The effect of tablet use on trunk posture while sitting

Abstract

BACKGROUND:

The use of tablet during the office work is on the rise, but the biomechanical response of tablet use under various sitting postures is not well understood.

OBJECTIVE:

This study quantitatively measured changes in trunk kinematics under three sitting conditions (raised leg, neutral leg, and lowered leg) while using a tablet.

METHODS:

Fifteen participants were asked to sit on a chair with three different postures while staring at a handheld tablet or gazing straight ahead with a bare hand, and the head flexion, lumbar flexion and trunk inclination were captured with electrical goniometers.

RESULTS:

The results revealed significantly less lumbar flexion (12.8%) and trunk inclination (28.0%) while using the tablet compared to the empty hand condition (p < 0.001), but at a significant cost of increased head flexion (90.8%; p < 0.001). Further, while using the tablet, participants showed less head flexion in the raised leg condition (p < 0.001) than in the others (9.7% and 7.5%, respectively), but larger trunk inclination and lumbar flexion were required (p < 0.001 in both).

CONCLUSIONS:

Collectively, the lower extremity sitting posture significantly changed the way to observe the tablet by adopting more head flexion in neutral and lowered leg conditions or more trunk flexion in raised leg condition.

Keywords

Lumbar flexion head flexion trunk inclination sitting posture

1 Introduction

Smart devices such as the tablet and smartphone have become indispensable and are now ubiquitous in our daily lives with a global penetration rate of 67% (5,112 billion people around the world) [1]. According to statistics published in 2012, office workers with a tablet in America use the device for 2.1 hours daily during their work [2]. Users are becoming increasingly and constantly reliant on their smart devices throughout the day, especially as an assistant device of the office work such as notepad, calendar etc. Hence, the effect of this growing use of smart devices on the musculoskeletal system is a critical issue in the area of office ergonomics and occupational health.

A recent study evaluated the head flexion angle while using a smartphone and showed negative influences (e.g., forwarded head posture) in both standing and sitting [3]. They revealed a larger head flexion angle in sitting and texting and highlighted the posture as a significant contributor to the development of neck and upper back discomfort.

Similarly, previous studies focusing on the prolonged use of a desktop computer pointed out two prevalent postures such as forward head posture (i.e., also referred to as text neck denoting the anterior movement of the cervical spine) and rounded shoulder posture (i.e., abduction of the scapulae) [4, 5]. The negative influence of those postures has already been studied and adopting non-neutral neck postures over time has been found to increase muscular efforts of the cervical paraspinals and trapezius, stabilizing the neck-shoulder system [5 –9]. To minimize neck or shoulder discomfort caused by the use of PC or smart device, it is critical to employ a neutral neck posture achieved by the keeping the spine vertical with the lordosis in lumbar and cervical region [10].

Using a smart device while sitting could also influence the low back biomechanics. Weston, Le, and Marras [11] investigated the effect of using a touch-screen tablet while sitting and showed increased muscle recruitment in the erector spinae and latissimus dorsi during tablet use. They suggested that the results are highly related to changes in the upper body postures such as increases in mean torso flexion and mean neck flexion while using a tablet, as compared to desktop computer use. Collectively, the increasing use of smart devices is expected to exert a deleterious effect on both the low back and upper back.

While previous studies show how smart device use affects the flexion angles in the sagittal plane of the head, neck and low back in both sitting and standing, none have examined the effect of various sitting postures while using a smart device. Sitting is the most commonly observed posture in tablet users (51%), according to a study surveying 3,600 students across 30 high schools in Shanghai [12]. It is well documented in the literature that the pelvic angle (i.e., hip angle) and lumbar lordosis can be influenced by various sitting factors such as the slope of the seat, the height of the chair and the location of hands [13, 14]. They suggested that sitting on a forward-sloping chair brings about a pelvic extension (i.e., anterior rotation of pelvis) and, consequently, better lumbar lordosis than a flat chair. Recent studies also confirmed a close interaction between the lower extremity and trunk via significant biomechanical linkages (i.e., origin and insertion of the low back muscles, thoracolumbar fascia, psoas major etc.) [15, 16]. Collectively, it could be hypothesized that the changes in lower extremity sitting posture (e.g., trunk-thigh angle) could influence the upper body postures throughout the interaction with hand-held smart devices. On this basis, the goal of the current in-vivo study was to quantitatively measure the effect of different sitting postures on the low back and neck while using a tablet.

2 Methods

2.1 Participants

Fifteen male participants were recruited from undergraduate and graduate students at Pusan National University throughout notices all over the building. All participants did not have any current or chronic pain or symptoms in the low back, upper back, and lower extremities. Prior to participation, each provided written informed consent in a form approved by the Pusan National University Institutional Review Board. The average and standard deviation of age, height and whole-body mass of the participants were 24.5 (SD 1.6) years, 176.7 (SD 7.0) cm, and 73.0 (SD 8.9) kg, respectively.

2.2 Apparatus

Two electrical goniometers were used to capture angular displacement in the low back and neck (Inline 2D Electrical Goniometer, Noraxon, Scottsdale, AZ). The goniometer sensors were aligned over the T12 and S1 vertebrae (capturing lumbar flexion angle) and the external occipital protuberance and C7 vertebrae (capturing the head flexion angle) by using straps and double-sided tape (data collected at 200 Hz) (Fig. 1).

Fig. 1

Electrical goniometers aligned over the T12 and S1 vertebrae (capturing lumbar flexion angle) and the external occipital protuberance and C7 vertebrae (capturing the head flexion angle) by using straps and double-sided tape.

A smartphone digital camera (Apple iPhone 6) was used to capture the lateral view of each participant for assessing the trunk inclination angle [10]. The camera was placed 1.5 m away from the participant and was fixed on a stable tripod. The height of the camera was adjusted to the shoulder height of each participant and each condition. Two markers were placed upon the acromion and iliac crest before experimental trials, and were used to develop a virtual line representing the trunk inclination angle from the global vertical line.

The experimental apparatus included a kneeling chair (NS750 AR Euro, Nistul International, Taiwan) for taking three experimental postures such as lowered leg, neutral leg and raised leg. A knee supporter was only used for the lowered leg condition. The postures assuming neutral leg and raised leg were performed after removing the knee supporter (Fig. 2). Also, a tablet (Apple iPad Air 2; 444 g, 169 mm×240 mm) was used to figure out the effect of using a smart device while sitting.

Fig. 2

Three sitting postures: (a) neutral leg, (b) raised leg, and (c) lowered leg. Use of the armrest was not allowed.

2.3 Experimental design

A 2×3 repeated measure design was used in the current experiment with two independent variables (IVs) as follows: 1) smart-device conditions (DEVICE): tablet use and empty hands; and 2) lower extremity sitting postures (POSTURE): raised leg, neutral leg, and lowered leg. No backrest or arm supporter was used in any of the three sitting postures, and the seat heights were determined by the height of the popliteal fossa of each participant, minus 1 cm (describe pressing by the seat cushion) [13]. The neutral leg posture was defined as positioning the thigh horizontally while sitting on a leveled chair. The lowered leg posture assumed a kneeling posture on the knee supporter while sitting on a forward-sloping chair. The raised leg posture was defined as placing two feet on a board which was half the height of the seat (see Fig. 2).

Four dependent variables (DVs) capturing the upper body postures were considered: lumbar flexion angle, head flexion angle, trunk inclination angle and smart device angle (i.e., tablet angle). Lumbar flexion angle focused on the movement of five lumbar vertebrae in the sagittal plane, denoting lumbar lordosis (see Fig. 3). Head flexion angle aimed to capture the movement of seven cervical vertebrae in the sagittal plane. Trunk inclination angle focused on the entire inclination of the trunk from the vertical plane. The angle of the hand-held tablet under three sitting conditions was measured to capture the pitch angle from the horizontal plane, recorded by an application on the smart device.

Fig. 3

Definition of head flexion angle and lumbar flexion angle.

2.4 Task and procedure

The experimental procedure was described in detail to the participants, and informed consent was obtained. Then, the participants were fitted with two electric goniometer sensors and two markers in the appropriate locations (See section 2.2. Apparatus). Before starting the experimental procedure, the participants were informed of the three sitting postures and were given the opportunity to sit on a chair. Then, a randomized sequence of the 18 sitting tasks (3 POSTUREs×2 DEVICEs×3 repetitions) was performed. During the sitting task, the participants were asked to sit on a chair with three different postures while browsing a handheld tablet or sitting down comfortably with their arms relaxed. In all trials, resting their arms on the arm support and their leg was not allowed. Under the tablet use condition, the participants were able to browse and read newspapers selected by themselves without any restriction. In each trial, the tablet was delivered to the participants with the main page of the news portal open, and the participants were asked to browse the news within the news portal only.

Each task continued for 30 seconds without any request or restriction on the trunk posture. Only the three sitting postures were assumed. The data were recorded in the last 5 seconds of each trial, because there was no significant changes in the trunk and neck postures while reading the article (after at least 10 seconds). Between trials, the participants were given a 1-min rest period. After the final trial, the electric goniometer sensors and markers were removed and the subjects were free to leave.

2.5 Data processing

The lumbar flexion angle, head flexion angle and smart device angle were the average angle (in the sagittal plane) over the 5-sec recording for each trial. The digital pictures of the trunk inclination were evaluated by finding the angle between the vertical lines and the line segment running from the acromion to the iliac crest.

2.6 Statistical analysis

All statistical analyses were conducted using SAS^® and Minitab^®. Prior to model analysis, model adequacy checking was performed on the data, including tests for homoscedasticity (Bartlett’s Test and Levene’s Test) and normality (Anderson-Darling Normality Test). The dependent variables which do not satisfy the model assumptions were transformed by using the Jonhson transformation, so that the ANOVA model assumptions were fully satisfied. Univariate ANOVA (p < 0.05 for F statistic) was conducted to test the effects of independent variables (POSTURE and TIME) on three dependent measures (head flexion angle, lumbar flexion angle, trunk inclination angle). Also, the Post-hoc test using the Bonferroni method was employed on the independent variable and its interaction that were found to be significant in the ANOVA test. Finally, the Pearson correlation was performed between three dependent variables for revealing associations among the variables of the upper body posture. A p-value less than 0.05 was the standard level for significance.

3 Results

The results of the analysis on upper body postures provided insight into the changes in the participants’ strategy to stare at the target device. First, in the lumbar flexion angle, ANOVA showed significant effects of POSTURE and DEVICE (p < 0.001 for both IVs) suggesting the influence of hand-held device and lower extremity sitting postures on the upper body postures, but there was no interaction effect between POSTURE and DEVICE (Table 1). The following Post-hoc test on POSTURE showed significantly less lumbar flexion in the lowered leg condition (i.e., more lordotic posture) as compared to the others, denoting the bigger trunk-thigh angle, the better the lumbar lordosis. Meanwhile, the Post-hoc test on DEVICE revealed better lumbar lordosis in the tablet condition as compared to the empty hands condition even the difference was minor (about 3°) (Fig. 4).

Table 1
The effects of sitting postures and hand-held device on the trunk kinematics: ANOVA results for three dependent variables

Head flexion angle Lumbar flexion angle Trunk inclination angle

POSTURE p = 0.193 p < 0.001 p < 0.001

DEVICE p < 0.001 p < 0.001 p < 0.001

Interaction p = 0.010 p = 0.123 p = 0.707

	Head flexion angle	Lumbar flexion angle	Trunk inclination angle
POSTURE	p = 0.193	p < 0.001	p < 0.001
DEVICE	p < 0.001	p < 0.001	p < 0.001
Interaction	p = 0.010	p = 0.123	p = 0.707

(Bold characters indicate significance at the p < 0.05 level).

Fig. 4

Effect of POSTURE and DEVICE on average lumbar flexion angle. Error bars represent the standard error of the sample mean. The letters represent the results of the Post-hoc tests. The average lumbar flexion angle with different letter indicates that they are statistically significantly different.

Second, in the head flexion angle, ANOVA showed significantly bigger head flexion while engaged with the smart device (p < 0.001) (Table 1 and Fig. 5). The negative effect of tablet use on the head flexion angle was also clear by showing 27° more flexion in 10th percentile, 25° more flexion in 50th percentile and 21° more flexion in 90th percentile (Fig. 6). Note that the 10th percentile of the tablet condition showed similar degree of the head flexion with the 90th percentile of the empty hands condition. In addition, ANOVA also showed a significant interaction between POSTURE and DEVICE (p = 0.010) (Fig. 5). Simple effect analysis, used to show a simple effect of an independent variable, revealed a significant main effect of POSTURE when it was sliced by each DEVICE under the smart device condition (p < 0.001). The following Post-hoc results showed that there is no difference between the lowered leg and the neutral leg conditions, but the lowered leg condition showed significantly bigger head flexion angle than the raised leg.

Fig. 5

Interaction plot between POSTURE and DEVICE for the average head flexion angle. Error bars represent the standard error of the sample mean. The letters represent the results of the Post-hoc tests. The average head flexion angle with the same letter indicates that they are not statistically significantly different.

Fig. 6

Head flexion angle with or without the smart device (Tablet PC). Error bars represent the standard error of the sample mean.

In the trunk inclination angle, ANOVA showed significant effects of POSTURE (p < 0.001) and DEVICE (p < 0.001), but there was no interaction (p = 0.707). The Post-hoc test on POSTURE confirmed greater forward trunk inclination in the raised leg than in the lowered and neutral leg conditions (Fig. 7). Similarly, Post-hoc result revealed less trunk inclination in the smart device condition than in the empty hand condition.

Fig. 7

Effect of POSTURE and DEVICE on trunk inclination angle. Error bars represent the standard error of the sample mean. The letters represent the results of the Post-hoc tests. The average trunk inclination angle with different letter indicates that they are statistically significantly different.

Additional analysis was performed for assessing the association among the variables of the upper body posture. The tests were performed to show that three DVs representing the upper body posture have a significant influence on each other. The Pearson correlation tests showed that the lumbar flexion angle had a significant correlation with the trunk inclination angle (r = 0.619, p < 0.001) and the head flexion angle (r = 0.229, p < 0.001), and that the head flexion angle also had a significant correlation with the smart device angle (r = –0.560, p < 0.001) (Fig. 8). Note that the raised leg condition mostly required greater lumbar and trunk flexion, and the lowered leg condition distributed on the smaller lumbar and trunk flexion, suggesting better sitting posture.

Fig. 8

Scatterplot between lumbar flexion angle and trunk inclination angle according to the POSTURE.

4 Discussion

With technological advances in wireless communications and ubiquitous computing, the smart device available anytime anywhere is now an indispensable information equipment for office work and daily activities. During office work, the sitting posture is most common, so a quantitative investigation on the effect of various sitting postures while using a tablet can provide a valuable insight into the nature of an interaction between sitting strategies and smart device use. Specifically, the previous studies revealed that both conditions have a significant impact on trunk postures. The sitting posture which controls lower extremity location changes the lumbar lordosis and thoracic kyphosis [13 , 17–19]. Browsing a hand-held smart device increases head flexion angle and lumbar flexion angle [3]. Considering that adjacent body parts such as the lower extremity, lower back, and upper back are interconnected and interact with each other throughout the lumbosacral interaction and pelvifemoral interaction, the lower extremity posture could give a significant effect on the trunk posture [15, 16]. Meanwhile, the hand-held smart device may also influence on the trunk posture. Thus, the current study aimed to reveal the changes in head flexion, lumbar flexion and trunk inclination under the various sitting conditions while using a tablet.

In regards of the effect of lower extremity sitting postures, the participants tended to lose the lumbar lordosis and bend the upper body forward by changing the sitting posture from a lowered leg condition to a raised leg condition. The result of better lumbar lordosis in the lowered leg condition is supported by previous studies in which the lumbar lordosis was increased with increasing seat inclination forward [13 , 17]. In terms of changes in trunk inclination, the current study revealed that the participants tended to keep the trunk more vertical in a lowered leg condition. In general, as the position of lower extremity rises from the lowered leg condition to the raised leg condition, the vertebrae were bowed forward. The findings are a clear indication of the interaction between the lower extremity and trunk and suggest that the adjacent body parts in the upper body and lower body significantly influence each other. Specifically, the raised leg pulled the hamstring muscles which are largely inserted into the pelvis, and then the pelvis could tilt backward while the lumbar vertebrae could be forced to be flexed (i.e., loss of lordosis in the low back). The flexed lumbar spine may result in curved spine. Collectively, the lowered leg condition could be recommended only if an independent effect of the sitting posture on trunk postures was considered.

Interestingly, when the participants engaged with a tablet, they were apt to have less lumbar flexion and trunk inclination, but at a cost of significantly increased head flexion angle in the smart device condition. The gap between the empty hand and the smart device condition in lumbar flexion and trunk inclination was minor (about 3 degrees in both), but the head flexion angle differed by more than 20 degrees between the two conditions. Increases in lumbar flexion angle and trunk inclination angle could increase the moment arm between the center of trunk mass and L5/S1, and hence the participants would have naturally adopted a strategy to achieve the goal of observing the tablet by having more flexion in the neck while reducing the burden on the trunk. Also, they were holding the tablet by locking their elbows into their sides. In other words, the posture adopted in the tablet condition may be the best to minimize the overall load on the body but at the expense of taking even more load on the neck.

Compared to the previous studies in the head flexion angle, the users tended to flex their head between 20° and 25° in the desktop or laptop PC condition [20, 21], between 15° and 25° (between 45° and 55° before neutralizing with the normal cranio-cervical angle reported in the literature) in the tablet condition [23], and between 35° and 45° in the smartphone condition [3]. The current study found a similar head flexion angle with Young et al. [22] when engaged with the tablet (between 45° and 51°). This suggests that the hand-held use of a tablet causes larger head flexion angle than the smartphone, possibly due to the difference in weight of the tablet (3.5∼5 times heavier). The participant may not be able to raise their upper extremities enough while holding the tablet because of the weight. Consequently, they naturally adopted more head flexion posture while holding the tablet than the smartphone. Considering that the findings of previous studies focused on the prolonged head flexion, it is reasonable to believe that the use of a tablet in a static posture could contribute to the development of symptoms of neck pain and upper back pain [23 –25].

There was a significant interaction between the sitting posture and the smart device use in terms of the head flexion angle. While using the smart device, participants showed less head flexion angle in the raised leg condition than in the lowered leg condition and in the neutral leg condition. This is directly attributable to the larger trunk inclination and lumbar flexion achieved in the raised leg condition. Regarding our hypothesis suggesting a significant interaction between the posture of the lower extremity and upper body, the participants may have been pushed to take a larger low back flexion and trunk inclination posture in the raised leg condition, because of the pulling force of the hamstring muscles on the pelvis. Meanwhile, in an attempt to minimize the overall trunk load, the participants reduced the lumbar flexion and trunk inclination and increased the head flexion in the lowered leg and neutral leg condition, so that they could adopt the posture without any significant pulling force of the hamstring muscles. The participants may have adopted more flexed head posture to make it easier to see the tablet, and hence more biomechanically beneficial, than adopting a larger lumbar flexion and trunk inclination posture. Collectively, the results suggest that the sitting posture significantly affected the strategy adopted to achieve the goal of observing the tablet by adopting more head flexion (in neutral and lowered leg conditions) or more trunk flexion (in raised leg condition). Meanwhile, it is possible that even with the least head flexion angle, the raised leg condition while using a tablet could be regarded as the worst posture, due to the larger moment arm caused by the bigger lumbar flexion and trunk inclination, among the six postures (3 sitting postures×2 device conditions) tested in the current study.

Investigating the effect of sitting strategy on the upper body postures was one of the focus in the current study. The results clearly revealed changes in the lumbar flexion and trunk inclination according to the lower extremity posture (i.e., sitting posture), but there was no significant difference in the head flexion angle. Additional correlation analysis suggested that the trunk inclination is highly affected by the lumbar flexion angle (i.e., lumbar lordosis). Regarding the current results, it could be reasonable to hypothesize that the effect of lower extremity posture is limited until the thoracic vertebrae. However, although the signal (i.e., correlation coefficient) was weak, the head flexion angle and lumbar flexion angle showed a significant correlation. Future research focused on the topic may reveal valuable insight into the interaction. Also, the correlation analysis revealed a significant relationship between the angle of the tablet and head flexion angle, indicating that the sitting posture affected the hand-held angle of the tablet. In other words, if the device is fixed on a table with an adequate angle, the head flexion angle could be minimized [22].

Care should be taken to generalize the current study results in that this study only measured the trunk kinematics. Muscle activation pattern of the agonist and antagonist while using a tablet may provide a better insight into the user’s strategy on the sitting posture. Also, the current study was limited in that there was only one simple task with the tablet which is web browsing and reading. Future studies are necessary to fully elucidate the effect of various smart device tasks such as message typing, application using, and video watching. Finally, the current study observed the participant’s posture for a short time. Observations over a long period of time could be able to provide an in-depth understanding.

5 Conclusions

The current in-vivo study has highlighted the importance of considering the sitting strategy when using tablet by revealing that tablet users experience a larger head flexion angle (about 5∼10° more) as compared to the smartphone and PC users tested in the previous studies [3 , 21]. Even though the raised leg posture significantly decreased the head flexion angle when using a tablet, the significant cost of the larger lumbar flexion and trunk inclination on the low back should be considered. Meanwhile, the participants tended to keep their trunk vertical in the neutral and lowered leg conditions but at a cost of increased head flexion angle in the tablet condition. Although a direct comparison among the three sitting postures while using a tablet was limited in the current study, the raised leg condition was the worst posture due to the larger moment arm between L5/S1 and the center of the trunk mass. Similarly, the lowered leg condition could be recommended in that the posture showed better lumbar lordosis and smaller trunk bending. However, the risk of greater head flexion should be minimized by placing a tablet on the table or supporter.

Conflict of interest

No potential conflict of interest was reported by the authors.

Footnotes

Acknowledgments

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2017058412).

References

We Are Social. [webpage on Internet]: Digital 2019 Global; 2019 [updated 2019 January 25]; Available from: https://wearesocial.com/ digital-2019-global.

Government Technology: State & Local Government News Articles. [webpage on Internet]: Infographic: Tablets in the Workplace; 2012 [updated 2012 December 10; cited 2018 July 5]; Available from: www.govtech.com/e-government/Infographic-Tablets-in-the-Workplace.html.

Lee

, Kang

, Shin

. Head felxion angle while using a smartphone. Ergonomics. 2015;58(2):220–6.

Yip

CHT

, Chiu

TTW

, Poon

ATK

. The relationship between head posture and severity and disability of patients with neck pain. Manual Therapy. 2008;13(2):148–54.

Szeto

, Lee

. An ergonomic evaluation comparing desktop, notebook, and subnotebook computers. Arch Phys Med Rehabil. 2002;83(4):527–32.

Douglas

, Gallagher

. The influence of a semi-reclined seated posture on head and neck kinematics and muscle activity while reading a tablet computer. Applied Ergonomics. 2017;60:342–7.

Kothiyal

, Bjornerem

. Effects of computer monitor setting on muscular activity, user comfort and acceptability in office work. Work. 2009;32(2):155–63.

Szeto

, Straker

, O’Sullivan

. A comparison of symptomatic and asymptomatic office workers performing monotonous keyboard work- Neck and shoulder muscle recruitment patterns. Manual Therapy. 2005;10(4):270–80.

Seghers

, Jochem

, Spaepen

. Posture. Muscle activity and muscle fatigue in prolonged VDT work at different screen height settings. Ergonomics. 2003;46(7):714–30.

10.

Shaghayegh Fard

, Ahmadi

, Maroufi

, Sarrafzadeh

. Evaluation of forward head posture in sitting and standing positions. European Spine Journal. 2016;25(11):3577–82.

11.

Weston

, Le

, Marras

. A biomechanical and physiological study of office seat and tablet device interaction. Applied Ergonomics. 2017;62:83–93.

12.

Shan

, Deng

, Li

, Zhang

, Zhao

. Correlational analysis of neck/shoulder pain and low back pain with the use of digital products, physical activity and psychological status among adolescents in Shanghai. PLoS One. 2013;8(10):e78109.

13.

Bendix

. Seated trunk posture at various seat inclinations, seat heights, and table heights. Human Factors. 1984;26(6):695–703.

14.

Bridger

, Wilkinson

, Houweninge

. Hip joint mobility and spinal angles in standing and in different sitting postures. Human Factors. 1989;13(2):229–41.

15.

Jin

, Mirka

. A systems-level perspective of the biomechanics of the trunk flexion-extension movement: Part I- normal low back condition. International Journal of Industrial Ergonomics. 2015;46:7–11.

16.

Jin

, Mirka

. A systems-level perspective of the biomechanics of the trunk flexion-extension movement: Part II- fatigued low back conditions. International Journal of Industrial Ergonomics. 2015;46:1–6.

17.

Claus

, Hides

, Moseley

, Hodges

. Different ways to balance the spine: subtle changes in sagittal spinal curves affect regional muscle activity. 2009;Spine 34(6):208–14.

18.

Dunk

, Kedgley

, Jenkyn

, Callaghan

. Evidence of a pelvis-driven flexion pattern: Are the joints of the lower lumbar spine fully flexed in seated postures? Clinical Biomechanics. 2009;24(2):164–8.

19.

De Carvalho

, Soave

, Ross

, Callaghan

. Lumbar spine and pelvic posture between standing and sitting: A radiologic investigation including reliability and repeatability of the lumbar lordosis measure. Journal of Manipulative and Physiological Therapeutics. 2010;33(1):45–55.

20.

Gold

, Driban

, Yingling

, Komaroff

. Characterization of posture and comfort in laptop users in non-desk settings. Applied Ergonomics. 2012;43(2):392–9.

21.

Turville

, Psihogios

, Ulmer

, Mirka

. The effects of video display terminal height on the operator: A comparison of the 15° and 40° recommendations. Applied Ergonomics. 1998;29(4):239–46.

22.

Young

, Trudeau

, Odell

, Marinelli

, Dennerlein

. Touch-screen tablet user configurations and case-supported tilt affect head and neck flexion angles. Work. 2012;41(1):81–91.

23.

Sommerich

, Joines

, Psihogios

. Effects of computer monitor viewing angle and related factors on strain, performance, and preference outcomes. Human Factors. 2001;43(1):39–55.

24.

Straker

, Skoss

, Burnett

, Burgess-Limerick

. Effect of visual dislay height on modelled upper and lower cervical gravitational moment, muscle capacity and relative strain. Ergonomics. 2009;52(2):204–21.

25.

Nordander

, Hansson

, Ohlsson

, Arvidsson

, Balogh

, Stromberg

, Rittner

, Skerfving

. Exposure–response relationships for work-related neck and shoulder musculoskeletal disorders – analyses of pooled uniform data sets. Applied Ergonomics. 2016;55:70–84.