Forward sloping chair effects on spinal shape in the Hong Kong Chinese and Indian populations

International Journal of Industrial Ergonomics 23 (1999) 9—21

Forward sloping chair effects on spinal shape in the Hong Kong Chinese and Indian populations Ravindra S. Goonetilleke*, Banna G. Rao Department of Industrial Engineering and Engineering Management, Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong Received 1 September 1996; received in revised form 14 April 1997; accepted 15 July 1997

Abstract Forward sloping seats are universally accepted based on their increased trunk-thigh angle during sitting. However, these seats are not preferred by some individuals due to reasons such as excessive pressure on knees, difficulties during ingress and egress, and postural fixity during sitting. Some researchers have claimed that forward sloped sitting preserves the lumbar lordosis, thereby making it more comfortable for the sitter. This claim has not been validated across all populations and, therefore, appears to have some disagreement among researchers. In this study, spinal shape during standing and sitting in forward sloping chairs is measured and quantified using a three-dimensional sonic digitizer. Twenty subjects (ten Hong Kong Chinese and ten Indian) have participated in the experiment. Fifteen points on the spine are digitized during standing and sitting in a forward-sloping seat with trunk—thigh angles of 70°, 80°, 90°, 100°, 110°, and 120°. Different measures are used to analyze and differentiate the spinal shape. The correlation between the length of spine during standing and a subject’s height is low, but significant. The behavior of the spinal shape change during sitting differs between the populations as shown by the maximum lumbar and maximum thoracic deviations. The Indian subjects seem to approach the standing curvatures in the thoracic region during 30° forward sloping sitting. The Hong Kong Chinese subjects, on the other hand, do not show any resemblance to the standing curvatures during forward sloping sitting. One possible reason could be the differences in arch angle between the two populations. The variations in spinal shape among subjects appear to be similar within a population. Relevance to industry Forward sloping seats may not be appropriate for all populations since changes in the spinal shape differ between populations. The arch angle may be an indicator and possibly a predictor of the appropriateness of forward sloping seats for different populations, if spinal shape is related to sitter discomfort. ( 1999 Elsevier Science B.V. All rights reserved. Keywords: Sitting; Spinal curvature; Lumbar curvature; Forward sloping seats; Spine

* Corresponding author. 0169-8141/99/$19.00 Copyright ( 1999 Elsevier Science B.V. All rights reserved PII S 0 1 6 9 - 8 1 4 1 ( 9 7 ) 0 0 0 9 6 - 6

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1. Introduction Sitting is generally preferred to standing since sitting requires less muscular activity and less energy consumption (Grandjean et al., 1982) compared to standing. However, many studies indicate a higher risk of low back pain with work performed in sitting postures (Andersson, 1987; Kroemer and Robinette, 1969; Grieco, 1986). It is also believed that prolonged sitting (Bendix, 1994; Kelsey and Hardy, 1975) in a slouched or kyphotic posture is closely linked with the incidence of low back pain (Keegan, 1953). Many researchers have investigated the effects of a seat on body posture (Bendix and Beiring-Sorenson, 1983; Branton, 1969; Bridger, 1988; Drury and Francher, 1985; Frey and Tecklin, 1986; Mandal, 1987). Related studies indicate that flat or rearward sloping seats promote lumbar kyphosis, while forward sloping seats preserve the lumbar lordosis. Hence, forward tilted seats have been proposed by many researchers (Burandt, 1969; Carlso¨o¨ 1963; Mandal, 1975, 1981). Mandal (1987) contends that comfort during sitting is achieved using forward tilted chairs or sitting with the thighs sloping at about 30° although Bendix (1986) concluded that the lordotic effect of a forward sloping seat is only slight. Similarly, Chaffin and Andersson (1991) (p. 337), stated that a “slight degree of lordosis can be present” in a forward sloping seat surface. Most variations in seat designs stem from Keegan’s (1953) claim that the lumbar spine goes into a neutral (reduced lumbar flattening), relaxed posture when the trunk-thigh angle is 135°. The variation in shape of the lumbar spine has been conventionally measured using the base angle (Mandal, 1981), i.e. the angle between a base line (or horizontal) and a line running sagitally through the fifth lumbar disc. Alternatively, Akerblom (1948), Schoberth (1962) and Andersson et al. (1979) used the angle between a line running sagitally through the first lumbar disc and the fifth lumbar disc to indicate the degree of lumbar flattening. Any uneven or “excessive” wear on the reference discs can significantly affect the calculated “lumbar angle”. In addition, lumbar flattening alone may be insufficient to predict upper back discomfort or pain. In this study, distances from a fixed (floating among subjects) vertical reference are used instead. The

primary reason for using distances is to easily regenerate the complete spine for comparisons between standing and the different sitting angles and also to minimize any errors attributed to wear on the lumbar vertebrae. To further understand spinal deformations and their effect on pain and discomfort, mathematical models have been proposed too. Two such models are the cantilever model and the more recent arch model proposed by Aspden (1988). The Aspden model produces bounds within which the spine lies and gives an indicator of the optimum solution in terms of minimizing the stresses generated by the spine. Accurate and reliable spinal shape information may help predict stresses better. However, quantifying these spinal deformations has been difficult due to variations between subjects and also due to the inherent structural variations. Meanwhile, differences across populations can contribute to different mathematical models. This study attempts to identify the total spinal changes between the Indian and Hong Kong Chinese populations while obtaining the shape variations during sitting.

2. Study objectives This research tests the validity of Mandal’s (1987) claim in two populations, Hong Kong Chinese and Indian. According to that claim, a forward slope of at least 10° is necessary to achieve a lumbar lordosis when sitting in an upright position without any lumbar support (Mandal, 1987). In addition, this study also quantifies the differences in the shape of the spine between these same populations.

3. Methodology 3.1. Subjects Ten Hong Kong Chinese and ten Indian male subjects were used in this experiment. All subjects were university students or employees. None of the subjects had any back pain or spinal abnormalities.

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Fig. 1. Three views of the experimental chair.

3.2. Equipment 3.2.1. Height and angle adjustment chair A chair with a flat sitting surface, no cushioning, incorporating a seat height and tilt adjustment mechanisms, without any backrest was fabricated (Fig. 1). The chair had a five-point base support. The height adjustment was performed with a conventional chair lever, while the seat pan tilt was adjusted through a variable stop mechanism. 3.2.2. The sonic digitizer Spinal shapes and lumbar lordosis can be measured and quantified using various techniques, e.g. radiographs (Farfan, 1973), photographs (Flint, 1963; Burdett et al., 1986), the Iowa Anatomical Positioning System (Day et al., 1984), posture

meters (Harris, 1955), inclinometers (Bridger et al., 1989; Gerhardt, 1994) specialized goniometers (Burdett et al., 1986) and flexible rules (Israel, 1959; Walker et al., 1987; Hart and Rose, 1986; Lovell et al., 1989). However, the safety, validity and reliability among the methods appear to differ significantly depending on the population studied (for example with or without low back pain), or the intra-tester and inter-tester variations, tester experience, and the posture of the subject (Lovell et al., 1989). Hence, a standard technique used in mechanical part design and CAD/CAM applications, i.e. three-dimensional digitization, was used to quantify the lumbar lordosis and obtain spinal shape data with better reliability and validity. The Science Accessories Corporation, GP-12 sonic-digitizer system was used for this purpose (Fig. 2).

Fig. 2. Sonic digitizer (a) triangular array (b) digitizing probe.

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Fig. 3. Mechanical goniometer.

Sonic digitizers take advantage of the fact that the time to traverse a distance is determined by the speed of sound in the surrounding environment. The GP-12 system utilizes an automatic and continuous calibration procedure to obtain the speed of sound. The digitizer consists of two-emitters fixed to a digitizing probe and a fixed frame with three sonic receivers located in a triangular pattern. When the time taken for the sound wave to travel from the emitter to a receiver is determined, and knowing the speed of sound, the distance is easily calculated. The three-dimensional co-ordinates are obtained using triangulation principles. When a sound wave travels from a single emitter to three or more known receivers of known orientation and the speed of sound and time duration between the emitter and receiver are known, using triangulation principles allows us to calculate the location of that emitter in a three-dimensional space. Some disadvantages of the digitizer are as follows: f Every emitter/receiver combination being used requires a direct line of sight. f The system has a digitizing volume of 2.4 m3. f A data validation tolerance of 0.05 cm was used. Although smaller values are possible, setting the tolerance too small may result in a long time spent to obtain the co-ordinates of a point. On the other hand, a setting too large may result in poor accuracy. 3.2.3. Mechanical goniometer A mechanical goniometer (Fig. 3) was used to measure the trunk—thigh and seat pan angles.

Fig. 4. Markings on spine in preparation for digitization.

3.3. Procedure 3.3.1. Subject preparation All subjects were asked to sign an informal consent and participation in the experiment was voluntary. No subject payment was made. Each subject was asked to remove their shirt, followed by the marking of fifteen points on the spine prior to digitization. The points marked were the bony prominences (vertebra) on the surface obtained by palpation. The fifteen points consisted of five markings in each of the three regions, cervical, thoracic and lumbar, starting from the C1 vertebra and ending with the L5-vertebra (Fig. 4). 3.3.2. Digitizer preparation The default coordinate system of the digitizing system is always relative to the triangular detector array. That is, an x-axis that goes from left to right, a y-axis that goes from down to up, and a z-axis that comes out from the array while the origin is positioned such that the detector array is centered in the X and ½ directions with respect to the 2.4 cm3 volume. However, the system allows the user

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Table 1 Descriptive statistics of Hong Kong Chinese subjects

Fig. 5. User-coordinate system defined on chair surface.

Statistic

Mean

Standard deviation

Range

Age (yr) Height (in) Weight (lb)

22.6 68.78 142.39

1.35 1.78 13.34

21—25 66.34—71 .26 125—170

Table 2 Descriptive statistics of Indian subjects

to define any User Coordinate System (UCS) by inputting points 1—3, as shown in Fig. 5. The XZ plane and the origin for the sitting posture was defined on the seat pan (in horizontal plane) of the chair while that for the standing posture was defined on the floor plane.

Statistic

Mean

Standard deviation

Range

Age (yr) Height (in) Weight (lb)

27 67.9 152.01

1.33 3.02 16.35

24—28 62.5—72 114—176

3.3.3. Digitization procedure Digitization during standing required the subject to stand in an erect posture. Each of the fifteen points was digitized using a hand held probe (Fig. 2b). The procedure during sitting was as follows:

from the digitizer for each subject was three-dimensional. However, to characterize the shape of the spine, all data analyses were limited to the twodimensions in the sagittal plane. In addition, all data were transformed such that the origin was at the C1 vertebra (the first digitized point) and the vertical axis (y axis) was perpendicular to the horizontal support surface. Fig. 6 displays the variations in spinal shape for the different sitting angles for each subject. This figure reveals that, at angles of 70° and 80°, the spine appears to be more kyphotic. When the trunk angle is 100°, 110°, and 120°, the cervical and thoracic regions seem to “approach” the standing shape in most subjects. However, this effect is not very clear in the lumbar region. To characterize the shape of the spine, the following notation is used:

1. The chair height was adjusted to each subject’s popliteal height. 2. The seat was tilted to obtain the predetermined trunk to thigh angle, as measured using a mechanical goniometer. The seat pan angles tested were 0° (horizontal), 10°, 20°, and 30° forward, as well as 10° and 20° backward (The respective trunk-thigh angles were 90°, 100°, 110°, 120°, 80°, and 70°). All results will be referenced with respect to these trunk—thigh angles. 3. The subjects sat erect at each pan angle, with thighs along the seat pan, and feet on the floor or a footrest (for the backward tilt angles) prior to digitizing the fifteen points. The above procedure was repeated for all twenty subjects.

4. Results and data analysis Tables 1 and 2 present the descriptive statistics of the two groups of subjects. The data obtained

¸ "length of spine (from C1 to L5) along h vertical (y) axis (length projection of spine on the y-axis) when subject has a trunk—thigh angle of h.

Hence, ¸ , ¸ , ¸ ,¸ ,¸ ,¸ ,¸ repres90 100 110 120 80 70 180 ent the “spinal lengths” at trunk to thigh angles of 90°, 100°, 110°, 120°, 80°, 70° and standing posture, respectively.

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Fig. 6. Spinal shape for sitting and standing postures for the two populations. All dimensions in mm. The deviation is measured from the vertical plane.

R.S. Goonetilleke, B.G. Rao / International Journal of Industrial Ergonomics 23 (1999) 9–21 Table 3 Correlation of ¸

180

vs. subject height

Case

Pearson correlation coefficient R2 and probability

For all 20 subjects Indian Hong Kong Chinese

0.55 (p(0.05) 0.56 (p(0.09) 0.52 (p"0.12)

0.30 0.31 0.27

4.1. Correlation analysis Correlation analysis was performed to evaluate the relationship between a subject’s height and the spinal length at standing, ¸ (projection on the 180 vertical plane). The Pearson correlation coefficient, although statistically significant (p(0.05) for all 20 subjects, is comparatively low (Table 3) but similar across the two populations. 4.2. Inter-correlation analysis The inter-correlation between spinal lengths measured at different postures (¸ ) is statistically h significant (p(0.05) with Pearson correlation coefficients ranging from 0.58 to 0.96 (R2 ranges from 0.34 to 0.92) for all 20 subjects (Table 4a). Higher correlations are found between the postures in the forward tilted seat pan positions (e.g., between ¸ and ¸ ). The lowest value for R2 is between 120 110 the 20° backward tilted (¸ ) seat pan posture and 70 the standing posture (¸ ), thereby suggesting that 180 the flexibility of the spine differs among subjects. The inter-correlation pattern slightly differs between the Hong Kong and Indian populations. For the Indian population, a higher R2 ('0.83) is observed between ¸ and ¸ or ¸ (Table 4c), 180 120 110 whereas for the Hong Kong population, the R2 appears to be similar (in the region of 0.75) between ¸ and all lengths at trunk—thigh angles 180 between 80° and 120° (Table 4b). 4.3. Arch angle analysis To examine the differences in the posture between the two populations in the standing configuration, an angular measure was used. This angle

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was defined as the angle between the line joining the first digitized point (C1 vertebra) and the last digitized point (L5 vertebra), and the vertical (Fig. 7). Restated, it is a measure of the total arch angle of the whole spine from the vertical plane. An analysis of variance on the arch angle revealed a significant (F(1, 18)"6.04, p(0.05) difference between the Hong Kong population and Indian population. The Hong Kong population had a higher angle (mean"6.5°) than the Indian population (mean"4.2°). This angle may be considered as a goodness index of standing posture or as an indication of pelvic “balance”. Further study is needed to comment on its value. 4.4. Lumbar and thoracic deviation comparisons To quantify the differences in the spinal shape in the various sitting configurations, two measures were defined: maximum thoracic deviation (d ) and 5 the maximum lumbar deviation (d ) from the vertical or y-axis (Fig. 8). The location of the turning point (i.e., point of maximum absolute deviation) was chosen such that the point corresponds to the maximum deviation in the thoracic and lumbar regions when standing. The same point was picked for the other postures, and the corresponding deviation from the vertical was used in the analysis to follow. Notably, the exact location of the maximum deviation was not of much concern in this study due to the difficulties involved in identification based on skin markings. For each deviation, two, 2-way analyses of variance were performed using the SAS package. The first included all angles, including standing while the second excluded the standing posture. The purpose of excluding the standing posture was to eliminate any bias as a result of an “outlier”. Tables 5 and 6 summarize the ANOVA results for lumbar deviation. Corresponding results are also shown in Fig. 9. No significant (p(0.05) interaction exists in either case (Tables 5 and 6). However, significant main effects are present at the p(0.05 level. The Hong Kong subjects show a significantly greater (absolute) lumbar deviation (i.e. a reduced level of lumbar lordosis as seen in Fig. 9a). A post-hoc Students Newman Keuls (SNK) shows that the

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standing posture is significantly different from the other sitting positions. In addition, no significant differences are seen between any of the trunk—thigh angles from 80° to 120°. This result can be found in Fig. 10.

When the standing deviation was excluded from the analysis, the pattern of significant differences in lumbar deviation for the various trunk—thigh angles is different, as shown in Fig. 11.

Table 4 Inter-correlation matrix (R2) between ¸ , ¸ , ¸ , ¸ , ¸ , ¸ , ¸ . Probability values are in parentheses 90 100 110 120 80 70 180 Variable

¸ 70

(a) All 20 subjects ¸ 1 70 (0)

¸ 80

¸ 90

¸ 100

¸ 110

¸ 120

¸ 180

0.72 (0.0001)

0.49 (0.001)

0.45 (0.001)

0.61 (0.0001)

0.61 (0.0001)

0.34 (0.01)

¸ 80

0.72 (0.0001)

1 (0)

0.72 (0.0001)

0.58 (0.0001)

0.83 (0.0001)

0.76 (0.0001)

0.69 (0.0001)

¸ 90

0.49 (0.001)

0.72 (0.0001)

1 (0)

0.45 (0.001)

0.72 (0.0001)

0.69 (0.0001)

0.71 (0.0001)

¸ 100

0.45 (0.001)

0.58 (0.0001)

0.45 (0.001)

1 (0)

0.77 (0.0001)

0.77 (0.0001)

0.62 (0.0001)

¸ 110

0.61 (0.0001)

0.83 (0.0001)

0.72 (0.0001)

0.77 (0.0001)

1 (0)

0.92 (0.0001)

0.76 (0.0001)

¸ 120

0.61 (0.0001)

0.76 (0.0001)

0.69 (0.0001)

0.77 (0.0001)

0.92 (0.0001)

1 (0)

0.74 (0.0001)

¸ 180

0.34 (0.01 )

0.69 (0.0001)

0.71 (0.0001)

0.62 (0.0001)

0.76 (0.0001)

0.74 (0.0001)

1 (0)

0.71 (0.002)

0.71 (0.002)

0.49 (0.02)

0.56 (0.01)

0.62 (0.01)

0.28 (0.1)

(b) Hong Kong Chinese subjects ¸ 1 70 (0) ¸ 80

0.71 (0.002)

1 (0)

0.92 (0.0001)

0.76 (0.001)

0.90 (0.0001)

0.83 (0.0003)

0.74 (0.001)

¸ 90

0.71 (0.002)

0.92 (0.0001)

1 (0)

0.77 (0.001)

0.83 (0.0002)

0.87 (0.0001)

0.76 (0.001)

¸ 100

0.49 (0.02)

0.76 (0.001)

0.77 (0.001)

1 (0)

0.94 (0.0001)

0.96 (0.0001)

0.74 (0.001)

¸ 110

0.56 (0.01)

0.90 (0.0001)

0.83 (0.0002)

0.94 (0.0001)

1 (0)

0.92 (0.0001)

0.76 (0.001)

¸ 120

0.62 (0.01)

0.83 (0.0003)

0.87 (0.001)

0.96 (0.0001)

0.92 (0.0001)

1 (0)

0.72 (0.002)

¸ 180

0.28 (0.1)

0.74 (0.001)

0.76 (0.001)

0.74 (0.001)

0.76 (0.001)

0.72 (0.002)

1 (0)

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Table 4 (Continued) Variable

¸ 70

(c) Indian subjects ¸ 1 70 (0)

¸ 80

¸ 90

¸ 100

¸ 110

¸ 120

¸ 180

0.83 (0.0003)

0.25 (0.15)

0.59 (0.01)

0.74 (0.001)

0.69 (0.003)

0.64 (0.005)

¸ 80

0.83 (0.0003)

1 (0)

0.46 (0.03)

0.50 (0.02)

0.74 (0.002)

0.67 (0.004)

0.76 (0.001)

¸ 90

0.25 (0.15)

0.46 (0.03)

1 (0)

0.27 (0.12)

0.58 (0.01)

0.48 (0.03)

0.71 (0.002)

¸ 100

0.59 (0.01)

0.50 (0.02)

0.27 (0.12)

1 (0)

0.72 (0.002)

0.67 (0.003)

0.61 (0.01)

¸ 110

0.74 (0.001)

0.74 (0.002)

0.58 (0.01)

0.72 (0.002)

1 (0)

0.90 (0.0001)

0.90 (0.0001)

¸ 120

0.69 (0.003)

0.67 (0.004)

0.48 (0.03)

0.67 (0.003)

0.90 (0.0001)

1 (0)

0.83 (0.0003)

¸ 180

0.64 (0.005)

0.76 (0.001)

0.71 (0.002)

0.61 (0.01)

0.90 (0.001)

0.83 (0.0003)

1 (0)

Fig. 7. Definition of arch angle. Fig. 8. Maximum (absolute) thoracic and maximum (absolute) lumbar deviations.

A simple effects analysis (one-way) was also performed on the data (Winer et al., 1991). The result (SNK) for the Indian subjects resembled that in Fig. 10 with no significant difference between 80°, 90°, 100°, 110° and 120° trunk—thigh angles. However, a subtle difference exists for the Hong Kong

subjects: no difference arose between any of the sitting postures including the 70° trunk—thigh angle. The results for the thoracic deviation slightly differ. The two-way analysis of variance including

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Table 5 ANOVA for maximum lumbar deviation including standing posture

Table 6 ANOVA for maximum lumbar deviation excluding standing posture

Source

Mean square (degrees of freedom)

F value (probability)

Source

Mean square (degrees of freedom)

F value (probability)

Angle Race Angle * Race

31 962 (6) 28 393 (1) 1567 (6)

20.09 (0.0001)* 17.84 (0.0001)* 0.99 (0.44)

Angle Race Angle * Race

6229 (5) 32 202 (1) 1112 (5)

3.45 (0.0062)* 17.83 (0.0001)* 0.62 (0.69)

*Significant at the p(0.05 level.

*Significant at the p(0.05 level.

the standing posture showed no statistically significant (p(0.05) interaction (Table 7). Here again, the Hong Kong subjects showed a significantly greater thoracic deviation (Fig. 9b). Notably, a higher absolute value denotes a higher level of kyphosis. However, the thoracic deviations associated with the different postures and the two populations significantly differed at the p(0.05 level. A post-hoc SNK analysis showed that the deviation in the standing posture significantly differed when compared to all sitting postures. When the standing posture was excluded, the only significant difference was between the two populations

(Table 8). The simple effects analysis for each population yielded different results. The trunk—thigh angle was significant in both cases. However, for the Hong Kong population, all seated postures did not significantly differ in the thoracic deviations; however, all of these significantly differed from the standing deviation. In comparison, for the Indian population the only significant difference in thoracic deviation was between standing and a trunk—thigh angle of 70°, and also between standing and a trunk—thigh angle of 80°. There was no difference between the angles of 180°, 120°, 110°, 100°, and 90° (Fig. 12).

Fig. 9. Mean thoracic and lumbar deviations (mm) for the Indian and Hong Kong populations.

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5. Discussion Fig. 10. SNK results for maximum lumbar deviation.

Fig. 11. SNK results for maximum lumbar deviation excluding standing.

Table 7 ANOVA for maximum thoracic deviation including standing posture Source

Mean square (degrees of freedom)

F value (probability)

Angle Race Angle * Race

3433 (6) 15 225 (1) 450 (6)

5.51 (0.0001)* 24.45 (0.0001)* 0.72 (0.63)

*Significant at the p(0.05 level.

Table 8 ANOVA for maximum thoracic deviation excluding standing posture Source

Angle Race Angle * Race

Mean square (degrees of freedom)

F value (probability)

712 (5) 15 414 (1) 403 (5)

1.08 (0.37) 23.47 (0.0001)* 0.61 (0.69)

*Significant at the p(0.05 level.

Fig. 12. SNK results for maximum thoracic deviation in the Indian population.

The arch angle, inter-correlation analysis among the spinal lengths for the different postures, and the thoracic and lumbar deviations seem to indicate two different spinal deformation patterns for the two populations. Above results seem to indicate a difference in the flexibility or deformation characteristics between the populations. The experimental results indicate that the lumbar spine is most kyphotic at a 70° trunk—thigh angle. At 70°, the pelvis tilts backwards. Hence, the lumbar spine becomes more kyphotic and at the same time the kyphosis of the thoracic spine is also increased although not significantly different from the other sitting postures. The thoracic deviations (hence curvature) in the sitting postures approach the standing curvatures, as the trunk angle increased from 90° to 120°, only for the Indian subjects. However, in the case of the lumbar region, the kyphosis reduces as the trunk—thigh angle increases but far from lordotic for both groups of subjects. The only difference between the two populations is at the 70° trunk—thigh angle. This difference can be attributed to the difference in the arch angles. With a higher arch angle (for example the Hong Kong subjects), the deformation of the spine at extremely low trunk—thigh angles is low, thereby allowing greater flexibility to the sitter with no more discomfort than any other sitting position.

6. Conclusions This study has not only measured spinal shape variations at different trunk angles non-invasively using a sonic digitizer, but also studied the variations in spinal length and shape. The digitization procedure used could quantify the shape of the skin overlying the spine, and should not be interpreted as an exact representation of the roentgenographic spine (Hart, 1989). The measurements may not be identical. In clinical studies, two operational definitions for lumbar lordosis appear: the position of the vertebral bodies obtained from traditional roentgengrams and the shape of the skin overlying the spine which can be quantified through digitization or flexible rules. Hence, any calculations performed

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to obtain lumbar curves as defined by Mandal (1981) or Akerblom (1948) may not be very accurate with surface measurements or digitizations. However, surface measurements have been shown to correlate well with angular measures taken from radiographs (Adams et al., 1986). One limitation of the study is the digitizing probe which may have caused slight skin movement during digitization. Although some researchers have claimed that a 105° trunk—thigh angle preserves the standing lumbar curvature, such a variation was not seen in this study with a forward sloping seat surface. The results obtained herein correlate with Bendix’s conclusions (1986) for the Hong Kong Chinese population. The thoracic deviations differ for the two groups of subjects, implying a different deformation pattern of the spine for the two populations. This finding suggests that Hong Kong subjects can experience back pain with forward sloping seats. Hence, caution must be exercised when seats and sitting surfaces are recommended based on research unique to a specific population. So one may ask the question as to whether “all” subjects from any population feel more comfortable when sitting on forward sloping seats as reported by Mandal. The answer may be, possibly, based on related pelvic tilt. With a forward sloping seat at the “correct” height, the pressure on the ischial tuberosities can be reduced, thereby limiting the pelvic tilt. This may result in a lumbar curve similar to that during standing presumably “restoring” the standing curvature. However, the same may not be said about the thoracic region of the spine for all cultures.

Acknowledgements This paper was made possible through a grant from the Research Grants Council (HKUST700/ 95E) of Hong Kong.

References Adams, M.A., Dolan, P., Marx, C., Hutton, W.C., 1986. An electronic inclinometer technique for measuring lumbar curvature. Clinical Biomechanics 1, 130—134.

Akerblom, B., 1948. Standing and sitting posture, with special reference to the construction of chairs. Doctoral dissertation, Nordiska Bokhandeln, Stockholm. Andersson, B.J.G., 1987. Biomechanical aspects of sitting: an application to VDT terminals. Behavior and Information Technology 6, 257—269. Andersson, G.B.J., Murphy, R.W., Ortengren, R., Nachemson, A.L., 1979. The influence of backrest inclination and lumbar support on lumbar lordosis. Spine 4, 52—58. Aspden, R.M., 1988. A new mathematical model of the spine and its relationship to spinal loading in the workplace. Applied Ergonomics 19, 319—323. Bendix, T., 1986. Seated trunk postures at various seat inclinations, seat heights, and table heights. Human Factors 26, 695—703. Bendix, T., 1994. Low back pain and sitting. In: Lueder, R., Noro, K. (Eds.), Hard Facts About Soft Machines. Taylor & Francis, London, pp. 147—155. Bendix, T., Beiring-Sorenson, F., 1983. Posture of the trunk when sitting on forward-inclining seats. Scandinavian Journal of Rehabilitation Medicine 15, 197—203. Branton, P., 1969. Behaviour and body mechanics and discomfort. Ergonomics 12, 316—327. Bridger, R.S., 1988. Postural adaptations to a sloping chair and work surface. Human Factors 30, 237—247. Bridger, R.S., Wilkinson, D., van Houweninge, T., 1989. Hip joint mobility and spinal angles in standing and in different sitting postures. Human Factors 31, 229—241. Burandt, U., 1969. Ro¨ntgenuntersuchung u¨ber die Stellung von Becken und Wirbelsa¨ule beim Sitzen auf vorgeneigten Fla¨chen. In: Grandjean, E. (Ed.), Sitting Posture. Taylor and Francis, London. Burdett, R.G., Brown, K.E., Fall, M.P., 1986. Reliability and validity of four instruments for measuring lumbar spine and pelvic positions. Physical Therapy 66, 677—684. Carlso¨o¨, S., 1963. Writing desk, chair and posture of work. Department of Anatomy, Stockholm, Sweden. Chaffin, D.B., Andersson, G.B.J., 1991. Occupational Biomechanics. Wiley, New York. Day, J.W., Smidt, G.L., Lehmann, T., 1984. Effect of pelvic tilt on standing posture. Physical Therapy 64, 510—516. Drury, C.G., Francher, M., 1985. Evaluation of a forward sloping chair. Applied Ergonomics 16, 41—47. Farfan, H.F., 1973. Mechanical Disorders of the Low Back. Lea and Febiger, Philadelphia. Flint, M.M., 1963. Lumbar posture: a study of roentgenographic measurement and the influence of flexibility and strength. Research Quarterly 34, 15—20. Frey, J.F., Tecklin, J.S., 1986. Comparison of lumbar curves when sitting on the Westnofa Balans Multi-chair, on a conventional chair, and standing. Physical Therapy 66, 1365—1369. Gerhardt, J.J., 1994. Documentation of Joint Motion. Isomed, Portland, OR. Grandjean, E., Hunting, W., Nishiyama, K., 1982. Preferred VDT work station settings, body posture and physical impairments. Journal of Human Ergology 11, 45—53. Grieco, A., 1986. Sitting Posture: An old problem and a new one. Ergonomics 29, 345—362.

R.S. Goonetilleke, B.G. Rao / International Journal of Industrial Ergonomics 23 (1999) 9–21 Hart, D.L., 1989. Commentary. Physical Therapy 69(2), 102—103. Hart, D.L., Rose, S.J., 1986. Reliability of a non-invasive method for measuring the lumbar curve. Journal of Orthopaedic and Sports Physical Therapy 8, 180—184. Harris, R., 1955. A simple posture meter. Annals of the Rheumatic Diseases 14, 90—91. Israel, M., 1959. A quantitative method of estimating flexion and extension of the spine: a preliminary report. Military Medicine 124, 181—186. Keegan, J.J., 1953. Alterations of the lumber curve related to posture and seating. Journal of Bone and Joint Surgery 35-A, 588—603. Kelsey, J.L., Hardy, R.J., 1975. Driving motor-vehicles as a risk factor for acute herniated lumbar vertebral discs. International Journal of Epidemiology 102, 63—73. Kroemer, K.H.E., Robinette, J.C., 1969. Ergonomics in the design of office furniture. Industrial Medicine Surgery 38, 115.

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Lovell, F.W., Rothstein, J.M., Personius, W.J., 1989. Reliability of clinical measurements of lumbar lordosis taken from a flexible rule. Physical Therapy 69 (2), 96—102. Mandal, A.C., 1975. Work-chair with tilting seat. Lancet, 642—643. Mandal, A.C., 1981. The seated man (Homo sedans): the seated work position, Theory and Practice. Applied Ergonomics 12, 19—26. Mandal, A.C., 1987. The influence of furniture height on back pain. Behavior and Information Technology 6, 347—352. Schoberth, H., 1962. Sitzhaltung, Sitzschaden, Sitzmobel. Springer, Berlin. Walker, M.L., Rothstein, J.M., Finucane, S.D., 1987. Relationship between lumbar lordosis, pelvic tilt, and abdominal muscle performance. Physical Therapy 67, 512—516. Winer, B.J., Brown, D.R., Michels, K.M., 1991. Statistical Principles in Experimental Design. McGraw-Hill, New York.