Ergonomics considerations for the conceptualization of the shape of body supports C.C.M. Moes, I. Horv´ath Delft University of Technology, Subfaculty of Industrial Design Engineering Jaffalaan 9, 2628BX Delft, the Netherlands C.C.M.Moes/I,
[email protected]
Abstract: This paper presents a methodology to incorporate ergonomics data and requirements of the pressure distribution between a person and a product into computer supported conceptual shape design. The methodology is applied to a flat sitting surface. Theoretical fundamentals for the algorithms are presented. Keywords: Pressure distribution controled shape design, pelvic tilt, somatotype, fuzzy surface definition, shape correction factor. 1. INTRODUCTION The area of IDE (Industrial Design Engineering) covers the development of products that will be manufactured in large quantities (mass products), suited for specific user groups. Although mass production reduces the costs of manufactoring, it shows also a disadvantage, since such products will be used by more persons. Here usually problems arise because people diverge with respect to their physical and mental properties and capabilities, e.g., anthropometry, somatotype, the physical constitution, cognition, experience, motivation. Therefore a mass product should be designed such that the unavoidable concessions are minimized. This optimization requires the knowledge of the product-relevant human capabilities and properties, as well as a continuous modification and adaptation of the whole design process to the current circumstances. Then products can be used in a comfortable, efficient and safe way by larger user groups. Within the current interest of the application of ergonomics knowledge into the computer supported design of the shape of consumer products, the relationships must be known of product characteristics, such as shape, weight, centre of mass, mechanical properties and surface texture, and the physiological and biomechanical consequences during usage and must be expressed in the programs of the ergonomics criteria, requirements and wishes. These relationships are operationalized by the pressure distribution between person and product. A slight modification of e.g., the shape, results in a corresponding change of the pressure distribution. Therefore the pressure distribution is selected to be the very evaluative quantity for the product shape. A second aspect of ergonomics concerns the variability of the human species: the anthropometric measures, skin composition, physical and mental constitution, motor skills, etc. These variables vary with age, gender, ethnicity, profession, level of training, and
Fig 1. Physiological descriptors for discomfort. many more. The description of a user group consists of the quantitative description of such variables. Thus, to obtain maximum comfort consumer products should be manufactored on an individual basis, which will inevitably end in expensive products that can hardly be sold. But, ergonomics also shows a way out of this problem. Ergonomics describes the acceptable ranges for comfort, effeciency and safety, so that products can be manufactored in a limited series of measures, each covering a subgroup of the intended user population. It is therefore a primary goal to determine the ranges of the product shape and other product characteristics. Here the field of fuzzy engineering is entered, for which the ergonomics statistical descriptives of human properties, and in particular the body characteristics, forms the main source of information. This paper describes (i) those aspects of the pressure distribution, which relate to the experienced comfort, as seen from physiological and biomechanical points of view, and to the product shape. Then the results of the measurements of the pressure distribution of persons sitting on a flat surface are given. Finally a solution is presented for the consideration of pressure distribution data in the calculation of an optimized product shape, that meets the ergonomics requirements. 2. PRESSURE DISTRIBUTION The pressure distribution is related to the experience of comfort via physiological processes (figure 1) and via biomechanical processes (figure 2). The physiological effects of the exertion of pressure can contribute to the experience of comfort via the following processes.
Fig 2. Biomechanical descriptors for discomfort (I)
The perception of pressure contributes not only to different tactile perceptions such as pain, pressure difference, touch or shear, but also to the discrimination of the size of the pressing object, directional information or the distance between spatial separated stimulations. Only limited literature was found about the quantification of the relations between such perceptions and the corresponding aspects of comfort. Yet the investigative efforts on tactile perceptions resulted in the determination of some levels such as the touch and discrimination thresholds for different parts of the body. (II) If pressure is exerted on skin capillaries, the blood flow can be constricted which deprives the skin of nutrients, inhibits the removal of waste products, and leads to local anaemia or tissue necrosis. The level of risk depends on the duration and the magnitude, and on the intermittent character of the load [Kosiak, M, 1985]. If a vibrating pressure is exerted diseases like VWF1 can result. Levels of risk are well known [ISO, 2631][NIOSH, 1989] and can be used to assess the requirements for e.g., the hardness of the product surface. The levels of risk depend on duration, frequency, intensity, etc. (III) The exertion of pressure on nerves can cause the inhibition of the transmission of pulse trains, resulting in numbness or even damage of nerve fibres [Dahlin, L.B. et al., 1986]. Only limited information was found about this subject. The biomechanical aspects of the exertion of pressure are usually worked out with the help of free body diagrams (FBD). A FBD depicts the forces on an isolated body, so that the equations of motion can be formulated. The pressure distribution contributes as such to: (I) The motion of the body, which is part of the product functionality and is therefore related to comfort. Example: dynamic tools. (II) The bare exertion of a force, without motion. The resulting functionality as well as the pressure perception contribute to the experience of comfort. Example: supports, static tools. 1
Vibration induced White Finger disease. A severe disease that has often led to amputation.
Fig 3. (III)
Measurement setup.
Tiredness. A force often arises from muscle activation, resulting in tiredness and muscle pain. It is difficult to quantify this relationship because of its dependency on training, being used to, constitution, gender, etc.
3. MEASUREMENTS AND RESULTS 3.1 Setup of the measurements The setup for the measurement of the pressure distribution is shown in figure 3. The measurements of the dependent variables included the assessment of the magnitude of the contact area and the pressure distribution. The independent variables included among other things the body mass, the stature, the distance between the SIPS-es (spina iliaca posterior superior), the percentage of subcutaneous fat [Durnin, J.U.G.A. at al., 1974] and the somatotype [Carter, J.E.L., at al., 1990]. In order to reduce complexity of the measurement the following setup was arranged: (i) the supporting sitting surface was flat and horizontal, (ii) the subjects did not use a backrest, (iii) horizontal force exertion was prevented. The subjects were 27 healthy students, aged from 18 to 24 years. None showed back complaints or had problems with sitting. The instrumentation consisted of two measuring devices. One device, which we called the ‘mirror box’ and is comparable with the paedobarograph, showed an extremely low pressure threshold and was used for the assessment of the form and the magnitude of the contact area. The second device, a 24×32 elements capacitive pressure transducer system, was used to obtain the pressure distribution data. The readings for the contact area and the pressure distribution data were taken at various pelvic positions: from maximum
reference position x ¯ Pmax αp βPm GT ∆T α β∆pT A0 α βAp
2
N/cm N/cm2 /◦ N/cm3 cm cm/◦ cm2 cm/◦
s
17.0 7.8 52.1 25.0 6.9 4.2 12.3 1.5 -0.038 0.016 702 104 -2.6 1.8
Fig 4.
multiple regression const
ecto
3.9 11.5 -1.03 4.6
4.2 13.1 2.55
51.3 1.32
gender stature
-3.3
%fat
∆SIPS r 0.73 0.70 0.80 0.91
0.053 10.2 44.3 -0.21
0.81 077
The results of the statistical analysis.
forward tilt to maximum backward tilt. The reference position was the middle position where the subject experienced maximum pressure under the seating bones. The angle of the pelvic tilt was referred to the ‘middle position’, which corresponds to sitting upright, while experiencing maximum pressure under the seating bones. The angle of pelvic tilt was obtained with the help of the ‘antenna method’ [Moes, C.C.M., 1998]. A small rod (the antenna) was fixed to the sacral skin. A pelvic rotation resulted in a corresponding movement of the antenna, which was recorded by a video camera. 3.2 Results of the measurements Data reduction was accomplished by expressing the pressure distribution in terms of a few parameters: (i) the left and right maximum pressure, (ii) the pressure gradient around the two areas of maximum pressure, (iii) the distance between the two points of maximum pressure, (iv) the magnitude of the contact area. The statistical analysis included among other things the average and standard deviation of the dependent variables and the application of multiple regression. The results of the statistical analysis are summarized in figure 4. The left two data columns give the average and the standard deviation of the variables in the leftmost column. The remaining columns give the regression coefficients of the pressure distribution variables with respect to the ectomorphic index, gender and stature. The coefficient of correlation is given in the last column. Multiple regression was applied to the pressure distribution data with respect to the measured body characteristictics. The maximum pressure (Pmax ), the pressure gradient (GT ) and the rate of change of the maximum pressure when the pelvic tilt (αp ) changes from the middle position (βPαmp ) depend mainly on the ectomorphic index, while the distance (∆T in cm) between the points of maximum pressure in the reference position (corresponding to the distance between the lower aspects of the ischial tuberosities) is mainly predicted by gender (0 for female, 1 for male) and stature (cm). The change of ∆T for αp varying pelvic tilt (β∆ ) showed an average value of -0.038 cm/◦ , indicating that a forward T ◦ tilt of 1 corresponds with a decrease of ∆T of 0.038 cm. The average maximum forward ◦ ◦ tilt was ca 20 and the average maximum backward tilt was ca 21 . Significant correlation αp of β∆ with body characteristics was not found. T
4. COUPLING SHAPE DESIGN AND ERGONOMICS 4.1 Issues relating to ergonomically controlled shape design Consideration of three-dimensional pressure distributions in the conceptualization of the shape of products, which are functioning as some kind of supports for the human body, is a multi-faceted research problem. Practical implementation of the process is challenging due to the following reasons: (I) Body forms and sizes varies in general in a very wide range. Therefore, a cluster oriented handling of the particular user groups is needed. (II) A wide distribution can be observed in terms of the physical body properties. This also needs some sort of generalization of the experimental data. (III) There are several difficulties in measuring 3D pressure distributions between deformed body parts and double-curved support surfaces. deformed human body
p
pressure distribution
x
shape of body support
a.
deformed human body
p x
b.
pressure distribution adapted shape of body support
Fig 5. The principle of consideration of pressure distribution Consequently, pressure distribution is measured on flat surfaces and compensated based on physical principles. (IV) Designers might want to apply a given (functionally and/or aesthetically pleasing) shape concept for the product which can be in conflict with the shape required for the ergonomically important contact surfaces. (V) There is no mathematically provable unique mapping between the deformed human body parts and the optimal shape of the support. The needed relationship have to be derived based on ergonomics knowledge, experiences and analogies. (VI) There is an inherent vagueness in the pressure distribution controlled shape generation. Therefore, fuzzy shape modeling has to be considered, rather than conventional crisp representations. The process of pressure distribution controlled shape conceptualization starts out of the assumption, that antrophometric data for the human body parts, which are in contact with the supports, are available. The measured data are used as primary information for the undeformed shape of the body part (figure 6a). The antrophometric database can depict either individual users or clusters of users. Furthermore, availability of information on the
s
stiffness distribution
measured pressure distribution
p
pc
x
x
undeformed body shape deformed body shape
a.
Fig 6.
flat support surface
b.
flat support surface
The effect of pressure distribution on the shape
stiffness distribution of the body parts is also supposed. In general, these stiffness data can only be determined by implicit measurements. Such data can be extracted from the literature and/or ergonomic databases. The shape conceptualization also needs pressure distribution data, which were obtained by concrete measurements during the experiments completed in the former phase of our research. Pressure distribution is described by a vector-vector function. The vector function defines the actual distribution values, while the dependent variables r epresent the reference points in the space. Since the magnitude and distribution of pressure determines the comfort of the user, the value of the permitted contact pressure must be reasoned out from the ergonomics knowledge. It definitely differs from the average pressure shown in figure 6b due to the differing loadability of the various regions of the body parts. Is has to be seen, that the connection between the undeformed shape of the body parts and the ergonomically pleasing shape of the body support is implicit. In the practice, the physical connection is expressed by the pressure distribution. 4.2 The framework of the shape conceptualization methodology The main activities in the shape conceptualization process are: (i) generation of a database of antrophometric data, (ii) measuring and generalizing pressure distributions, (iii) consideration of additional ergonomics knowledge, (iv) vague geometric modeling of the shape of the contact regions of the human body, (v) vague geometric modeling of the supporting regions of the product, (vi) deriving shape correction functions from body characteristics and pressure distribution data, (vii) application of the relevant shape correction function to the initial shape, and (viii) verification of the generated shape of the contact region with a view to the entirety of the product. The reference shape is defined as the shape of the unloaded skin so that the reference pressure is zero2 . If the spatial location of the skin elements (i, j) is symbolized as sij then for a person sitting on a flat surface the pressure equals: T F R T F pF ij = cij (sij − sij ) = cij ∆sij {z } |
below tubers 2
) F c(T ) R F and pFij = cc(T ij (sij − sij ) = cij ∆sij
|
{z
}
(1)
outside tubers
Actually the atmospheric pressure, but this pressure is in equilibrium with the hydrostatic pressure within the body.
where, of course, the symbol ‘∆’ refers to the reference situation. Further, the index ‘T ’ refers to the tuberal area, ‘c(T )’ to the complement of T , ‘F ’ to flat, ‘R’ to the reference shape. The equations (1) assume that the elastic properties depend on the skin area: there exists a difference between the tuber area and elsewhere. Pilot measurements reveiled that the maximum pressure of the whole contact area is usually found below the tubers. Outside this area the pressure rapidly decreases. Therefore the pressure gradient shows locally high values. The areas of high and low pressure are therefore symbolized as AT and Ac(T ) . It is reminded that the pressure gradient constitutes the very cause of the displacement of tissue liquids. Since the Poisson’s ratio of body tissues has a value not far from 0.5 the material necessarily shows sideway expansion, leading to a spread of the pressure lines. This effect depends strongly on the aspect ratio (quotient of the tuber width and the tissue thickness), the relative impression depth (the ratio of impression and impressibility), the friction and the Poisson’s ratio [Chow, W.W. et al., 1978][Zhang, M., et al., 1997][Reddy, N.P. et al., 1982]. The shape conceptualization methodology worked out by the authors does not assume any concrete measured data for the deformed shape of the body parts. The reason is that the actual deformation is influenced not only by the original shape, the rearrangement of the internal structures and stiffness distribution of the body part, but also by the shape of the applied body support [Turner, R. et al., 1998]. Therefore, the deformed shape is computed based on mechanical modeling that starts out of the measured stiffness distributions. This approach is frequently applied in the literature [Chen, D. T. et al., 1992]. We followed the deformable (spatial) lattice concept, discussed among other in [Jee, H. et al., 1998]. A deformable lattice is a set of regular lattice point in the 3D space. The sub-system of the active surfaces (containing the actual contact surfaces) corresponds to the nominal (unloaded) surfaces of the human body parts. When a particular pressure distribution is applied on the contact region of the deformable lattice, the resulting deformation will approximate the real-life deformation behavior of the human body. The total elastic energy will be equal to the work of the force and moment raised by the body weight and the tilt of the pelvis, respectively. Finding the deformed state is a variational problem, which is solved by numerical gradient method. The subset of the contact surface points of the deformable lattice is used to generate the system of contact surfaces of the designed product. Here, however, further considerations have been involved in the shape conceptualization methodology. As it is shown in figure 7.a, at the beginning of the process the geometric description of the distribution of the shapes of the body parts and the initial product shape are given. The latter is not supposed to be the ergonomically pleasing contact surface for the body support. First, the designer has to select that cluster of the nominal shape that corresponds to the targeted users of the body support. Then, this considered range of body shapes is substituted by a sufficiently dense cloud of points, which forms the basis of the further shape conceptualization. Similarly, the initial shape of the product is described by a consistently dense set of points. Based on (i) the undeformed shape of the concerned body part, (ii) the stiffness distributio n of this body part as well as (iii) the pressure distribution measured on flat surface, a shape correction function is constructed. This correction function is more em-
measured body shapes
C
shape correction function x
fuzzified body shape
ergonomically optimized product shape
fuzzified product shape
p
pressure distribution
x
initial product shape b. a.
Fig 7. The principle of fuzzy shape conceptualization pirical than theoretical. At the same time, it can be expressed by mathematical formula (namely, by a vector-vector function). The vector function values define the deformation vectors, and the dependent variables represent the reference points in the space. When applied, the correction function modifies the spatial position of the points that describes the initial shape specified by the designer. The result of applying the shape correction function is the ergonomically optimized contact surface of the product with a pressure distribution satisfying the ergonomic criteria. In the case of support devices, long lasting physical contact exists between the human skin and domains of the product. To increase the ergonomic comfort, the contact regions are often separated by intermediate layers (e.g., clothing). In the first phase of our research, however, this option has not been considered. The contacts are taken as static, the human body elastic and the support surface rigid. The solution presented above does not consider application of elastic (soft) materials to cover and/or cushion the rigid support surfaces. Nevertheless, the mathematical computation reckons with uncertainty due to the stochastic nature of antrophometric data and body characteristics. The pressure distribution is principally determined by the body weight and position, the geometry of the contact domain, and the stiffness distribution of the concerned domain of the human body. It was mentioned earlier, that consideration of the body characteristics of a group of humans introduces fuzziness into the description of the contact geometry. The initial point cloud modeling of the contact surfaces both of the human body and the product allows as to care for this fuzziness. The resultant point cloud forms the basis of generation of interpolating surfaces. There have been several algorithms published for general surface fitting to spatial data points and smooth parametric surface interpolation [Vergeest, J.S.M., 1989]. Algorithms for some specific requirements have also
been developed [Bj¨orkenstam, U.C., 1987]. The authors decided on the application of NURBS-based surface interpolation, for it provides possibility for local manipulation. 5. CONCEPTUAL SOLUTION 5.1 The general reasoning model based on causal aspects Now the question of a closer description of the cause of the pressure distribution arises. The following aspects take a central position in the analysis: product properties such as the product shape and the mechanical characteristics of the product surface (P ), body characteristics such as the mechanical characteristics and the shape of the unloaded skin of the contacting part of the human body (B ), and the type of product usage (U ). Figure 8 shows these aspects at the left side. These aspects result in the actual interaction, and thus in the pressure distribution.
Fig 8. General model. If the pressure distribution Π is expressed as a function of P , B and U , then a change of the pressure distribution can mathematically be expressed, in first order approximation: ∆Π =
∂f0 ∂P
B,U
∆P +
∂f0 ∂B
P,U
∆B +
∂f0 ∂U
P,B
∆U
(2)
This equation defines various quantities. As a matter of fact it is at this moment not possible to describe these variables quantitatively. But their qualitative meaning will be discussed. ∆Π can also be explained as the deviation from the ‘ideal’ pressure distribution (∆Π = 0). The ideal pressure distribution leads to (i) the intended product motion or force exertion, (ii) the right experience of skin pressure (tactile perception), (iii) no extreme physiological load of the skin. It depends of course on the type of the product in question and on the circumstances during product usage. ∆P refers to the deviation of product characteristics from a certain ‘product standard’. This standard includes various product properties such as surface hardness, location of the centre of mass, inertia and the product form. ∆B is the deviation from the ‘average’ user. The expression ‘average’
refers to the intended user group, where different aspects, such as the amount of subcutaneous fat, motor skills or anthropometric measures, play a role. ∆U refers to variations in product usage with respect to the intended type of usage. For example, different ways of handling, different velocity of movements, and even unintended usage. The linear coefficients describe the unit of variation of the pressure distribution for a unit of variation of P UB with respect to their reference ‘values’ (because of the first order approximation). These coefficients must be derived for each type of product under investigation. Thus equation (2) can be represented, within the constraints of the first order approximations, as a flat surface in a three dimensional space. This space is spanned by three axes that represent the (assumed independent) variables P UB . 5.2 Formulation of the criteria The most important ergonomics criteria for right product functioning relate to comfort, efficiency and safety. The pressure distribution and the criteria are mutually related. On the one hand the pressure distribution induces a certain level of ces. On the other hand these criteria put requirements to the pressure distribution. We concentrate on the first: c = fc (Π), e = fe (Π), s = fs (Π). The underlying most fundamental criteria for an acceptable pressure distribution depend, as a matter of fact, on the physiological consequences of pressure load. If the criteria and the acceptable regions for comfort, efficiency and safety as well as their relationships with the pressure distribution are known then these equations lead in first order approximation to: ∆c = (∂fc /∂Π)∆Π, ∆e = (∂fe /∂Π)∆Π, ∆s = (∂fs /∂Π)∆Π so that: ∆(Π) =
∆c + ∆e + ∆s ∂/∂Π(fc + fe + fs )
(3)
Equation (3) defines the acceptable fuzziness for the pressure distribution with reference to the criteria comfort, efficiency and safety. Now the problem of the definition and quantification of ces arises. In order to quantify the concept ‘comfort’, it must first be investigated which aspects are included. This depends on the type of product, the intended user group, etc. If this is known then the mentioned aspects must be described in such a way that it is possible to judge them on for instance an (at least) ordinal scale so that they can be quantified. Then the ‘value’ of comfort must be defined based on a weighted combination of these quantities. A comparable reasoning can be done for efficiency and safety. 5.3 Combining causality and criteria If the coefficients of equation (2) and the acceptable range of the pressure distribution, as it follows from equatuin (3), are known and if P UB are independent, it should in principle be possible to derive acceptable combinations of (∆P, ∆U ,∆B ): ∂f0 ∆c + ∆e + ∆s = ∂/∂Π(fc + fe + fs ) ∂P
∂f0 ∆P + ∂B B,U
∂f0 ∆B + ∂U P,U
P,B
∆U
(4)
The right part of equation (4) defines the direction of the above mentioned surface spanned by the P -, B - and U -axes. The left part determines its position in this space. The tolerances (∆c, ∆e, ∆s) define the limits of the range of acceptable positions. Together with
reasonable ranges of (∆P, ∆U ,∆B ) the design space is then defined. The next section explores the (im)possibilities to adequately quantify the different terms of equation (4). Provisionally we assume linearity and independency of the different terms of (4). Nevertheless these assumptions must be investigated. Another complication is whether the different terms, both variables and coefficients, can indeed be quantified either based on ranking or even on an interval or ratio scale. Adequate research is needed for decisive answers. Inclusion of the ergonomics conceptual design in the balanced comprehension program for the conceptualization of the product shape, requires precise definition and quantification of the terms of equation (4). Until now no reported research to this field has been found. 6. ALGORITHMS FOR PRESSURE CONTROLLED SHAPE DESIGN The developed methodology starts out of the sorted antropometric data, the pressure distributions measured between flat surfaces and body parts of a group of humans, and an initial shape of the product which provides the requested functionality. The designed support surface is discretized as a flexible net. Specific compensating functions have been formulated as vector-vector fields to compensate for the differences of body stiffness with a preliminary deformation of the initial surface(s). From the consideration of the difference between the shape of the unloaded and the loaded body, a displacement field is applied on the nodes of the flexible net that describes the initial shape of the product. The pressure distribution is considered as concentrated forces applied on the net’s nodes and the resultant displacement is calculated from the static equilibrium of the net. The cloud of spatial points representing the nodes of the net are used for parametric surface interpolation which produ ces the optimum shape. Below a high level description of the algorithm for pressure distribution controlled shape design is presented with the aim of clarifying the main steps. The limited extent of the paper does not allow us to consider programming and data management aspects. The algorithm advances in the following steps. 1 Let Bi be a point set representing the 3D point data of the considered body part measured for one individual. Identify one characteristic point r of the point cloud and take it as a reference point. Generate the point cloud B that describes the undeformed shapes of the body parts for a cluster C of the humans by arranging all concerned point sets Bi such a way that their reference points ri are coincidental and the orientation of the clouds is identical. 2 Generate the point set boundary Bb of the point cloud B by selecting the closure points. 3 Generate the medial surface transform (MAT) of the point set boundary Bb . Consider this sufficiently dense point set S as the undeformed shape of the respective body part for the cluster C of humans. 4 Convert the initial geometric model of the contacting region of the product into a sufficiently dense point set P . 5 Let A be the reference surface lattice for the pressure distribution. Let the nodes of
the lattice defined by vectors a. Let π(a) be a vector-vector function that specifies the pressure distribution π at the nodes a. 6 Generate the 3D deformable lattice L of the human body part based on the stiffness distribution data. 7 Project the reference surface lattice A of the pressure distribution onto the point set S . Based on the deformable lattice L compute the deformed shape of the body part as the point set S 0 . 8 Apply the shape compensation function Φ for the deformed shape S 0 to generate the ergonomically pleasing shape for the contact region of the product as a point set Se . 9 Apply singularity recognition for the point set Se . Generate the natural representation (N-rep) of the point set Se as a composition of shape singularities δ and natural surfaces ω , as discussed in [Horvath, I., et al., 1998]. This procedure is then repeated for varying pelvic tilt, other subjects of the user group, and other user groups. 7. SUMMARY AND CONCLUSIONS The paper presented a methodology and a computer-based solution for consideration of pressure distribution in conceptualization of body supports providing ergonomic comfort for the users. All important aspects of the ergonomics knowledge have been considered. At the development of the algorithm it was assumed that (i) the scanning and measuring of the different body parts provide sufficiently dense, everywhere consistent point sets, (ii) the pressure distribution data are available as vectors assigned to a dense surface lattice, (iii) the initial product shape is represented as a well arranged system of NURBS surfaces. The link between the shape of the body parts and the shape of the support structures is established based on pressure distribution. Shape correction functions are generated and applied to modify the initial shape proposed by the designers into an ergonomically pleasing system of supporting surfaces. To encounter the vagueness in description of the shapes, an initial point set modeling is proposed. This is converted into a natural representation that provides flexibility in conceptual shape modeling. The on-going phase of the research is focusing on the software-level implementation of the pressure distribution controlled shape conceptualization methodology. ACKNOWLEDGMENTS The research work reported in this paper relates to the Integrated Concept Advancement (ICA) project of the Subfaculty of Industrial Design Engineering, FDEP, of the Delft University of Technology. 8. REFERENCES [Kosiak, M, 1985] Kosiak, M.: “Etiology of decubitus ulcers”, Archives of Physical and
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