||The effects of different tablet tilt angles on posture control and muscle activity in tablet computer users
||Department of Physical Therapy
Perceived exertion/ comfort
研究背景與目的：由於平板電腦獨特的性能，使用者可以躺著，坐著，站著或於行走時使用平板電腦。平板電腦使用者可以在不同的姿勢下來操作平板電腦。平板電腦也可以在不同的傾斜角度下使用。根據過去研究，當平板電腦的傾斜角度減少時，頸部姿勢愈漸彎曲，頸部肌肉的肌肉活動程度增高。而當傾斜角度增加時，手腕姿勢愈漸伸直，伸腕肌的肌肉活動程度增高。過去研究也發現使用電腦有較高的電腦視覺症候群(Computer vision syndrome)的發生。此外，較高的自覺費力度和自覺舒適度也和肌肉骨骼疾病(Musculoskeletal disorder)的形成有關。就性別而言，以前研究也認位女性相比於男性有較高罹患肌肉骨骼疾病的風險。然而，平板電腦的使用和上述參數，例如姿勢、肌肉活性、視覺、自覺費力及自覺舒適度之間的關係仍未得到詳盡的研究。因此，本篇研究的目的有三，第一，探討平板電腦傾斜角度對平板電腦使用者姿勢及肌肉活動的影響;第二，探討時間效應對平板電腦使用者的影響;第三，比較不同性別使用者在使用平板電腦時的差異。
研究方法：本實驗收取15名男性和15名女性，皆具備使用過觸控式電子儀器的經驗。所有的參與者皆會在不同的傾斜角度情況下(0˚，15˚，50˚，及自選角度)進行平板電腦作業。在研究過程中，透過三度空間紅外線攝影機(Qualisys)記錄頭部，頸部，軀幹和上肢姿勢。另外會以肌電圖記錄雙側頸豎脊肌(Cervical erector spinae）、雙側上斜方肌(Upper trapezius）、雙側前三角肌(Anterior deltoid）、右手腕伸指肌(Extensor digitorum）和右手腕屈指淺肌（Flexor digitorum superficialis）的肌肉活動。六個身體部位的自覺費力度和舒適度，以及視覺不適和眨眼頻率。本研究統計方式使用三因子重複測量變異數(傾斜角度，性別和時間)分析姿勢和肌肉活動。二因子重複測量變異數(傾斜角度和性別)分析自覺費力、自覺舒適度和視覺不適。曼-惠特尼 U檢定比較傾斜角度偏好順序和眨眼頻率。魏可遜排序檢定用於比較性別間的傾斜角度偏好順序和眨眼頻率。另外，為了控制身高對於姿勢及肌肉活性的干擾效果，使用重複測量共變數分析姿勢的差異。
Background and purpose: The tablet computers offer a mobile computing experience and users operate it while lying, sitting, standing, or walking. Thus, the tablet computer users may perform different postures to operate the tablet computer. According to previous studies, neck flexion increased significantly and muscle activity of neck increased as the tablet tilt angle decreased. Wrist extension increased and muscle activity of wrist extensor increased as the tablet tilt angle increased. Previous studies demonstrated that computer works were associated with a high prevalence of computer vision syndrome. The higher perceived exertion and perceived comfort were also associated with developing musculoskeletal symptoms (MSDs) in the computer users. Besides, females were found to be at higher risk of MSDs than males on computer work. However, the relationship between the tablet computer and aforementioned parameters are still under investigation. Three purposes of the current study were: first, to investigate the effects of tablet tilt angle on body kinematic and kinetics in tablet computer users during performing tablet task; second, to explore the time effects on tablet computer users; third, to compare the gender differences in tablet computer users.
Method: Fifteen male and fifteen female adults who experienced with touch display inputs were recruited. All participants executed the tablet computer task at four tablet tilt angles (0˚, 15˚, 50 ˚, and self-chosen tilt angle). While performing the tablet computer task, posture of head, neck, trunk and upper extremity were recorded by motion captured cameras (Qualisys). Muscle activity of bilateral cervical erector spinae (CES), bilateral upper trapezius (Trap), bilateral anterior deltoid (AD), right extensor digitorum (RED), and right flexor digitorum superficialis (RFDS) were recorded. Perceived exertion and comfort in six body regions as well as visual discomfort and blink rate were rated. Also, the preference of tilt angle in tablet computer users was recorded. A three-way repeated measures analysis of variance (RMANOVA) on tilt angle, gender and time was used to evaluate posture and muscle activity. A two-way RMANOVA on tilt angle and gender was used to evaluate perceived exertion, comfort and visual discomfort. Whitney U test was used to compare the preference of tilt angle and blink rate. Wilcoxon rank sum test was used to compare the preference of tilt angle and blink rate between gender groups. In order to control for the confounding effects of body height, a repeated measures analysis of covariance (RMANCOVA) was also used between genders.
Results: The head and neck postures became more erect as the tablet tilt angle increased (p<0.001). The trunk side bending angle increased as tilt angle increased (p=0.028). The posture of thoracic, trunk and lumbar did not significantly change as the tablet tilt angle increased; however there was a trend toward extension on these postural parameters. For upper extremity, when the tilt angle increased, the shoulder posture became more flexed (p<0.001). The elbow posture became extended (p<0.001) and the wrist posture became more extended (p<0.001). There was no significant difference on the wrist deviation posture at different tilt angles. For bilateral CES, the muscle activities decreased as tilt angle increased, but it did not reach the significant level (p=0.080). For RAD (p<0.001) and RED (p=0.028), the muscle activities significantly increased as the tablet tilt angle increased. Both exertion and comfort scores on the neck (both side, p<0.05) and upper back (both side, p<0.05) significantly decreased. Both scores on the wrist (right side, p<0.05) significantly increased as the tilt angle increased. For the time effect, the gaze angle significantly decreased (p=0.025), and the cervical-thoracic angle significantly increased (p=0.007). In addition, the muscle activities of Ltrap (p=0.004) and LAD (p=0.025) significantly decreased as time progressed. For gender differences, males had greater head, neck and trunk flexion postures than females, especially for thoracic flexion (p=0.006). Although, females had slightly greater shoulder flexion, elbow extension and wrist extension. However, there were no significant differences (p>0.05). The female group tended to have a higher muscle activity than the male group, except for bilateral CES. For the perceived scores, females had a higher scores than males, especially for neck region (both side, p<0.05).
Conclusion: From analysis of body kinematic, kinetic, and perceived score, the results showed that there was a lower risk level of MSDs at intermediate angle, such as 15˚ tablet tilt angle, than at extreme angle, such as 0 ˚ and 50 ˚ tablet tilt angle, due to no extreme postures demonstrated on neck, trunk and upper extremity as well as lower muscle activities on neck and upper extremity. Moreover, the results also showed that females might have a higher risk of MSDs than males during the tablet computer task due to higher muscle activities and higher perceived scores. For the time effect, not many significant time effects were found in this study. However, if the experimental time can last longer, we speculated that more significant time effects might be found on body kinematic and kinetic parameters.
Table of Contents VI
List of Tables VI
List of Figures VI
Chapter 1. Introduction 1
1.1、The prevalence of tablet computer use 1
1.2、The touchscreen interfaces and tilting angles 2
1.3、Liquid crystal displays and computer vision syndromes 4
1.4、Tablet computer users’ posture and muscle activity 7
1.5、Gender differences on computer users 12
1.6、Study limitation of previous studies 14
1.7、Motivation and purpose 15
Chapter 2. Methods 17
2.1、Study population 17
2.2、Experimental procedure 18
2.3、The workstation 18
2.4、Touchscreen tilt angle and Touchscreen task 20
2.5、Outcome measures 20
2.5.1 Posture 20
2.5.2 Electromyography (EMG) 23
2.5.3 Perceived exertion and perceived comfort 24
2.5.4 Perceived visual discomfort and blink rate 25
2.6、Data analysis 26
Chapter 3. Results 28
3.1、Participants’ characteristics 28
3.2、Mean posture 28
3.2.1 Gaze distance 28
3.2.2 Gaze angle 29
3.2.3 Cranial-cervical angle 29
3.2.4 Cervical-thoracic angle 29
3.2.5 Head angle 30
3.2.6 Neck angle 30
3.2.7 Thoracic angle 30
3.2.8 Trunk angle 31
3.2.9 Trunk side bending angle 31
3.2.10 Lumbar angle 31
3.2.11 Scapular elevation angle 32
3.2.12 Shoulder angle 32
3.2.13 Elbow angle 32
3.2.14 Wrist angle 33
3.2.15 Wrist deviation angle 33
3.3、Movement range 33
3.3.1 Gaze distance 33
3.3.2 Gaze angle 34
3.3.3 Cranial-cervical angle 34
3.3.4 Cervical-thoracic angle 34
3.3.5 Head angle 35
3.3.6 Neck angle 35
3.3.7 Thoracic angle 35
3.3.8 Trunk angle 36
3.3.9 Trunk side bending angle 36
3.3.10 Lumbar angle 36
3.3.11 Scapular elevation angle 36
3.3.12 Shoulder angle 37
3.3.13 Elbow angle 37
3.3.14 Wrist angle 37
3.3.15 Wrist deviation angle 37
3.4、Electromyographic data 38
3.5、Perceived exertion and perceived comfort 39
3.6、Perceived visual discomfort and blink rate 40
3.7、The order of preference of tilt angle 41
Chapter 4. Discussion 42
4.1、Effects of the tilt angle 43
4.1.1 Postural analysis 43
4.1.2 EMG analysis 50
4.1.3 Analysis of perceived score 54
4.1.4 Analysis of visual parameters 56
4.2、Effects of the time 58
4.2.1 Postural analysis 58
4.2.2 EMG analysis 60
4.2.3 Analysis of visual parameters 61
4.3、Effect of the gender 63
4.3.1 Postural analysis 63
4.3.2 EMG analysis 65
4.3.3 Analysis of perceived score 67
4.3.4 Analysis of visual parameters 69
4.4、Explanation for the interactions 70
4.4.1 Analysis of the time × gender interaction 70
4.4.2 Analysis of the tilt angle × gender interaction 73
4.5、Limitations and further study 74
Chapter 5. Conclusion 76
Appendix 1 Perceived exertion scale 95
Appendix 2 Perceived comfort scale 96
Appendix 3 Perceived visual discomfort scale 97
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