Evaluation Of Postural Changes Using Body Pressure Distribution System Levente DÉNES a* Noémi TAKÁCS a a Institute of Product Development and Manufacturing, Faculty of Wood Sciences, University of West Hungary, Abstract The primary objective of this paper was to evaluate the pressure distribution characteristics of five working chairs. More specifically the study reveals the following aspects: evaluation of differences associated with chair dimensions, adjustability features and analysis of body pressure distribution in different seating postures. Postures, movements, chair parts inclination and comfort rating data were collected from 4 subjects. Considerable effects of tasks built and chair type on pressure distribution and pressure changes were demonstrated. The results indicate an additional research to further explore the effect of other factors, i.e. foam type, elasticity, etc. on pressure distribution and discomfort measures. Keywords: posture, pressure map, work chair, comfort 1. INTRODUCTION A recent study shows that more than 50% of employees in the EU perform high- or lowskilled whitecollar work, predominantly in offices (PARENT-THIRION et al., 2007). As far as working chairs are one of the most important elements of every office environment the functionality of the office chairs is crucial to facilitate the workers in their jobs. Statistics and several studies reveal the importance of ergonomic seating which all office employers should consider. The right ergonomic chair with proper use can help to reduce injuries and the socalled cumulative trauma disorders (CTDs), which often results from repetitive keyboard work for prolonged periods of time (MIN-YONG et. al., 2000). Consequently a work chair should encompass adequate support, task efficiency, posture adjustability and comfort (TREASTER and MARRAS, 1987). Several studies showed that the performing of different tasks in office work causes variations in user postures and movements (ADAMS et al., 1986; BABSKI-REEVES et al., 2005; COMMISSARIS and REIJNEVELD, 2005; DOWELL et al., 2001; ELLEGAST et al., 2007; VAN DIEËN et al., 2001). During posture changes the surface pressure of chair s seat and back vary leading to comfort or discomfort while sitting. People of different body weights and builds distribute their weight on a chair in similar patterns, but pressure intensity and areas of distribution differ from person to person. Good pressure distribution in a chair focuses peak pressure under the sitting bones in upright postures and in the lumbar and thoracic areas in reclined postures. Correct pressure distribution is critical to seated comfort (GRANDJEAN et al. 1973). A high level of surface pressure can constrict blood vessels in underlying tissues, restricting blood flow, which the sitter experiences as discomfort. During sitting large scale pressure levels over the thighs, buttocks and genitals are experienced which weakens the circulation in soft tissues and the legs. Pressure that is levelled predominantly over the area of the genitals also squeezes the inner pudental arteries, responsible for the blood supply for erectile tissue. As a consequence sitting postures leads to
discomfort and over time to health disorders (e.g. back, neck and shoulder pain, etc.) which curing price is paid by society through missed work and reduced workeffectiveness/productivity (JOHANSEN and JOHREN, 2002). 2. MATERIALS AND METHODS Four male subjects participated in this study with the body characteristics presented in Table 1. Both Body Mass Index (BMI) and Reciprocal Ponderal Index (RPI) were determined to analyze the relationship between subjects built and pressure distribution. The BMI is a common measure of body fat based on height and weight and it is defined as the individual's body weight divided by the square of his or her stature. The Reciprocal Ponderal Index is other index for adiposity; height divided by cube root of weight. Table 1. Subjects body characteristics Subject A B C D Age 30 28 28 21 Weight 98 66 75 83 Stature 175 185 173 192 BMI 32,0 19,3 25,1 22,5 RPI 38,0 45,8 41,0 44,0 BMI: Body Mass Index = (mass (kg)/stature (cm) 2 )x(10,000); RPI: Reciprocal Ponderal Index = stature (cm)/mass (kg)1/3. For pressure distribution measurements the Body Pressure Measurement System (BPMS ) developed by Tekscan was utilized using two sensor foils Model 5315, with a sensing area of 488mm x 427mm and 2016 sensing points (1 sensel/cm 2 ), pressure range of 0-250 mmhg. The sensor mat s accuracy was rated at +/- 3.5mmHg for observed pressure measurements. The system is used to "pressure map" people sitting on seats and chairs, beeing both dynamic and very portable, so it can be just as easily used in a laboratory as in field measurments. The information gathered by system provides input for improving chair designs and comfort. Pressure distribution and the location of the points of maximum pressure, both on chairs seat and back was measured for subjects sitting on five different office chairs (Figure 1). The pressure data were analyzed for force- and pressure-related quantities. Three postural treatments were created specific for office work: the general office posture i.e. typing on a computer keyboard, the laying back posture when the subjects adopt a more relaxing position and an asymmetrical pose, i. e. the body mass having transferred on the right side and a part of the body weight is supported by the chair arm thorough forearm (Figure 2.). 1. 2. 3. 4. 5. Figure 1. Chairs selected for tests 2
a. b. c. Figure 2. Postures analyzed (a.: typing on a keyboard; b.: laying back; c.: laying right) 3. RESULTS AND DISCUSSION Postures and movements of body parts, chair part movements and body pressure distribution data were gathered. In this study the latter, i.e. pressure distribution maps are analyzed. Peak pressure values for different chairs and postures both for the right and left side of the subjects buttock and back are summarized in Table 2. Table 2. Summary table of maximum pressures B, Chair Posture Side A Seat/Back, C D N/cm 2 N/cm 2 N/cm 2 N/cm 2 1. a. left 1,189/- 1,831/- 1,063/- 1,350/- right 0,973/- 1,379/- 1,247/- 1,668/- b. left 0,962/0,298 1,212/0,976 0,673/0,796 1,673/0,608 right 0,947/0,738 0,842/0,951 0,724/0,617 1,849/0,496 c. left 0,884/0,188 0,802/- 0,561/0,132 0,764/0,108 right 1,078/1,574 1,651/0,397 1,067/2,204 0,673/0,841 2. a. left 0,980/- 1,245/- 0,940/- 1,512/- right 0,909/- 0,519/- 0,846/- 1,675/- b. left 0,664/0,887 0,699/0,760 0,818/0,686 0,936/0,590 right 0,713/0,991 0,648/0,709 0,706/0,525 0,920/0,529 c. left 0,976/0,774 0,719/- 0,702/0,498 1,056/- right 0,851/0,150 1,020/- 0,722/0,531 1,135/- 3. a. left 1,214/0,215 1,544/- 0,829/- 1,473/- right 1,354/0,303 1,432/- 0,842/- 2,277/- b. left 0,704/0,321 0,789/1,211 0,673/0,383 0,851/0,383 right 1,261/0,354 0,965/1,355 0,590/1,346 0,965/1,346 c. left 1,488/0,215 0,715/- 0,541/0,605 0,764/0,605 right 1,702/0,617 0,920/0,608 0,751/0,682 1,978/0,682 4. a. left 0,842/- 1,227/- 0,760/- 2,272/- right 0,878/- 1,463/- 1,058/- 2,262/- b. left 1,535/0,509 1,083/1,007 0,851/0,372 1,401/0,440 right 1,475/0,433 1,145/0,937 1,007/0,334 1,210/0,440 c. left 0,931/0,226 1,058/0,191 0,691/0,164 1,506/1,346 right 1,069/2,063 1,662/1,637 1,294/0,576 1,036/0,352 5. a. left 1,212/0,584 1,501/- 1,107/- 1,660/- right 1,35/0,702 1,731/- 1,203/- 1,726/- b. left 2,054/0,209 1,345/- 0,548/0,807 1,697/- right 2,016/0,211 1,334/- 0,515/0,727 1,666/- c. left 1,025/0,453 0,532/- 0,699/0,543 1,470/0,904 right 1,129/0,572 2,272/- 1,573/1,453 1,152/0,713 3
The values show differences in pressure distribution from chair to chair and posture to posture. Furthermore the body built of subjects reveals the uneven allocation of the pressure, for example not the most hevier person has the maximum pressure but the thinnest one, demonstrating that contact area play a significant role in the seat comfort. The maximum pressure value we find for person D using chair 3 for writing on keyboard however, the pressure distribution is uneven with a difference of 0,8 N/cm 2. It can be concluded as a general rule that considerable difference exists between the right and left size of the pressure values no matter the subject is typewriting on a computer keyboard or relaxing by laying back.subject B with the lowest weight had the highest pressure map values irrespectively og chair type. Figure 3. shows the pressure distritubion maps of the same person in two different postures Figure 3 (D-3-b and D-3-a) and the maps for the same chair with two users in reclined position. When the subject bends in front using a keyboard the pelvis will rotate and high pressure values appear under ischium. Figure 3. Pressure distribution map for different postures and subjects Similar pressure maps are shown on Figure 4. on which the same person using two different chairs exhibit distinct pressure distributions (A-5-b and A-4-b). When the subjects adopt a right laying position most part of the loads fall on the seat and the back load is close to zero. Figure 4. Pressure distribution map for different chairs and postures 4
4. CONCLUSIONS The aim of this study was to analyze the pressure distribution of four subject with various body built on five working chairs and in three postures. The results of the body pressure distribution showed that the reclining back posture is associated with the more dispersed pressure reducing this way the fatique of the body. The pressure distribution at the working chairs was measured using a flexible pressure sensing mat, the contour maps and peak values compared in function of subjects stature or working positions. The comparison of five work chairs with subject adopting three postures revealed significant differences in terms of peak pressure values. As was expected, the heavier subject exhibited relatively larger effective contact area and lower peak pressure values. The results of the study emphasize that many aspects of workchair design, such as variability of tasks and work organizational factors, should be considered in order for physical inactivity at computer workplaces to be avoided and musculoskeletal disorders thus prevented. Acknowledgements The authors acknowledge the financial support of TAMOP-4.2.1/B-09/1/KONV-2010-0006 V. subproject Research Grant - The technical and innovation background development of the regional economy growth and special thanks for the support of Linea Furniture Ltd. in the mainframe of GOP -1.1.1-08/1-2008-0041 project. The TAMOP-4.2.1/B and GOP projects are supported by European Regional Development Fund and the Hungarian Government. References ADAMS, M., DOLAN, P., MARX, C., HUTTON, W. (1986). An electronic inclinometer technique for measuring lumbar curvature. Clinical Biomechanics 1, 130e134. BABSKI-REEVES, K., STANFIELD, J., HUGHES, L., (2005). Assessment of video display workstation set up on risk factors associated with the development of low back and neck discomfort. International Journal of Industrial Ergonomics 35, 593e604. COMMISSARIS, D.C.A.M., REIJNEVELD, K., (2005). Posture and movements during seated office work; results of a field study. In: Veiersted, B., Fostervold, K.I., Gould, K.S. (Eds.), Ergonomics as a Tool in Future Development and Value Creation; Proceedings of the 37th Annual Conference of the Nordic Ergonomics Society NES and NEF, Oslo (Norway), 10e12 October, pp. 58e61. DOWELL, W.R., YUAN, F., GREEN, B.H., (2001). Office seating behaviors: an investigation of posture, task and job type. Proceedings of the Human Factors and Ergonomics Society, 1245e1248. ELLEGAST, R., HAMBURGER, R., KELLER, K., KRAUSE, F., GROENESTEIJN, L., VINK, P., BERGER, H., (2007). Effects of Using Dynamic Office Chairs on Posture and EMG in Standardized Office Tasks. Springer, Berlin. GRANDJEAN, E.; HUNTING, W.; WOTZKA, G.; AND SCHARER, R, (1973). An ergonomic investigation of multipurpose chairs. Human Facton, 15:3,247-255. JOHANSEN, U., JOHREN, A., (2002). Personalekonomi Idag. Uppsala Publishing House. ISBN 9170052026 PARENT-THIRION, A., FERNÁNDEZ, M.E., HURLEY, J., VERMEYLEN, G., (2007). Fourth European Working Conditions Survey. European Foundation for the Improvement of Living and Working Conditions, Dublin. TREASTER, D.; AND MAMAS, W.S., (1987). Measurement of seat, pressure distributions. Human Factors 295, pp. 563-575. 5