International Journal of Automotive Technology, Vol. 15, No. 4, pp. 667−672 (2014)

DOI 10.1007/s12239−014−0069−8

Copyright © 2014 KSAE/ 078−16

pISSN 1229−9138/ eISSN 1976−3832

667

EMPIRICAL ANALYSIS ON THE EFFECT OF DESIGN VARIABLES

OF AUTOMOTIVE SEAT LUMBAR SUPPORT ON THE INITIAL

SITTING DISCOMFORT

J. T. YANG1)*, H. Y. CHOI2), J. H. LEE3) and S. U. LEE4)

1)Hankook ESI, 357 Yangcheon-ro, Gangseo-gu, Seoul 157-801, Korea2)Department of Mechanical and System Design, Hongik Univercity, Seoul 121-791, Korea

3)Department of Business Administration, Konkuk University, Seoul 143-701, Korea4)Department of Rehabilitation Medicine, Seoul National University Boramae Medical Center, Seoul 156-707, Korea

(Received 26 December 2012; Revised 1 June 2013; Accepted 10 September 2013)

ABSTRACT−An empirical analysis was performed in order to examine whether the height of the lumbar support installed on

the driver’s seat in a vehicle makes a difference to the initial discomfort when sitting, and among the design variables of the

seat lumbar support, which one is the main variable that affects the initial discomfort most when sitting. The subjects who

participated in this study were 50th percentile male in their 30s and 40s from Korea and America, had driving experience, and

had not experienced any back pain in the last 12 months. In order to figure out the level of discomfort felt by a driver

depending on the movement or the shapes of the lumbar support, subjective sensibility evaluation was conducted and sitting

body pressure was measured, and the change of lumbar spine angle was observed using X-ray image. Based on the results,

design variables that affect the initial discomfort when sitting on a car seat and the correlation among these design variables

were verified through statistical significance testing. The empirical analysis suggested that, among design variables of car seat

lumbar support, the degree of prominence is the main variable that affects the initial discomfort when sitting for both Koreans

and Americans, while the height and support width have very little to do with it.

KEY WORDS : Body pressure distribution, Comfort, Discomfort, Lumbar spine curvature, Lumbar support, Prominence

1. INTRODUCTION

Among the parts that compose a car, a seat is as important

as a powertrain in terms of its significance and cost. In

developing a vehicle, seats have a direct impact on the

comfort of the passengers (Jung and Lee, 2000). As people

spend more time in a car, they are paying more attention to

the seats as much as the crash safety and appearance.

Therefore, customers have more expectation regarding the

quality and comfort of seats, which are closely related to

passengers’ fatigue level as well as their safety in the car.

According to a report by UMTRI (University of

Michigan Transportation Research Institute), the primary

element related to a driver’s discomfort when sitting is the

position of the lumbar support and the second element is

the distribution of pressure on the cushion (Schneider and

Ricci, 1989). The market research agency J. D. Power and

Associates published a report in 2006 that suggests that the

second important issue in car seat quality evaluation is the

lumbar support, of which the problems have been

reoccurred for a number of years. The most frequently

noted problem relating to a car seat is that “the waist part of

the back support is uncomfortable” which is attributed to

low quality and uncomfortable lumbar support.

In evaluation the seat comfort of car seats, the lumbar

support, which is installed on the back support of the

driver’s seat plays a very important role (Frohriep and

Petzel, 2007; Zenk et al., 2006; Nanae Michine et al.,

2005). Also, optimizing lumbar support has been a big

subject in the car industry during the last decade.

The car seats currently produced by Korean carmakers

or seat manufacturers are made based on North American

body sizes (Jung et al., 1999). In this study, Koreans and

Americans who have different lifestyles and body sizes

were compared. Then, based on a mid-size Korean car,

subjective sensibility evaluation, body pressure, and

lumbar angle changes were observed regarding the initial

sitting discomfort according to the design variables

(prominence, height, support width).

The study was conducted after a preview by IRB

(Institutional Review Board) of Seoul National University

Boramae Medical Center, in order to ensure the subjects’

rights and safety.*Corresponding author. e-mail: yjt@esi.co.kr

668 J. T. YANG, H. Y. CHOI, J. H. LEE and S. U. LEE

2. EXPERIMENTAL SETUP AND METHODS

2.1. Selection of Subjects

The subjects were 50th percentile Korean and American

men in their 30s and 40s who have driving experience, had

no back pain in the last 12 months, and satisfied the height

and weight condition. The body measure data of Koreans

was based on the 50th percentile body size research

performed between 2003 and 2004 by SizeKorea, and that

of Americans was measured based on the SizeUSA

between 2002 and 2003 (Kim et al., 2010).

As shown in Figure 1, the lumbar spines of all applicants

was x-rayed to determine their participation after

examining presence of back pains such as back distortion

or disc problems.

Finally, 12 Koreans and 6 Americans who satisfy all of

the conditions above were selected for the research, and

their body data is presented in Table 1.

2.2. Mockup Design and Build

The car seat mockup made for this study modeled after the

driver’s seat of a mid-sized sedan available in the Korean

market. Such as the photographs (b) and (c) in Figure 2, the

steel frames were all made with wood, considering the

image interference during X-ray imaging, and the joints

were also made with wooden board in order to minimize

the interference. The seat cushion, foam, and cover were all

the products used in the actual models and they were all

made of fabric, and the hog-ring and the steel wire that

connect the foam and the cover were removed due to X-ray

interference.

DSC company’s product was used for the lumbar

support installed on the back of the seat to support the back

part like (a) in Figure 2. As the lumbar supports of mid-

sized cars in Korea can only adjust the prominence, it was

modified so that it can be moved up and down as well as

back and forth by installing an additional motor. The newly

modified lumbar support can move between 0 mm and

45 mm back and forth, and +30 mm and -30 mm vertically.

The mockup parts’ location (wheel point/heel point/rail

mounting location) and angles(cushion/back) were adjusted

equally to the Korean and American 50th percentile seat

Package data. Most importantly, the position of the hip point

was measured by H-point machine, which was applied to the

mockup.

The seat mockup (frame, foam, cover, lumbar support)

had very little interference to the image from the X-Ray test.

2.3. Sensibility Evaluation

To measure the initial discomfort felt by the subjects when

sitting in the mockup, a sensibility evaluation was

performed. The prominence and height of the lumbar

support installed in the mockup can be adjusted by 5 levels

using its motor. Additionally, the support width can be

changed by two levels. Therefore, the subjects are asked to

rate the initial sitting discomfort among the combination of

25 positions based on the prominence and the height levels

of the lumbar support and the two support widths, giving

higher scores to the more comfortable locations. In order to

maintain the initial sitting posture during the sensibility

evaluation, some landmarkers were attached to the major

points on the subjects’ bodies. The evaluation was recorded

with a 3D motion camera. Figure 3 shows the 3D motion

capture environment to maintain the sitting position of the

subjects. The sensibility evaluation was conducted in the

next two ways: First, the subjects found the most and the

least comfortable locations on their own. Second, based on

the reference position, they rated relative comfort and

discomfort. Before the test, the subjects had become

Figure 1. Pretest for back pains.

Table 1. Mean data of the test subjects.

Age(SD) years Height(SD) cm Weight(SD) kg

Korean 39.3(2.2) 169.7(1.1) 71.3(1.5)

American 40.5(5.0) 177.9(1.3) 83.5(2.7)

Figure 2. Test mockup.

Figure 3. 3D motion capture environment to maintain the

sitting position of subjects.

EMPIRICAL ANALYSIS ON THE EFFECT OF DESIGN VARIABLES OF AUTOMOTIVE SEAT LUMBAR 669

familiar with the level of discomfort according to different

lumbar support positions, and the test was repeated three

times for reproducibility.

The sensibility evaluation was rated on a scale of 1 to 10.

2.4. Body Pressure Measurement

Body pressure is, along with subjective sensibility

evaluation, the most frequently used method for objective

measurement. Many papers have studied on the

distribution of sitting body pressure to research the seat

discomfort so far (Demontis and Giacoletto, 2002;

Oudenhuijzen et al., 2003). As in Figure 4, the pressure

sensor system (X-sensor, 43 × 43 cells) was installed on the

back of the seat, and the pressure on the back of the seat

was measured according to the different prominence,

height and width of lumbar support when subjects sat in the

mockup. Then, the pressure distribution measured on the

seat back when subjects are sitting was studied as to how it

affects the comfort and sensibility evaluation, and their

correlation was analyzed.

2.5. X-radiograph Imaging

X-ray image can be used to analyze the lumbar spine

curvature according to its prominence (Choi et al., 2010).

In this study, to observe the lumbar spine shape of the

subjects according to the design variables of the lumbar

support, the change in lumbar in 25 positions (five changes

in prominence, and five in height) was imaged by X-ray.

During the imaging, the subjects maintained the initial

sitting position and the tester changed the position of the

lumbar support. The X-ray was used to accurately detect

the support location of the lumbar support and the changing

shape of the lumbar spine according to changing the

prominence. Figure 5 shows the lumbar spine images

according to the 25 positions of the lumbar support.

3. RESULTS

3.1. Subjective Discomfort Rating

In sensibility evaluation, both Koreans and Americans

expressed discomfort with the change in the prominence of

the lumbar support, but they did not feel much discomfort

with the different heights. In Figure 6, more than half of the

Koreans and Americans felt the least discomfort when the

lumbar support was low and little prominent, and when

sitting in the driver’s seat of a mid-sized car. Here, the

prominence ranged between 15 and 25 mm, which is close

to the effective lumbar prominence providing comfort,

23~27 mm (Hughes et al., 1998). Also, except for some

subjects, both Koreans and Americans said it was most

uncomfortable when the lumbar support was most

prominent (45 mm) and highest (30 mm).

In the sensibility evaluation, Koreans suggested that they

felt the least discomfort in the prominence and height that

is one level (15 mm) below than what Americans felt most

comfortable with.

In the sensibility evaluation, there was no significant

difference between when the subjects found uncomfortable

positions on their own and when they rated relative

discomfort.

Figure 4. Subject on test mockup and measured sitting

pressure distribution.

Figure 5. 25 X-ray images for a subject (left to right in row:

prominence increase, top to bottom: Height increase).

Figure 6. Discomfort map from the subject.

670 J. T. YANG, H. Y. CHOI, J. H. LEE and S. U. LEE

3.2. Body Pressure Distribution (BPD)

Many studies have already assessed the sitting discomfort

based on the body pressure distribution on the seat cushion

and the back. Furthermore, a body map was made by

dividing the cushion section and back section, the

discomfort was studied based on the body map that shows

the body pressure on the cushion and back of the seat

(Mergl et al., 2005). In this paper, the actual lumbar length

(including T12-S1) was exactly measured from the X-ray

image and this range was compared to the vertical cell size

of the pressure sensor. The distance between T12 and S1

was similar to that of 16 cells in the pressure sensor as

shown in Figure 7, and these 16 cells were divided into

four zones in order to analyze the body pressure

distribution.

As shown in Figure 8, comfort corridor of Koreans and

Americans were completed by calculating the 4 regions of

body pressure distribution (BPD) by using the calculation

methods described previously. Least discomfort in the

lumbar area, which was induced by pressure between L2

and L4, was reported when the pressure was 2.1~3.3 kPa

for Koreans, and 2.8~4.6 kPa for Americans. The figure

shows a body pressure distribution when the Koreans and

Americans felt the least discomfort.

3.3. Cobb’s Angle Change

A line was drawn on the each even side of the top and the

bottom end of the lumbar spine and the lines were

connected perpendicularly. The angle of the cross line was

defined as the cobb’s angle. In order to reduce measurement

error, 5 measured the vertebra angle of the subjects for 3

times; the angle was measured through x-ray images. The

figure on the left side in Figure 9 shows the lumbar angles

of Koreans and Americans that were measured in the

position that was evaluated to be the least uncomfortable.

The figure on the right shows the lumbar angles of the

subjects when they moved to the most uncomfortable

position.

The angle change was measured larger for Americans

than for Koreans, and the lumbar angle changed most in the

L4/L5, which is the lower area, when the prominence of

the lumbar support increased.

However, it was very difficult to determine the least and

most uncomfortable posture by the lumbar angle.

The results in section 3.1 showed that there was a 15 mm

difference in height of the lumbar support that was rated as

the least uncomfortable by both Koreans and Americans.

As shown in Figure 10, X-ray image readings proved that

lumbar support is placed between L3 and L4 of both

Koreans and Americans. This result is considered to be

caused by different body sizes of Koreans and Americans.

(weight, seated height, back waist).

3.4. Statistical Analysis

In this study, the difference in initial sitting discomfort was

Figure 7. Body pressure zone.

Figure 8. BPD Corridor from a subject.

Figure 9. Cobb’s angle of the lumbar.

Figure 10. Lumbar support position of Koreans and

Americans.

EMPIRICAL ANALYSIS ON THE EFFECT OF DESIGN VARIABLES OF AUTOMOTIVE SEAT LUMBAR 671

analyzed according to the design variables of the lumbar

support on the car seat. And, in order to analyze how the

difference varies between Koreans and Americans, the

correlation of the design variables, i.e., the prominence, the

height, the support width, and the sitting discomfort of the

lumbar support was verified based on Pearson Correlation

Coefficients. As a result, although the prominence and the

height had statistically significant negative correlation with

initial sitting discomfort for Koreans (p < 0.001), the

correlation between the support width and the initial sitting

discomfort were not statistically significant. Furthermore,

for Americans, only prominence had a very high negative

correlation with the initial sitting discomfort (p < 0.001).

Therefore, the correlation coefficient between prominence

and initial sitting discomfort was -0.423 for Koreans, and -

0.255 for Americans (p < 0.001), which suggests that both

Koreans and Americans perceive less discomfort during

initial sitting with less prominence of the lumbar support.

Also, the effect of the design variables of the lumbar

support on initial sitting discomfort was analyzed using

Stepwise Multiple Regression Analysis. For both Koreans

and Americans, among the independent variables including

prominence, height, and support width, prominence

affected initial sitting discomfort, which is a dependent

variable. All explanatory power of regression equation

(adjusted-R2) was 0.393 for Koreans (F = 65.607, p < 0.001),

and 0.129 for Americans (F = 15.781, p < 0.001). In each

case, only the design variables that were significantly

effective on the initial sitting discomfort, the dependent

variable based on stepwise multiple regression analysis,

were included in the regression equation. Although the

coefficient of determination is somewhat low, this

regression equation can be interpreted as significant. As the

previous results of analysis confirmed negative correlation

between initial sitting discomfort and the amount of

prominence in car seat lumbar support, this result shows

that among the design values of car seat lumbar support,

the amount of prominence is the major design value that

affects the initial sitting discomfort. There is no direct

effect between the initial sitting discomfort and the height

of lumbar support, but there is a low correlation between

the height and the amount of prominence. On the other

hand, the influence of lumbar support's support width were

found to be statistically insignificant on the initial sitting

discomfort. A regression equation was derived only with

the independent variables that were significantly influential

on discomfort, which was the dependent variable.

Regression equation (1) is Koreans, and (2) is Americans.

Ln(Yi) = 1.775+0.021Prominence_i-0.001Prominence_i^2

-0.0001Prominence_i*Height_i (1)

Ln(Yi) = 1.107+0.028Prominence_i-0.001Prominence_i^2

-0.0001Prominence_i*Height_i -0.003Cobb’s Angle_i (2)

(Yi : predicted discomfort value, i : subject(i=1,2,…,n))

4. CONCLUSION

When designing car seats, the amount of prominence,

height, and support width of lumbar support were selected

as the major design variables that were expected to affect

initial sitting discomfort.

In sensibility evaluation, the degree of discomfort

according to changes in prominence amount was easily

evaluated in both Koreans and Americans, but the subjects

showed relatively dull responses to the changes in height and

width of lumbar support. The results of statistical analysis

showed that the amount of prominence significantly affected

initial sitting discomfort, but the factors of height and width

in lumbar support were not significant, and no correlation

was found between these factors.

In order to evaluate initial sitting discomfort in

accordance with lumbar support changes in vehicles,

zoning of lumbar body pressure was completed. Compared

to Americans, the lumbar pressure was measured to be

lower in Koreans when they were in the least

uncomfortable position on the lumbar support, and they

preferred less amount of prominence and lower position of

lumbar support.

In sensitivity evaluation, the position of lumbar support

that was rated to be the least uncomfortable was different in

Koreans and Americans, but consequently the X-ray

readings showed that both Koreans and Americans had the

lumbar support placed between L3 and L4.

Future work will analyze the correlation between lumbar

disc pressure and discomfort by using human models and

FE analysis, and researches evaluating long-term sitting

and dynamic comfort will be conducted.

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