INFLUENCE OF BACKREST
INCLINATION ON THE READING
PERFORMANCE UNDER RANDOM
WHOLE BODY VIBRATION
Vikas Kumara, V.H. Saranb
Mechanical and Industrial Engineering Department, Indian Institute of Technology, Roorkee, India
a. [email protected] b. [email protected]
Tel.: +91 1332 285682; fax: +91 1332 285665. ABSTRACT:
Reading newspaper while traveling in rail cars can occasionally become a demanding task. This sedentary activity is not only influenced by the font size and type but also to a large extent by factors like vibration magnitude, direction, and backrest support. The present experimental study explores the influence of backrest inclination of the seated subjects while reading a newspaper in vibration environment. Thirty healthy male subjects in age group of 20-30 years participated in the study in which they were subjected to random vibrations of 1-20 Hz frequency range at three levels (0.5, 1.0 and 1.5 m/s2) of vibration magnitudes in the three independent axes (x, y and z). Perceived difficulty and percentage reduction in reading performance have been found to increase with the increase in vibration magnitude of whole-body vibration. The measured responses have been observed to be increased with the increase in backrest inclination during the exposure in vertical direction. The obtained results indicate that maximum difficulty was percieved for vibration in the fore-&-aft direction with vertical backrest support. The minimum perceived difficulty and percentage reduction in reading performance was found under the latral vibration exposure in the postures: with vertical backrest support and with backrest support inclined 30º with vertical.
Keywords: Whole body vibration, posture, reading performance
1. Introduction
The train passengers often prefer to perform some sedentary activities like reading, writing, typing etc. while travelling. Various factors such as vibrations, seat design, posture, backrest, temperature etc. affect the performance of sedentary activities. Adverse effects of whole body vibration (WBV) on the performance of various sedentary activities have been observed in the previous studies (Corbridge and Griffin: 1986, Corbridge and Griffin: 1991, Sundström and Khan: 2008, Bhiwapurkar et al., 2010). The field studies (Bhiwapurkar et al., 2010b and Khan and Sundström: 2004) on the Swedish and Indian trains have found that a large number of the passengers prefer reading (newspapers, magazines or books) over other activities. Passenger’s responses to questionnaire in these field studies have shown that on board vibration affects their reading performance. Vibration magnitude and vibration frequency have a strong effect on reading and writing activities (Griffin and Hayward: 1994, Bhiwapurkar et al., 2011, Khan and Sundström: 2006 & 2008).The performance of reading and writing has been observed to be typically decreased in the frequency between 1 and 8 Hz and with an increase in vibration magnitude & duration of vibration (Corbridge and Griffin, 1991; Griffin, 2003; Westberg, 2000). Recent laboratory studies (Bhiwapurkar et al., 2010, Bhiwapurkar et al., 2011, Bhiwapurkar et al., 2012) on reading a word chain in English , a Hindi newspaper and an English e-paper in vibration environment by measuring the subjective as well as objective responses, have shown a strong influence of the WBV on the reading activity. Variables like vibration magnitude, frequency, direction of vibration and subject posture affect the reading performance of the subjects which were evident from the subjective as well as objective measures.
attempt has been made to study the effect of vibration magnitude, vibration direction and backrest inclination on the reading activity in a controlled laboratory environment.
2. METHODOLOGY
2.1. Experimental Setup
The experiment was conducted on the vibration simulator installed in the Vehicle Dynamics Laboratory, Mechanical and Industrial Engineering Department, IIT Roorkee, India. This vibration simulator was developed as a mock-up of a railway coach and used in many previous studies (e.g., Bhiwapurkar et al., 2010-2012; Kumar & Saran, 2014). The vibration simulator consists of a platform made of stainless steel sheets (size 2m x 2m,) which can be excited with the help of three Electro-Dynamic Vibration shakers, in three directions: vertical (Z-axis), lateral (Y-axis) and fore-&-aft (X-axis). The Gaussian random vibrations can be generated by the closed- loop computer controlled vibration exciters, each having a force capacity of 1000 N and maximum stroke length (peak-peak) of 25 mm. A table and two chairs have been rigidly fixed on the platform of the vibration simulator (Figure 1). The cushion-less chairs have rigid and flat backrests with adjustable inclination. All parts of the platform has been found to have no resonance in the frequency range 1 to 20 Hz (considered in this study) for all the three axes of vibration.
The vibration data was acquired through a tri-axial accelerometer (PCB PIEZOTRONICS-356A32) along with a data acquisition device (NI cDAQ-9174) withthe Labview software .
The simulator provides a controlled and comfortable environment with a working illumination well above 250 Lux. The illumination from all the direct and indirect light sources is well-distributed over all the seats and table. The test subjects were seated on the rigidly mounted chairs on the platform of the Vibration Simulator.
2.2. Vibration Parameters
The vibration simulator was excited with a continuous Gaussian random vibration in each axis of vibration (x, y and z), with the help of computerized control system. The three levels of vibration magnitude: 0.5, 1 and 1.5 m/s2 rms (unweighted) were excited in each axis independently. The frequency of random vibrations ranged from 1 to 20 Hz, which is considered a critical range for the human beings. Power Spectral Density (PSD) of vibrations generated by one of the vibration exciters over the required frequency range at 1 m/s2 rms vibration magnitude is shown in Fig. 2.
3. Volunteer Subjects
A total of 30 healthy male subjects voluntarily participated in the experimental study, whose Anthropometric data are shown in Table-1. The test subjects were all students (graduate/postgraduate or research scholar) of the Institute having reading fluency in English and with normal eyesight (normal visual acuity 6/6 vision). The subjects had the freedom to withdraw themselves from experimentation without any reason and at any point of time. An approval from the Institute Human Ethical Committee was obtained for carrying out the present study.
Table 1: Anthropometric data of subjects participated in the experiment
Age (Years)
Height (cm)
Seated Height (cm)
Weight (Kg)
Seated weight (Kg)
Arm length (cm)
Lower leg length (cm)
Mean 26.4 171.7 133.9 70.8 53.3 58.0 50.8
Standard deviation
Figure 1: Schematic diagram of computerized controlled Vibration Simulator
3.1.Subject Postures
The experimental task was performed by the subjects in four postures (Fig. 3). The subject was seated on a chair in an erect posture holding the reading material in the hands in following postures (P):
(i) Without backrest (P1)
(ii) With vertical backrest support (P2)
(iii) With backrest support inclined 30º with vertical (P3)
(iv) With backrest support inclined 45º with vertical (P4)
The normal viewing distance 40 ±2 cm between the subject forehead and reading material [Legge et al., 1985a and 1985b] was maintained throughout the experiments. In order to maintain consistent conditions throughout the experimental process, the inner angle between the upper arm and lower arm of each subject was maintained 95 ±2 degrees.
Figure 3: Four seated postures assumed by the subjects while performing the experiment task
3.2. Experimental Task
The experimental task consisted of reading aloud, paragraphs of 300 words length, from leading Indian English national newspapers, selected randomly in order to avoid learning effect. Nimrod MT font of size 8 was selected for the reading task as it was one of the most commonly used font type by Indian English Newspapers with varying font sizes (7 to 10) depending upon the news content. The reading performance was objectively evaluated by the experimenter based on the time duration of the reading task. The subjective evaluation was performed by assessment of the perceived difficulty in reading task on the Borg’s CR-10 scale (Borg: 2007), which consists of nine labeled and eight unlabeled points (Table-2). The value ranges from minimum’0’ to maximum ‘10’ for this scale, but greater values can be chosen depending upon their perceived difficulty.
Table 2: Borg’s CR-10 scale (Borg, 2007)
0 Nothing at all 0.3
0.5 Extremely weak 0.7
1 Very weak
1.5
2 Weak 2.5
3 Moderate 4
5 Strong 6
7 Very strong 8
9
10 Extremely strong
3.3.Experimental Procedure
Written voluntary informed consent of the subjects was obtained before experimentation. The test subjects also filled up a health screening questionnaire related to their personal background, level of education, fitness and musculoskeletal disorders (Kuorinka et al., 1987). All the subjects were free from any musculoskeletal disorders and were suitable for the experimental task.
For the convenience of each participant, the whole experiment was split into three sessions (one direction each). Every participant was exposed to total 36 test conditions: three directions of vibration × three levels of vibration magnitude × four backrest postures, with one minute break between the consecutive conditions. All the conditions were randomized in order to minimize the order effects. The experimenter started the digital stop watch at the commencement of the Reading task and stopped it on hearing the word "stop" from the subject at the end of task.
Response Data Analysis
A factorial analysis of variance (ANOVA) was performed with the help of software Statistical Package for Social Sciences (SPSS Inc., Chicago, USA, version 16) to evaluate the collective responses of participants., The results at levels of p<0.05 and p<0.01 were considered as ‘significant’ and ‘highly significant’ respectively. The repeated measure module was selected to evaluate the responses, since the experimental procedures involved repetitive collection of responses from the test subjects for each test condition. In ANOVA, within-subjects module was applied for all the independent variables: Vibration direction, magnitude, and seated postures.
The analysis of variance was also interpreted in terms of two other statistical parameter ‘partial eta square’ and ‘observed power’ which provide the capacity to avoid masking effect of the powerful variables and raise the assurance to right detection of a variable effect, respectively.
4. RESULTS
The reading performance in vibration condition was evaluated by determining the reduction in performance with respect to the reading performance without vibration exposure.
4.1. Subjective Evaluation of Perceived Difficulty in Reading
Perceived difficulty in reading under WBV exposure is affected by many parameters e.g. vibration direction, vibration magnitude, backrest inclination etc. and is one of the key factors in reduction of reading performance. For the subjects seated in different postures under the X-, Y- and Z- axes vibrations for all magnitudes, the mean values of perceived difficulty in reading and percentage reduction in reading performance are illustrated in Figures (4a, 5a and 6a) and Figures (4b, 5b and 6b) respectively. In general, both the perceived difficulty and percentage reduction in reading performance increased with an increase in random vibration stimulus, in all the axes and for all postures.
Highly significant differences were observed in perceived difficulty for all the considered vibration magnitudes in all the postures for each axis of WBVs (p < 0.01). The differences between levels of perceived difficulty were also observed to be highly significant for the considered postures in each axis of WBV (p<0.01).
(a) (b)
Figure 4: Effect of vibration magnitude and postures on the (a) perceived difficulty in reading (b) percentage reduction in performance under the WBV exposure in Z-axis.
(a) (b)
Figure 5: Effect of vibration magnitude and postures on the (a) perceived difficulty in reading (b) percentage reduction in performance under the WBV exposure in Y-axis.
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0
0.5 1.0 1.5
P1 P2 P3 P4
Vibration Magnitude (m/s2)
Perceived difficulty
in
reading
0 5 10 15 20 25 30
0.5 1 1.5
P1 P2
P3 P4
Vibration Magnitude (m/s2)
%
Reduction in perform
ance
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0
0.5 1.0 1.5
P1 P2
P3 P4
Perceived difficulty
in
reading
Vibration Magnitude (m/s2)
0 5 10 15 20 25 30
0.5 1 1.5
P1 P2
P3 P4
Vibration Magnitude (m/s2)
%
Reduction in perform
(a) (b)
Figure 6: Effect of vibration magnitude and postures on the (a) perceived difficulty in reading (b) percentage reduction in performance under the WBV exposure in X-axis.
Effects of vibration directions and postures on the perceived difficulty under the WBV exposure at 1 m/s2 vibration magnitude are shown in Figure 7(a). Higher level of perceived difficulty has been observed in the P2 and P3 postures as compared to the other postures (p<0.01) during the exposure to X-axis vibrations. While the level of perceived difficulty is lower in the postures P2 and P3 under the exposure to lateral vibrations.
(a) (b)
Figure 7: Effect of vibration direction and postures on the (a) perceived difficulty in reading (b) percentage reduction in performance under the WBV exposure at 1 m/s2 rms vibration magnitude
4.2. PERCENTAGE REDUCTION IN READING PERFORMANCE
The mean values of percentage reduction in performance of test subjects for the vertical, lateral and fore-&-aft vibrations have been displayed in Figures 4 (b), 5(b) and 6(b). These figures also describe the effects of three levels of vibration magnitude and four postures.
Effect of vibration magnitude on the percentage reduction in reading performance in three independent axis of WBV is illustrated in Figures 4 (b), 5(b) and 6(b). It has been observed that the percentage reduction in
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0
0.5 1.0 1.5
P1 P2
P3 P4
Perceived difficulty
in
reading
Vibration Magnitude (m/s2)
0 5 10 15 20 25 30
0.5 1 1.5
P1 P2
P3 P4
Vibration Magnitude (m/s2)
%
Reduction in perform
ance
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0
P1 P2 P3 P4
Z Y X
Perceived difficulty
in
reading
Posture
0 5 10 15 20 25 30
P1 P2 P3 P4
Z Y X
%
Reduction in perform
ance
performa considere Highly S of WBV percentag performa vibration posture h
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SCUSSION
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ing activity in n in reading p l analysis of t ge reduction i vibration. This hiwapurkar et measured res ss the retina w vibration mag
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al. 2011,]. Th ponses of alm which is due t gnitude, there
to WBV. T ures adopted sure. It has be d fore-&-aft vi
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f the phase co ttern has been ration. In the en found by th
teract with eac ve interaction o
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al variables in rep
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ing also redu subjects have hat measured icipants affect creases, there reading perf petitive interac
n vertical WB more perceive without the su the test subje ot move in pha s more difficu or the percent
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nd percentage e responses: WBV expos est. More diff
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stures and found on percentage ted by the iculty and BV in each Hancock, is evident tina move At the high ect’s eyes during the responses nclination reduction perceived sure. This ficulty has which may ure. There the lateral er body of ection and nce in the duction in ead of test
head moves almost in the vertical direction but in P1 posture, the head moves in mid-sagittal plane, that means subjects face more problem to perform the task in the P1 posture compared to P2 posture in vertical direction [Bhiwapurkar et al. 2010 , Griffin, 2003, Mansfield, 2005]. Maximum perceived difficulty to read and percentage reduction in reading performance has been found in the fore-&-aft direction in the P2 posture assumed by the subjects.
This study has been done to quantify the effect of various parameters e.g. vibration magnitude, direction and postures with backrest inclination. The result presented may be useful for the train manufacturers to consider the parameters while designing the train compartment and seat components to provide activity comfort to the passengers so that they can utilize their time effectively and efficiently.
6. CONCLUSION
The results of current experimental study may be useful while designing the seat system and backrest for the railway passengers. Various parameters : vibration magnitude, direction and postures have been investigated in view of the reading activity performance. The perceived difficulty to read and percentage reduction in reading performance have been found to be affected by vibration magnitude in three independent directions of vibration. Both the responses have been found to increase with the increase in vibration magnitude of each axis of whole-body vibration. Backrest inclination has also been found to affect the reading activity, but varies according to the direction of vibration. The lower perceived difficulty to read and percentage reduction in reading performance has been observed in P2 Posture for the test subjects under the exposure of vertical and lateral vibration independently. Maximum levels of perceived difficulty and large percentage reduction in performance have been observed in the fore-&-aft direction of vibration in the P2 posture. The minimum perceived difficulty and percentage reduction in reading performance have been found under the latral vibration exposure in the P2 and P3 postures.
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