An Ergonomic Evaluation Comparing Desktop, Notebook, and ...

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An Ergonomic Evaluation Comparing Desktop, Notebook, and Subnotebook Computers Grace P. Szeto, MSc, Raymond Lee, PhD ABSTRACT. Szeto GP, Lee R. An ergonomic evaluation comparing desktop, notebook, and subnotebook computers. Arch Phys Med Rehabil 2002;83:527-32. Objective: To evaluate and compare the postures and movements of the cervical and upper thoracic spine, the typing performance, and workstation ergonomic factors when using a desktop, notebook, and subnotebook computers. Design: Repeated-measures design. Setting: A motion analysis laboratory with an electromagnetic tracking device. Participants: A convenience sample of 21 university students between ages 20 and 24 years with no history of neck or shoulder discomfort. Intervention: Each subject performed a standardized typing task by using each of the 3 computers. Main Outcome Measures: Measurements during the typing task were taken at set intervals. Results: Cervical and thoracic spines adopted a more flexed posture in using the smaller-sized computers. There were significantly greater neck movements in using desktop computers when compared with the notebook and subnotebook computers. The viewing distances adopted by the subjects decreased as the computer size decreased. Typing performance and subjective rating of difficulty in using the keyboards were also significantly different among the 3 types of computers. Conclusions: Computer users need to consider the posture of the spine and potential risk of developing musculoskeletal discomfort in choosing computers. Key Words: Computers; Ergonomics; Man-machine systems; Neck; Posture; Rehabilitation. © 2002 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation OMPUTERS PLAY an increasingly important role in our daily life and, because of their portability and conveC nience, notebook and subnotebook computers are becoming widely used. Prolonged and intensive use of the visual display terminal (VDT) is an important risk factor for work-related musculoskeletal disorders.1-4 Previous studies5-7 have identified a strong association between neck and shoulder discomfort and static neck posture in viewing the computer display. According to 1 study,8 workers tend to increase their forward head posture by approximately 5% to 10% when working with VDT, compared with their relaxed sitting posture. Such forward head

From the Department of Rehabilitation Sciences, Hong Kong Polytechnic University, Hong Kong. Accepted in revised form March 28, 2001. No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit upon the authors(s) or upon any organization with which the author(s) is/are associated. Reprint requests to Grace Szeto, MSc, Dept of Rehabilitation Sciences, Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, e-mail: [email protected]. 0003-9993/02/8304-6689$35.00/0 doi:10.1053/apmr.2002.30627

posture is associated with increased activities of the cervical extensor muscles and increased compressive loading of the spine.9,10 In previous studies11-16 evaluating the effects of different screen heights on the mechanical loading of the neck and shoulder regions, neck flexion angles and cervical muscle activities were found to increase with lower screen heights. Only a few studies17-20 have examined the posture assumed in using the portable notebook computers. The cathode-ray tube of a desktop computer and the flat-panel display of a notebook computer are obviously of different heights relative to the user. Because the overall dimensions of the screens, font display, and keyboard locations also differ, it is likely that a person using a notebook computer will adopt a different spinal posture than that used to operate a desktop computer. Straker et al17 reported significantly greater neck flexion and head tilt angles during notebook use compared with desktop computer use. In addition, the average discomfort score after using the notebook computer for 20 minutes was greater than that found after using the desktop computer. Villanueva et al18 compared the postures of subjects among 5 different sizes of computers including the standard desktop computer and 4 notebook computers of various sizes. They reported that as computer size decreased viewing angles and neck angles both increased progressively and the trunk inclined more forwardly. However, the postural parameters were captured only for a 5-minute duration with each computer, and the posture of the spine was only examined in 1 anatomic plane. The previously mentioned literature review clearly shows that there is inadequate ergonomic information available on the use of notebook and subnotebook computers. No information exists on the 3-dimensional posture of the spine. Typing performance and ergonomic factors, such as chair height and computer screen angles, have also not been fully evaluated. The purpose of the present study was to evaluate and compare 3 variables: (1) postures and movements of the cervical and upper thoracic spine, (2) typing performance, and (3) workstation ergonomic factors when using different types of computers (desktop, notebook, subnotebook). Our intent was to provide findings that would lead to a better understanding of the factors contributing to work-related musculoskeletal disorders resulting from computer use. METHODS Participants Selected from a sample of convenience, 21 university students (11 women, 10 men; mean age, 21.2y; mean height, 165.9cm; mean weight, 60.8kg) participated in the present study. Candidates were excluded if they had a history of shoulder or cervical, or thoracic spine pain requiring medical treatment within the previous 3 months; had a history of surgery of the shoulder, neck, or back, or had any spinal deformities. At the start of the testing session, each participant was given standard information explaining the study’s purpose and procedures and was asked to sign an informed consent form. This study was approved by the human ethics committee of the local university. Arch Phys Med Rehabil Vol 83, April 2002

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ERGONOMIC EVALUATION OF NOTEBOOK COMPUTERS, Szeto Table 1: Physical Characteristics of the 3 Computers Computer Model

Computer Classification

Keyboard Dimensions (width ⫻ height)

No. of Keys

Screen Size (width ⫻ height)

Palmax PD-1000 ILufa 486 DX2/66 AST Desktop

SNB NB DT

205 ⫻ 80mm 273 ⫻ 112mm 440 ⫻ 125mm

77 87 106

125 ⫻ 95mm 192 ⫻ 145mm 280 ⫻ 217mm

Abbreviations: SNB, subnotebook; NB, notebook; DT, desktop.

Research Design The present study used a repeated-measures design with each participant performing the same standardized typing task on each of the 3 computers. The testing order was determined by drawing lots at the start of the testing procedures. The independent variable was the type of computer. The dependent variables were (1) postures of the cervical and thoracic segments, (2) excursions of the cervical and thoracic segments, (3) VDT workstation assessment, and (4) typing performance. Mean posture was defined as the average position of the spinal segment in each trial. Excursion was defined as the difference between the minimum and maximum range of spinal movements the participant experienced in the trial. Instrumentation We tested 3 types of computers in the present study: a desktop computer (AST Bravo MS-T 5166a), a notebook computer (ILufa 486 DX2/66b), and a subnotebook computer (Palmax PD-1000 Plusc). Their physical characteristics are described in table 1. These models were selected for the study because their physical dimensions were the most typical of these 3 types of computers. The Polhemus 3Space Fastrakd movement analysis system was used to measure the postures and movements of the cervical and upper thoracic segments. This system consisted of the systems electronic unit, 3 sensors, and a source. The source generated low-frequency electromagnetic signals that were picked up by each sensor. The system’s electronic unit performed calculations to determine the position and orientation of each sensor relative to the source. The sampling frequency was 40Hz per sensor. We determined anatomic angles of the cervical and the thoracic spine from the sensor orientation by using the method of Grood and Suntay.21 Typing Task Each participant performed a standardized screen-based, copy-typing task for 15 minutes in each of the 3 experimental conditions: desktop, notebook, and subnotebook. Each participant was given 5 minutes to type with the particular computer to be used in the experimental trial before it began to familiarize him/herself with the computer. Each typing trial lasted 15 minutes. The software TypingMaster, version 4.5,e was used to display an essay on the computer screen, and the participant was instructed to copy the essay by typing at his/her normal speed. Within each experimental condition, three 15-second samples of spinal movements were acquired at 2, 6, and 9 minutes after the participant started typing. The participant was instructed to continue typing for the 15-minute duration and was not aware of the data collection process. Kinematic Analysis of the Cervical and Thoracic Segments Because the Fastrak system uses electromagnetic technology, signals would be distorted by the presence of metals; Arch Phys Med Rehabil Vol 83, April 2002

therefore, participants removed all metallic jewelry before the testing session, and no metal objects were allowed within the vicinity of the experimental area. Sensors were attached to the participant’s forehead (midpoint) and the C7 and T8 spinous processes. The zero reference anatomic position of the spine was recorded with the participant standing in relaxed position and looking directly ahead. Spinal movements were calculated with reference to this “zero” position. Movements of the cervical segment were determined from the movements of the forehead sensor with respect to the C7 sensor and movements of the upper thoracic segment from the movements of the C7 with respect to the T8 sensor. VDT Workstation Setup and Assessment All participants were provided with the same standard VDT workstation furniture, which included an adjustable pneumatic chair and a standard office desk (fig 1). They were instructed to adjust their chair heights to achieve a comfortable sitting posture. The chair had a height adjustment range of 35 to 49cm. The computers were placed on the desk at a floor-to-tabletop distance of 72cm. The participants were allowed to adjust the position and angle of the screen and keyboard according to their own preference and comfort. The VDT settings selected by various subjects were recorded. The angles of the screens were recorded by using a goniometer. The viewing distance was measured from the top of the viewable part of the screen to the midpoint between the eyes with the participant in a relaxed sitting position. Typing Performance and Subjective Assessment The TypingMaster program determined the participant’s typing speed, accuracy, and overall efficiency. Typing speed was measured as words per minute and then expressed as a score on a scale from 1 to 100, assuming a score of 100 for typing speed of 300 characters per minute. Typing accuracy was evaluated on a scale of 0 to 100 according to the number of errors made during typing. The overall efficiency of typing was then ex-

Fig 1. Experimental setup of the Fastrak equipment and the VDT workstation.

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ERGONOMIC EVALUATION OF NOTEBOOK COMPUTERS, Szeto Table 2: Mean Postures in the Cervical and Thoracic Spines Segment

Movement Plane

Cervical

Sagittal (flexion/extension)

Coronal (lateral flexion)

Transverse (axial rotation)

Thoracic

Sagittal (flexion/extension)

Coronal (lateral flexion)

Transverse (axial rotation)

Experimental Condition

Mean ⫾ SD

DT NB SNB DT NB SNB DT NB SNB DT NB SNB DT NB SNB DT NB SNB

2.9 ⫾ 7.7 7.3 ⫾ 7.1 10.6 ⫾ 6.3 1.7 ⫾ 4.3 1.6 ⫾ 3.8 3.0 ⫾ 4.6 ⫺2.3 ⫾ 5.0 ⫺3.7 ⫾ 3.6 ⫺4.1 ⫾ 4.4 ⫺3.4 ⫾ 6.4 ⫺1.5 ⫾ 7.3 ⫺0.3 ⫾ 6.8 ⫺0.6 ⫾ 5.1 ⫺0.3 ⫾ 4.5 0.3 ⫾ 6.4 0.5 ⫾ 5.2 0.5 ⫾ 5.3 0.1 ⫾ 7.2

F

P

17.526

.000*

1.999

.149

7.274

.020*

6.682

.003*

.64

.553

.123

.884

Post Hoc Analysis

F

P

DT vs NB DT vs SNB NB vs SNB

19.8 26.5 5.7

.000* .000* .027*

DT vs NB DT vs SNB NB vs SNB DT vs NB DT vs SNB NB vs SNB

8.3 10.7 0.9 6.2 11.4 1.9

.009* .004* .358 .022* .003* .188

Abbreviation: SD, standard deviation. * Statistically significant (P ⬍ .05).

pressed as the average of the speed and accuracy scores. At the end of the experiment, participants rated each computer keyboard on a numeric rating scale from 0 (very easy to use) to 10 (extremely difficult to use). Data Analysis Because preliminary data analysis showed no significant changes in the postures and excursions of the spine at 2, 6, and 9 minutes of typing in each experimental condition, we used the overall mean of each variable from the different time intervals to make statistical comparisons among the 3 experimental conditions (ie, the 3 types of computers). Repeatedmeasures analysis of variance (ANOVA) was used to examine the differences in the dependent variables among the 3 different types of computers. When significant differences were found, we used linear contrasts to identify which computers differed significantly from the others. The same procedures were used to examine the other variables such as typing performance and workstation measurements. The effect size of the statistical comparison was assumed to be .25, based on the data gathered. With the repeated-measures ANOVA model, the sample size of 21, and a significance level (␣) of .05, the power of the statistical tests was at least .90, which was considered sufficient to detect significant differences in the dependent variables among the 3 types of computers. RESULTS Kinematic Analysis A summary of the mean postures of the cervical and thoracic segments (table 2) shows significantly different (P ⬍ .05) flexion postures among the 3 computers. Significant difference also existed in the mean posture of axial rotation in the cervical spine between the desktop and the notebook (P ⫽ .009) and between the desktop and the subnotebook (P ⫽ .004). Table 3 summarizes the excursions of the cervical and thoracic segments in the 3 planes during the use of the 3 computers. In the cervical spine, we found generally significant dif-

ferences in the excursion of movements among the 3 computers (P ⬍ .05). There was little movement in the thoracic spine, suggesting that subjects had a fairly static posture. We found no significant differences in the excursion of thoracic movements among the 3 computers. VDT Workstation Assessment As shown in table 4, the mean chair height chosen by the subjects was about 44cm, regardless of the type of computer used. The viewing distance adopted by the participants was greater for the desktop than for the notebook or subnotebook. The mean viewing distance was 47.2 ⫾ 7.3cm for the subnotebook, 54.5 ⫾ 6.6cm for the notebook, and 53.6 ⫾ 7.4cm for the desktop computer. The screen angles were quite different between the desktop (9.5° ⫾ 3.9°) and the 2 smaller computers (notebook, 28° ⫾ 6.9°; subnotebook, 37.0° ⫾ 9°). Typing Performance and Subjective Assessment Participants’ typing performance (table 5) showed that the desktop computer produced the highest scores in all 3 aspects of typing. Participants typed more efficiently on the desktop computer than on the notebook or the subnotebook computers. Before the study, all the participants were asked about their previous experience in using the various types of computers. All used a desktop computer regularly at home and at the university. Their average weekly computer use was 10 ⫾ 6.2 hours (range, 2–28h). Six participants reported that they had used a notebook computer before but that their usage was limited to less than 5 hours a month. None of these notebook users had experience in using the subnotebook computer. All subjects reported being more familiar with the desktop computer than the notebook-type computers. Statistically significant differences were found among the 3 computers in the perceived level of difficulty in their use (P ⬍ .05). The mean scores of difficulty for the desktop, notebook, and subnotebook computers were 1.6 ⫾ 1.2, 5.0 ⫾ 2.0, and 7.5 ⫾ 1.8, respectively. Arch Phys Med Rehabil Vol 83, April 2002

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ERGONOMIC EVALUATION OF NOTEBOOK COMPUTERS, Szeto Table 3: Mean Excursions in the Cervical and Thoracic Spines

Segment

Movement Plane

Cervical

Sagittal (flexion/extension)

Experimental Condition

Mean ⫾ SD

DT NB SNB DT NB SNB DT NB SNB DT NB SNB DT NB SNB DT NB SNB

9.5 ⫾ 4.9 6.6 ⫾ 3.8 4.3 ⫾ 2.4 7.9 ⫾ 3.6 8.6 ⫾ 4.7 5.4 ⫾ 3.2 4.4 ⫾ 1.8 5.7 ⫾ 4.8 2.6 ⫾ 1.1 2.5 ⫾ 1.4 2.5 ⫾ 1.9 2.3 ⫾ 1.7 2.0 ⫾ 0.8 2.1 ⫾ 1 1.8 ⫾ 0.9 2.1 ⫾ 1.1 2.2 ⫾ 2.0 1.6 ⫾ 0.8

Coronal (lateral flexion)

Transverse (axial rotation)

Thoracic

Sagittal (flexion/extension)

Coronal (lateral flexion)

Transverse (axial rotation)

F

P

13.78

.000*

6.94

.030*

6.42

.040*

2.21

.810

1.16

.320

1.98

.150

Post Hoc Analysis

F

P

DT vs NB DT vs SNB NB vs SNB DT vs NB DT vs SNB NB vs SNB DT vs NB DT vs SNB NB vs SNB

8.1 23.0 7.1 0.6 13.7 9.1 1.3 25.6 9.7

.010* .000* .020* .470 .000* .000* .250 .000* .000*

* Statistically significant (P ⬍ .05).

DISCUSSION In the present study, participants assumed increased forward flexion of their cervical and thoracic spines when using smaller-sized computers. The results are consistent with the findings of Straker et al,17 which showed increased neck flexion angle in using notebook compared with desktop computers. Villanueva et al18 also found that subjects’ viewing angles and neck flexion angles progressively increased and the trunk became more forwardly inclined when they used the portable computers. Previous studies14-18 revealed that lower screen heights and increased neck flexion were associated with increased muscle activities in the neck and shoulder region. This posture would increase mechanical loading of the spine, possibly contributing to musculoskeletal discomfort. Hence, the use of the notebook and especially the subnotebook computer, with their characteristically increased neck flexion, may increase the risk of pain and injuries. It has been proposed that the eyes of the computer users should be at an approximate 45° downward angle when looking at the computer screen.14,15 This posture requires the computer screen to be lowered and tilted at 45°. However, the monitor of most desktop computers can only swivel to a limited extent on a standard desk. On the other hand, although the screen angles of the notebook and subnotebook computers are more easily adjustable to accommodate eye angle, their smaller screen size may place greater demands on the eyes. This issue was evident in the shorter viewing distances that participants in the present study assumed when using the notebook and subnotebook computers. They had to lean forward to see the smaller fonts more clearly. The present results support these arguments

Table 4: VDT Workstation and Anthropometric Measurements Measurement

DT

NB

SNB

Chair height (cm) Sitting height (cm) Viewing distance (cm)

44.2 ⫾ 2.6 46.7 ⫾ 3.0 53.6 ⫾ 7.4

44.4 ⫾ 2.4 46.6 ⫾ 2.7 54.5 ⫾ 6.6

44.4 ⫾ 3.4 45.9 ⫾ 3.1 47.2 ⫾ 7.3

NOTE. Values are mean ⫾ SD.

Arch Phys Med Rehabil Vol 83, April 2002

about the effect of computer screen size on posture. When using notebook and subnotebook computers, participants leaned forward and adopted significantly increased neck flexion angles. Increased flexion in the thoracic spine also contributed to the increased forward postures that participants assumed when using the smaller computers. The change in posture was greater in the cervical segment than in the thoracic segment. This means that the participants tended to adjust their posture in the cervical spine rather than the thoracic spine. This adjustment was probably because the trunk was mainly used for stabilizing the upper body in the typing tasks, and most subjects preferred to have their backs resting against the chairs during the tasks. Additionally, the excursions of movements in the thoracic segment were very small, indicating that the upper thoracic segment was relatively static during the typing task, regardless of the type of computer used. The thoracic spine’s relatively static posture might increase the risk of musculoskeletal discomfort in this region. It has been suggested that a major cause of musculoskeletal disorder in VDT users is the lack of physical motion6 despite the type of computer used. We found differences in the anthropometric and workstation measurements among the 3 computers. The viewing distance was the shortest for the subnotebook and the screen angles selected for it by the participants were also notably more inclined. Because viewing distances for subnotebook were shorter than the appropriate distance recommended by previous research,22,23 greater eyestrain and discomfort could occur when using the subnotebook computer for a prolonged time. The computer’s design affects typing performance as well as posture. The smaller size and the lower position of the notebook and subnotebook screens may negatively affect the user’s typing performance. The keyboard’s smaller size provides little space for the wrists to rest, making hand positioning difficult and requiring a high degree of finger dexterity and concentration to type appropriately. All these factors may account for the lower scores in typing speed, accuracy, and overall efficiency in notebook and subnotebook computers when compared with desktop computers. Interestingly, Straker17 reported no significant difference in their subjects’ typing performance between desktop and laptop

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ERGONOMIC EVALUATION OF NOTEBOOK COMPUTERS, Szeto Table 5: Typing Performance Comparing the 3 Computers Typing Performance Measure

Speed (word/min)

Accuracy

Overall efficiency

Experimental Condition

DT NB SNB DT NB SNB DT NB SNB

Mean ⫾ SD

28.7 ⫾ 9.8 26.6 ⫾ 9.1 22.6 ⫾ 7.1 75.4 ⫾ 14.3 66.6 ⫾ 28.9 56.1 ⫾ 31.4 41.4 ⫾ 11.8 36.2 ⫾ 17.5 27.1 ⫾ 15.5

F

P

36.45

.00*

5.92

.00*

9.97

.00*

Post Hoc Analysis

F

P

DT vs NB DT vs SNB NB vs SNB DT vs NB DT vs SNB NB vs SNB DT vs NB DT vs SNB NB vs SNB

21.5 51.4 24.5 2.3 14.7 3.0 2.9 24.5 5.8

.000* .000* .000* .144 .001* .101 .106 .000* .025*

* Statistically significant.

computers. The difference between their experimental results and those of the present study might be related to the type of subjects recruited. Their study was conducted among experienced professional typists whereas we recruited university students. Most participants in the present study had relatively little experience with the notebook and none with the subnotebook computer. The lack of previous experience may have contributed to their poorer typing performance on those 2 computers, which tend to demand more skills and require more postural adjustments. Our participants’ typing scores were compatible with their perceived difficulty in using the desktop keyboard compared with the difficulty they reported with the notebook and subnotebook. A limitation of the present study is that only the postures and movements of the cervical and thoracic region were analyzed. In future studies, electromyographic activities of spinal muscles should be examined concurrently to provide a more complete picture about the biomechanical loadings of the spine in using the various types of computers. Also, the present study compared performance on a fixed support system (fixed chair, desk), and the results might not be applicable in other situations in which the chair and desk could be adjusted. In the latter condition, users may adopt different postural adjustments to alleviate the postural strains. associated with the use of computers. Further research is recommended to compare the computers under different support conditions. CONCLUSION In the present study, participants adopted increased cervical and upper thoracic flexion when they typed on smaller-sized computers. Excursions of the cervical spine were significantly greater in all 3 anatomic planes when compared with the thoracic spine. The assessment of the VDT workstation showed that participants tended not to change their chair heights or positions, but rather they adjusted their postures to use the smaller computers. The study also showed that the participants typed faster and more accurately on a desktop computer than on the notebook or subnotebook computers. We conclude that the desktop keyboard may be a better choice of typing instrument than the notebook and subnotebook computers. Prolonged use of notebook and subnotebook computers may induce a more flexed spinal posture and possibly increased spinal loading, resulting in an increased risk of musculoskeletal discomfort. Acknowledgment: The authors thank Andy Wong, Selina Yip, Shum Siu Ling, Ka Man Ho, and Dorcas Chan for their assistance in the data collection and analysis process.

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