THE SIGNIFICANCE OF CONTACT PRESSURE DISTRIBUTION ON THE SOFT TISSUE BY MEN SITTING

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1 INTERNTIONL DESIGN CONFERENCE - DESIGN 2002 Dubrovnik, May 14-17, THE SIGNIFICNCE OF CONTCT PRESSURE DISTRIBUTION ON THE SOFT TISSUE BY MEN SITTING Todor Ergic, Zeljko Ivandic and Drazan Kozak Keywords: cair, body, contact, pressure distribution, experient 1. Introduction Te weigt of te uan body transfers by sitting over sitting bones (Tuber ossis iscii) and surrounded soft tissue on te cair and vice versa. Te pressure by cair acts on te bulk uscular and bulk bones, so its long tie activity ay cause canges on te soft tissue and skeleton. Terefore, design of te cair suitable for te uan body fro te ergonoic point of view becoes necessary. Tis is not related just wit te cofort. It is connected wit ealt carrying and avoiding of consequences connected wit long tie sitting. lot of tie te only relation between te cair and uan body by cair designing was te agnitude and distribution of sitting pressure. Upolstery over te seat as te role to distribute te pressure under Tubera ossis iscii on te larger area and to reduce te influence of te antropological and bioecanical properties of te individual. However, any probles today related to sitting and te seat are still inadequately solved. One of tese probles is connected wit long tie sitting in fixed position, wat in soe circustances of reduced circulation ay lead to trobosis [Stranden 2000]. lso, young drivers ay loaded very often its veins in fixed condition, so tey ay becoe ill fro trobosis. Internationally accepted diagnosis for suc cases calls as effort trobosis. Usually are coplaints of aircraft pilots on te uncoforted cairs in cockpit, wat is anifested by pains during iddle- and long-range fligts [Goossens 2000]. Tese pains caused by uncofortable seat could be a reason of decreased pilot s concentration wat are connected wit fligt safety. Te pilots are seated in te cairs, wic ave te good possibility of geoetry positioning. But, tis is not enoug. New solutions of te cusioning over te seat and cover fro te seepskin are designed to iprove air circulation between pilot s body and aircraft cockpit s cair. Neverteless, pilot s coplaints are not eliinated by tese solutions. Te ain goal of tis work was not just te easureent of te pressure to design new cair sape. Te ai of perfored easureents is also to find out te relevant values of single paraeters tat ay good describe pressure agnitudes and its distribution by en sitting. Tis investigation of te pressure distribution between te skeleton and te cair sould better describe its influence on te soft tissue of te uan body. 2. Te contact pressure easureent 2.1 Experiental set-up Te original exained cair for te easureent of te contact pressure distribution (Fig. 1) as been designed and fabricated. MULTIDISCIPLINRY SPECTS OF DESIGN 743

2 Te ai was to deterinate te influence factors on te agnitude and distribution of te pressure during sitting. Design of te cair ake possible continuously displaceents of its backrest troug te dept fro 220 to 540 easured fro te cair front. Te size of te backrest is 445 x 940. It is possible to put up te backrest for 5 fro te elastic eleents. Te angle of te backrest could be canged continuously fro -80º to 80º related to vertical position. Te cair is equipped wit te arrests displaced for 200 fro its front. Te distance between arrests is equal to 485. Designated eigt of te seat is 340 fro te ground, but te seat eigt ay be altered by adding of wasers below te legs. Figure 1. Experiental set-up 2.2 Principal of te pressure easureent Contact pressure easureent as been perfored by elastic eleents deforation assessent. Te elastic eleents in te for of cylinders wit diensions Ø8 x 10 are seated on te down plate. Te seat of te cair as te size of 20 x 480 x 635 and it consists fro 20 layers beec s veneer. On te seat te es wit 2914 square divisions wit diensions of 10±0,1 is attaced. Eac es division on te seat is perforated wit te ole sized of Ø1, were elastic eleents are fastened. Under seat te easured lat is placed. Measured lat as two degrees of freedo (orizontal and vertical displaceents). Horizontal displaceent u enables te easureent of elastic deforations of elastic eleents. Vertical displaceent w ay record deforations successively line by line. Deforation of elastic eleents appears during en sitting. Strain easureent is realised by erection of easured lat in te w direction for desired value, wat could be easured by clip gauge. Moving of easured lat in te u direction results wit contacts supplying under elastic eleents and recording of easured signals. Step otors are used to anage wit displaceents tat are easured by clip gauges, aplified and processed by PC. One easured lat erection after anoter wit displaceent easureent enable to deterinate te agnitude and distribution of te pressure by sitting. Te results of te perfored easureents are presented on te figures 2, 3 and 4 and tables 1, 2, 3 and 4, respectively. 2.3 Te position of single person by easureent Te sin bones of individual during experient were vertical and supported on te front edge of seat under knees. Te feet were leaned on te ground for 340 below te upper edge of te seat. ny person tried to keep te spine in te vertical position by pressure easureent. Te buttock of individual is leaned on te vertical support. Soulder-blades and nape are supported on te top of backrest. ll easureents are perfored witout any upolstery. 3. Te results wit easureent analysis Te easureents were perfored on te saple of 37 ale and 33 feale persons. Soe caracteristics of saples are given in te tables 1 and 2. Sport type of constitution wit larger uscle 744 MULTIDISCIPLINRY SPECTS OF DESIGN

3 ass produces also larger pressures, but saller tan very tin constitution. Tick persons ave te sallest pressure variation. Extra tick persons (especially older woan) ave te sae pressures tat are a little bit canged wit different sitting conditions. Tin individuals ave saller areas under tig bones and front edges are displaced toward Tuber ossis iscii. lso on te back side te edge of sitting is displaced toward Tuber ossis iscii. Te widt of sitting is canging relatively a little. It is iportant to explore all relevant factors tat ay influence on te agnitude and distribution of sitting pressure. Because of tat, antropological and bioecanical analyses of sitting were perfored. 3.1 Bioecanical aspect of sitting It was observed tat even sall sifting of te ead, trunk or any extreity as substantial cange of te pressure field as a consequence. s an exaple of suc beaviour, te ead turning for ~ 90 o in te left side is presented on te Fig. 2 (pressure easureent was perfored witout backrest). Fro te Fig. 2 is obviously tat weigt of body is transferred on te rigt side. Maxial rigt side pressure is 60% greater tan left side pressure. One can conclude fro tis picture tat individual position drastically influences te easureent results. If te sitting plane is so low tat legs ay touc te ground, te greatest pressures appear under Tuber ossis iscii. Here te region below te tig is unloaded wat is te case also by sitting on te leading part of te seat. If te sitting plane is too ig and feet don t touc te ground, te pressure is transferred troug te large field under tigs and axial pressure appears on te edge under knee. Twisting of te backrest turns te gravity centre of te upper part of body to back, so te contact surface is displaced also to back. Maxial pressures under Tuber ossis iscii are ten displaced for 1 to 2 and back edge of te contact surface for 3 to Figure 2. Grapical exaple of easured pressure by sitting 3.2 Bioecanical principal of te sitting Radiograpic studies all confir tat te pelvis rotates backwards and te lubar spine flattens wen sitting [ndersson 1986]. Wen oving fro a standing to an unsupported sitting position, lubar lordosis decreased by an average of 38. Tis ainly occurred by backward rotation of te pelvis (average 28 ). Te reaining 10 were canges in te vertebral body angles of te two lower lubar segents. kerblo, Keegan, Scobert and oters tat flattening of te lubar lordosis in sitting can be prevented by te use of well-designed low-back support. ndersson et al studied in wat way different types of lubar supports placed at different levels of lubar spine influenced a nuber of angles, including te lubar lordosis angle, as well as te influence of canges including te lubar lordosis angle, as well as te influence of canges in backrest inclination on tose angles. Groups of uscles on te buttock ave influence on te lubar vertebrae configuration and pelvis. Tey are connecting tig and knee cap. Te rotation of kneecap to back brings to flatten of lubar vertebrae and to canging of agnitude and distribution of pressure. MULTIDISCIPLINRY SPECTS OF DESIGN 745

4 3.3 ntropoetrics Te easureent results are sown in te tables 1 and 2, were te ratio of individual eigt and body ass are given. Te average values of axial pressure p ax and area were calculated. verage value of axial sitting pressure produced by en body is 16,77% greater tan one produced by woen. But, average value of sitting area by woan is for 20,8% greater tan by en. Te agnitude and distribution of te pressure is individual caracteristic, wat can be seen fro te easureent results (fig. 3, 4, 5 and 6). Te constitution of te single person as significant influence on te pressure field. s uscles are stronger, te pressures are greater. Te people, wo spend long tie in te sitting postures or are not exploded to pysical efforts, sow lower pressures. Fro antropoetric results index of prettily and uscles index are calculated, but teir values by statistical analysis give not accepted results. Body ass index only gives acceptable results (tables 3 and 4). Except tis, it as been trying to replace coplex pressure easureent wit single antropoetrical easureents, wat is depicted on te fig. 5 and 6. Te pressure is lower wen te contact surface of uan body by supporting is greater (fig. 3 and 4). Te seat could be adjusted to te sape of body, wat is valid for te resting body only. Canging of te sitting postures sould not influence on te cofort of sitting. Suova sown tat a an during 5-ours sitting canges its posture for 1000 ties. Table 1. Statistical values of easured paraeters for ale kg ge years verage value 180,3 82,1 35,5 2,248 0,798 42,88 17, Standard deviation 6,63 13,6 15 0,355 0,158 1,58 2, Minial value ,59 0, , Maxial value ,015 1,104 59,5 22, ln p ax Table 2. Statistical values of easured paraeters for feale kg ge years verage value 165,6 66,1 31,6 2,605 0,941 36,7 12,1 546 Standard deviation 7,21 14,2 11,3 0,439 0,188 7,30 2, Minial value ,398 0, , Maxial value ,2 54 3,237 1,174 55,5 19, P p = * r = 0,5777> (p < 0.01) r 2 = t = 5,1306> 2,72 (p < 0.01) X = Y = σ 2 p = SR = σ 2 = ln p ax p ave p ave 60.0 p p = * r = 0,55044 > (p < 0.01) 55.0 r 2 = t = 3,671 > 2,72 (p < 0.01) X = Y = σ 2 p = SR = σ 2 = Figure 3. Maxial pressures by sitting depending on sitting s surface (for en) Figure 4. Maxial pressures by sitting depending on sitting s surface (for woen) 746 MULTIDISCIPLINRY SPECTS OF DESIGN

5 Table 3. Statistical values of antropoetrical values - woen O 1 O 2 O 3 O 4 verage value 31,12 29,5 80,01 78,72 101,6 88,8 5,57 0,763 24,15 Standard deviation 5,21 4,99 13,57 14,27 10,95 8,17 2,59 0,070 5,28 Minial value ,6 61, ,7 1,72 0,672 17,86 Maxial value 47, , ,5 0,934 42,59 O 1 circuference of te upper ar in flexion and by fist contraction, O 2 circuference of te upper ar of relaxed fist, O 3 circuference of abdoen on te level of ubilicus in te orizontal line, O 4 circuference of abdoen 2-3 fingers over ubilicus in te orizontal line, O 5 circuference over ips, O 6 circuference over breasts. O 5 O 6 M index Table 4. Statistical values of antropoetrical values - en O 1 O 2 O 3 O 4 O 5 O 6 M index verage value 33,2 30,8 90,5 88,6 101,9 98,76 7,81 0,863 25,29 Standard deviation 4,6 4,21 12,2 12,96 7,33 9,21 3,56 0,086 4,11 Minial value 25,8 23, ,54 0,722 16,92 Maxial value 43, ,7 118,5 15,99 1,152 34,94 I prettily I prettily p p = exp( */ 2 ) * r = 0,47608> (p<0.01) r 2 = 0, p = 25,2932 log / 2 = 3,73795 σp 2 = 0, SR = 1,17185 σ 2 = 0, p p = exp( */ 2 ) * r = 0,569655>0.418 (p<0.01) r 2 = 0, p = 24,1527 log / 2 = 3,58512 σ p 2 = 0, SR = 0, σ 2 = 0, / 2 35 Figure 5. Maxial pressures by sitting depending on Body ass index by en 20 / Figure 6. Maxial pressures by sitting depending on Body ass index by woen 3.4 Te pressures on te blood vessels Many studies are sown tat inadequate designed cair as unfavourable influence on blood vessels as direct consequence. kerblo and Scobert proved tat pressure on te edge of te cair do not stop circulation even in te arteries wit sallest pressure. Landis study describes 4,3 as axial pressure tat soft tissue of te en could tolerate witout any probles. Tis level of pressure doinates on te border between arterioles and capillary. Tis value is not applicable by sitting [Jürgen 1997] fro siple reason. Naely, wen te weigt of te body divides by sitting area, ten average value is greater tan Landis value. Suc sitting is not pysiologically acceptable for te en. Terefore, Jürgen proposed tat pressure appearing during sitting on te cair wit upolstery sould not overcoes 10 under Tuber ossis iscii. MULTIDISCIPLINRY SPECTS OF DESIGN 747

6 4. Conclusions On te basis of perfored easureents and bioecanical and antropoetrical analysis ay be concluded tat agnitude and distribution of sitting pressure is individual caracteristic of any person. For te sae age and sex of te person wit te sae ratio of eigt and body ass different pressure fields were obtained. Individual caracteristics of agnitude and distribution pressure are related to: ratio of te soft tissue and skeleton, ratio of te uscles and fat tissue, tickness and distribution of te fat tissue, sape and size of bones (spine and pelvis sape) etc. Tere is no analytical law for axial pressures. lso, axial pressure is not only doinant value for te cair design. If tat were te case, air or water bag would be an appropriate solution. Muc iportant role by cair design plays te distribution of te pressure. Terefore, optial cair design would be acieved wit individual approac to any single person. Te goal of designers today sould be to realise cair sape wit suc agnitude and distribution of pressure, wic will ensure legs uscles and uan body relaxation by sitting. Tis will provide long tie sitting witout any ealt consequences. References ndersson, B. J. G., Loads on te Spine during Sitting, Te Ergonoics of Working Postures, Te Proceedings of te First International Occupational Ergonoics Syposiu, Zadar, Taylor & Francis, London and Piladelpia, 1986, pp Barocas, V. H., Tranquillo, R. T., n nisotropic Bipasic Teory of Tissue Equivalent Mecanics: Te Interplay ong Cell Traction, Fibrillar Network Deforation, Fibril lignent, and Cell Contact Guidance, Journal of Bioecanical Engineering, Vol. 119, No.2, 1997, pp Bros, T., rcan, M., Modeling te body/cair interaction an integrative experiental nuerical approac, Clinical Bioecanics, No. 15, 2000, pp Cristiansen, K., Subjective ssessent of Sitting Cofort, Collegiu ntropologicu Founded on te occasion of te 1 st Congress of European ntropologists eld in Zagreb in 1997, Vol. 21, No2, pp Goossens, R.. H. M., Snijders, J. C., Fransen, T., Bioecanical analysis of te diensions of pilot seats in civil aircraft, pplied ergonoics, No. 31, 2000, pp Jürgen, W. H., Seat Pressure Distribution, Collegiu ntropologicu Founded on te occasion of te 1 st Congress of European ntropologists eld in Zagreb in 1997, Vol. 21, No.2, pp Stranden, E., Dynaic leg volue canges wen sitting in a locked and free floating tilt office cair, Ergonoics, Vol. 43, No.3, 2000, pp Todor Ergic, MSc, BSc Mecanical Engineering Faculty in Slavonski Brod Josip Juraj Strossayer University of Osijek Tel , Fax: Eail: tergic@sfsb.r url: ttp:// 748 MULTIDISCIPLINRY SPECTS OF DESIGN