spine deformations of other back related problems. Having someone sit straight on the chair may be performed by asking the person on the chair to sit-up straight.

When a person with a second sensor located on its head is sitting on the chair, the second and first sensors provide information about the weight distribution of the person over the legs of the chair, as well as the inclination of the head. Combined with the determined distance between the second sensor with respect to a vertical axis in the middle of the chair, the device determines the weight distribution and (at least part of the) posture of a person on the chair. The invention will be explained by means of the non-limiting illustrative

embodiments shown in the figures below, in which:

- figure 1 shows schematically a chair according to the present invention;

- figure 2 shows a schematic side view of a seating surface of a chair

according to the present invention;

- figure 3 shows a schematic front view of a seating surface of a chair

according to the present invention;

- figure 4 schematically shows a side view of a person sitting on a chair

according to the invention;

- figure 5 schematically shows a transmitter defining a central vertical axis according to the invention;

- figure 6 schematically shows the definition of a central vertical axis by the centre of gravity of a polygon according to the invention;

- figure 7 schematically shows a side view of a person sitting on a chair with laser assisted distance determination according to the invention;

- figures 8a and 8b schematically show seating of a person on a convex

seating surface according to the invention;

- figure 9 schematically show an anatomic representation of a person on a convex seating surface according to the invention;

- figure 10 schematically shows a footprint of a device according to the

invention;

- figure 1 1 schematically shows a wireframe for a mathematical model

according to the invention.

Figure 1 shows a chair (1 ) according to the invention comprising an upward facing convex seating surface (2); four mutually spaced legs (3), for supporting the seating surface (2) on a surface, wherein each of the legs (3) is provided with a first sensor (4), for detecting a force exerted on the leg (3).

The shown seating surface (2) is substantially elongated, symmetrical and has convex shape, such that the curvature of the seating surface (2) in longitudinal direction (L) differs from the curvature of seating surface (2) in width wise direction, perpendicular to the longitudinal direction (L), in that the curvature in longitudinal direction (L) is relatively small. Figure 2 shows a schematic side view of a seating surface (20) of the chair, and shows the curvature thereof in longitudinal direction (L). A line from the highest point (HP) of the seating surface (20) towards an end (EL) of the seating surface (2) in longitudinal direction is at an angle (a) of about 5 degrees with respect to the horizontal (H).

Figure 3 shows a schematic front view of a seating surface (20) of the chair, and shows the curvature thereof in width wise direction (W), perpendicular to the longitudinal direction (L). A line from the highest point (HP) of the seating surface (20) towards an end (EW) of the seating surface (20) in width wise direction is at an angle (β) of about 45 degrees with respect to the horizontal (H).

Figure 4 schematically shows a side view of a person (30) sitting on a chair (31 ) according to the invention. The person (30) is sitting on the seating surface (32) of the chair (31 ). In the middle of the legs (33) of the chair (31 ) a central vertical axis (CV) is defined. The person (30) is equipped with a glass-like frame (34) with a second sensor (35), used to determine the position of the second sensor (35) with respect to the central vertical axis (CV), in this case the distance (D) between the sensor (35) and the axis (CV). Given the known dimensions of the frame (34), the relative position between the second sensor (35) and the axis (CV) can be calculated.

Figure 5 schematically shows a side view of a person (40) sitting on a chair (41 ) similar to figure 4. The central vertical axis (CV) in figure 5 is defined by a transmitter (46). The transmitter is located underneath the highest point of the seating surface (42).

Figure 6 schematically shows a chair (50) according to figure 1 , comprising an upward facing convex seating surface (52); four mutually spaced legs (53), for supporting the seating surface (52) on a surface, wherein each of the legs (53) is provided with a first sensor (54), for detecting a force exerted on the leg (53).

The first sensors (54) are located at the bottom of each leg (53) and define the corners of a polygon (55), in this case a rectangle (55). The centre of gravity (CG) of this rectangle (55) can be determined by the crossing of the two diagonals of the rectangle (55), which defines the location of the vertical central axis (CV).

Figure 7 schematically shows a side view of a person (60) sitting on a chair (61 ) according to the invention. The person (60) is sitting on the seating surface (62) of the chair (61 ). In the middle of the legs (63) of the chair (31 ) a central vertical axis (CV) is defined. The person (60) is equipped with a glass-like frame (64) with a second sensor (65), used to determine the position of the second sensor (65) with respect to the central vertical axis (CV).

The central vertical axis (CV) is determined by a locator (66), comprising a reflector

(66) . From a fixed horizontal position (FP) a laser (67) sends a laser beam (68) which reflects on the reflector (66). Based on common laser technology, the laser

(67) receives the reflected beam and determines the distance between the fixed point (FP) and the reflector (66), and therefore the distance between the fixed position (FP) and the central vertical axis (CV).

Similarly, the second sensor (65) comprises a reflector (65). From the same fixed position (FP), another laser (69) emits a beam (70) which reflects on the reflector (65) to determine the distance between the fixed position (FP) and the reflector (65). The difference between the distance between the fixed position (FP) with respect to consequently the lower reflector (66) and the upper reflector (65) determines the distance between the central vertical axis (CV) and the second sensor (65).

Figures 8a and 8b schematically show a pelvis (80) of a person and two sitting bones (81 ), as well as a convex seating surface (82). In figure 8b also forces (F) are indicated which show how the load exerted by the person through the sitting bones (81 ) is distributed over the convex seating surface (82).

Figure 9 schematically shows an anatomic representation of the sitting bones (81 ) and the surrounding structures, and a convex seating surface (82).

Figure 10 schematically shows a footprint (90) of a device according to the invention, with four legs (91 , 92, 93, 94). The width (W) of the footprint is typically about 508mm and the depth (D) is typically about 308mm. The centre of the polygon (95), in this case a rectangle (95), determined by the legs (91 , 92, 93, 94) ends is considered as the origin (O) in a coordinate system. In this coordinate system, the first leg (91 ) is placed at (-0.5W, 0.5D), the second leg (92) is places at (0.5W, 0.5D), the third leg (93) is placed at (-0.5W, -0.5D) and the fourth leg is placed at (0.5W, -0.5D).

Sensors in the legs (91 , 92, 93, 94) each measure loads (91 m, 92m, 93m, 94m), or weights, exerted on the legs (91 , 92, 93, 94). The feet (F) of a person sitting on the device are assumed to be located in the middle, at position (0, F) in the coordinate system.

When a person is sitting on the device, with its feet in the air, all the weight of the person is supported by the legs of the chair, and the mass of the subject (Sm) on the device can be measured by combining the loads measured by the sensors of the legs. When the person places its feet on the ground, also the feet will support at least some of the weight of the person. This can be determined by subtracting the loads measured when the feet were in the air by the loads measured when the feet are on the ground. The load supported by the feet, Fm, can thus be referred to as the mass of the subject (Sm) - (91 m+92m+93m+94m).

Based on this information, the centre of mass of the load on the chair can be determined. The centre of mass would be the sum of the loads measured by the sensors (91 m, 92m, 93m , 94m) and the feet (Fm), multiplied by their location (r), and subsequently divided by the total mass of the subject (Sm).

In the above describes coordinate system, the centre of mass (R) can be divided, and written, in width (W) and depth (D) direction as follows:

-0.5W * 91m + 0.5W * 92m - 0.5W * 93m + 0.5W * 94m + 0 * Fm

R W) =

Sm 0.5W * ((92m - 91m) + (94m - 93m))

R(W) =

Sm

0.5D * 91m + 0.5D * 92m - 0.5D * 93m - 0.5D * 94m + F * Fm

R(D) =

Sm

0.5D * ((91m + 92m) - (93m + 94m)) + F * Fm

R(D) = ^ - - - J Sm

Figure 1 1 schematically shows a model representation (100) of a person (101 ) sitting on a chair (102) with a convex seating surface (103). The person (101 ) is represented by a hip joint (104), a knee joint (105) and an ankle joint (106), as well as by a back (107), upper leg, or femur (108) and lower leg, or tibia (109).

Due to the convex shape of the seating surface, the person (101 ) is sitting on top and in the middle of the seating surface (103) of the chair (102). The distance between the seating surface (103) and the hip joint (104) is HO. The distance between the hip joint (104) and the knee joint (105) is UH. The height of the knee joint (105) from the floor is LH. The height of the ankle joint (106) from the floor is AH. The height of the chair (102), more in particular the height of the seating surface (103), is ZH. The angle (a) is the angle the upper leg (108) makes with the horizontal, whereas the angle (β) is the angle the lower leg (109) makes with the vertical. The angle (a) has a preferred angle of about 45 degrees, such that the angle between an upright spine, or back (107) and the thighs, or femur (108) is about 135 degrees.

Due to the known location of the person (101 ) located on top of the convex seating surface (103) of the chair (102), and an estimation of the lengths of the several body parts, for instance based on the total height of the subject, a geometrical model of a person sitting on the chair can be established. This model in turn may be used to determine the optimal sitting height of the person, and thus the optimal dimensions of the chair (102).

It will be clear that the invention is not limited to the illustrative embodiments illustrated and described herein, but that countless variants are possible without departing from the scope of the attached claims which will be obvious to the person skilled in the art.