Ergonomics
ISSN: 0014-0139 (Print) 1366-5847 (Online) Journal homepage: http://www.tandfonline.com/loi/terg20
Head flexion angle while using a smartphone Sojeong Lee, Hwayeong Kang & Gwanseob Shin To cite this article: Sojeong Lee, Hwayeong Kang & Gwanseob Shin (2015) Head flexion angle while using a smartphone, Ergonomics, 58:2, 220-226, DOI: 10.1080/00140139.2014.967311 To link to this article: http://dx.doi.org/10.1080/00140139.2014.967311
Published online: 17 Oct 2014.
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Date: 10 October 2017, At: 06:04
Ergonomics, 2015 Vol. 58, No. 2, 220–226, http://dx.doi.org/10.1080/00140139.2014.967311
Head flexion angle while using a smartphone Sojeong Lee, Hwayeong Kang and Gwanseob Shin* Department of Human and Systems Engineering, Ulsan National Institute of Science and Technology, Ulsan, Korea
Downloaded by [Australian Catholic University] at 06:04 10 October 2017
(Received 13 June 2014; accepted 15 September 2014) Repetitive or prolonged head flexion posture while using a smartphone is known as one of risk factors for pain symptoms in the neck. To quantitatively assess the amount and range of head flexion of smartphone users, head forward flexion angle was measured from 18 participants when they were conducing three common smartphone tasks (text messaging, web browsing, video watching) while sitting and standing in a laboratory setting. It was found that participants maintained head flexion of 33 – 458 (50th percentile angle) from vertical when using the smartphone. The head flexion angle was significantly larger ( p , 0.05) for text messaging than for the other tasks, and significantly larger while sitting than while standing. Study results suggest that text messaging, which is one of the most frequently used app categories of smartphone, could be a main contributing factor to the occurrence of neck pain of heavy smartphone users. Practitioner Summary: In this laboratory study, the severity of head flexion of smartphone users was quantitatively evaluated when conducting text messaging, web browsing and video watching while sitting and standing. Study results indicate that text messaging while sitting caused the largest head flexion than that of other task conditions. Keywords: head flexion; smartphone; neck pain; text neck
1. Introduction Smartphones are mobile information and communicative devices that have an operating system, more powerful computing capacity and diverse software applications than low-end mobile phones that contain a limited set of functions. Since the release of Apple iPhone on the market, the number of smartphone users has continuously increased around the world. According to the recent data of market research institutions, the penetration rate of smartphone has already exceeded 56% in top 15 countries in 2013 (Google 2013). Frequency of use and reliance on the smartphone has also been on the rise. Per market research reports, more than 65% of smartphone owners in USA use their phones every day, and spend one to 2 hours daily on average (Analytics 2013; Google 2013). Specifically, a recent survey reports that mobile device users spend more than 20 hours a week emailing, text messaging and using social network services, indicating their heavy reliance on smartphones to connect and communicate with others (eMarketer 2013). With the growing use of smartphones, concerns of musculoskeletal problems associated with the intensive use of the smartphone have also increased. Recent investigations showed that mobile device users tend to report pain symptoms on the neck, shoulder and thumb, and the severity of the symptoms increased with the total time spent using their mobile devices (Berolo, Wells, and Amick 2011). Pain in the neck of smartphone users, which has often been referred to ‘text neck’, has received more public attention recently due to the growing use of mobile devices in head forward flexion postures. Smartphone users typically hold the device using one or two hands below their eye height, look down at the device and type or touch the touch-screen display mostly using the thumb (Gold et al. 2012). They tend to maintain the head forward flexion posture even with the existence of pain symptoms or discomfort on the neck (Maniwa et al. 2013), and it has been suggested that the prolonged and/or frequent use of the smartphone with the severe head flexion posture could be one of main contributing factors to the prevalence of neck pain symptoms of smartphone users. Physical stresses on the neck and resultant health concerns associated with the frequent and/or prolonged head flexion have been frequently studied previously in research about the position of personal computer (PC) displays. Placing a computer display too high or low is known to cause awkward postures of the user and produce neck muscle fatigue and discomfort after a prolonged period of use (Kothiyal and Bjornerem 2009; Seghers, Jochem, and Spaepen 2003). To avoid or minimise the occurrence of the neck pain of PC users, it has often been recommended to keep the head posture near neutral by placing the top of the display at or slightly below eye level (ANSI/HFES 2007).
*Corresponding author. Email: [email protected] q 2014 Taylor & Francis
Ergonomics
221
While head posture and movement patterns of PC users have been studied frequently, research regarding the posture of smartphone users has been relatively scarce. Quantitative evaluation of the duration, frequency and severity of the head flexion posture during the use of smartphone can produce important information for studying the associations between the intensive use of smartphone and the occurrence of neck pain symptoms. In addition, such information may be used to develop ergonomics guidelines that can reduce the risk of neck pain symptoms associated with the use of smartphone. As an attempt to explore the head posture of smartphone users, this study was aimed to quantify the amount of head flexion of smartphone users in a controlled laboratory setting where participants completed three common smartphone tasks (texting, web browsing and video watching) in two different posture conditions (while sitting and standing). The three task conditions were specifically chosen as they were known as the most frequently used application categories of smartphone use (IDC 2013). This study also sought to determine whether there were any differences in the amount of head flexion between the three tasks and/or two posture conditions. The null hypothesis was that the head flexion angle in the sagittal plane was the same between the three tasks and between the two posture conditions.
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2. 2.1.
Methods Participants
Eighteen young participants (nine females, nine males) who had no physical difficulties in using smartphones while sitting and standing were recruited (see Table 1). All participants had at least one year of experience with touch-screen smartphones. Prior to the beginning of data collection, each participant provided an informed consent on a protocol approved by the institutional review board. 2.2.
Data collection
In an isolated laboratory space, each participant performed text-messaging, web-browsing and video-watching tasks in a fixed order using his/her own smartphone, while the angle of head forward flexion was measured using a three-dimensional motion capture system. Each task continued for 2 minutes and repeated twice, once while sitting on a chair and then while standing. A rest break of at least 1 minute was given after each task to minimise any carry-over effects between tasks. While conducting each task, head flexion angle was measured using a three-dimensional motion capture system. Reflective markers were attached to the skin at the forehead, and left and right tragus, and the markers were tracked by 12 motion capture cameras (OptiTrack, Naturepoint, Oregon, USA) at the sampling rate of 100 Hz. A head rigid body was created from the three markers and its forward tilt angle from global vertical on the sagittal plane was registered as the head flexion angle (Psihogios et al. 2001; Sommerich, Joines, and Psihogios 2001; Young et al. 2012). A reference angle (08 head flexion) was captured when the participant was sitting up straight at the beginning of the experiment. The subject was instructed to sit on a chair without slouching the back or leaning against the backrest of the chair, and look straight ahead. During the text-messaging task, the participant was asked to communicate with an experimenter using a text-messaging application of his/her choice for 2 minutes continuously. In the web-browsing task, the participant conducted simple web browsing with self-selected contents and a web browser. The participant was instructed to avoid continuous watching or repetitive texting to differentiate the web-browsing task from the text-messaging or video-watching tasks. In the videowatching task, the participant chose a video clip prior to data collection and then watched the clip continuously without any touch gestures during the 2-minute task duration. While conducting a task in sitting, the participant was allowed to make minor head and upper extremity movements, but asked to remain seated without leaning backwards against the backrest of the chair to avoid any confounding effects associated with the angle or shape of the chair backrest (see Figure 1). In standing conditions, the participant was asked to maintain a comfortable standing posture without lower limb movements. 2.3. Data analysis Independent variables of this study included the three smartphone tasks (text messaging, web browsing, video watching) and two posture conditions (sitting, standing). For each of the six conditions (3 tasks £ 2 posture conditions), the 10th, 50th Table 1.
Female Male All
Mean and standard deviation of participant information. Age (yrs)
Height (cm)
Weight (kg)
19.8 (0.92) 20.7 (1.77) 20.2 (1.48)
161.1 (4.17) 175.3 (5.39) 168.2 (8.56)
55.0 (5.55) 66.4 (6.05) 60.7 (8.12)
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Figure 1.
Text messaging while sitting on a chair.
and 90th percentile values of the head flexion angle were computed from the probability distribution. Effects of ‘task’ and ‘posture’ on the head flexion angle data were evaluated using the repeated measures two-way analysis of variance (ANOVA). Differences in the head flexion angle between the three tasks were further examined by Tukey’s post hoc analysis. A significance criterion of p , 0.05 was used for all statistical analyses. 3.
Results
Head flexion angle ranged from 37.28 (10th percentile, standing) to 46.88 (90th percentile, sitting) when conducting text messaging, from 33.48 (10th percentile, standing) to 42.58 (90th percentile, sitting) in web browsing and from 30.28 (10th percentile, standing) to 44.38 (90th percentile, sitting) in video watching (see Figure 2). In general, participants flexed the head more while using the smartphone in sitting compared to standing. Text messaging resulted in greater head flexion than that of the other tasks. Results of ANOVA show that the head flexion angles were significantly different between standing and sitting (e.g. posture) across all tasks (see Table 2). When conducting text messaging while sitting, participants flexed 10– 14% more in average than in standing. Similarly, sitting posture resulted in 4 – 6% and 23– 24% more head flexion than standing in webbrowsing and video-watching tasks, respectively. Head flexion angle also varied between the three tasks, but significant differences were found only for the 10th percentile and 50th percentile angles. Tukey’s post hoc analysis results show that the 10th percentile and 50th percentile head flexion angles of text messaging were significantly greater than that of web browsing and video watching. 4.
Discussion
The main findings of this study include the range of head flexion of smartphone users when they were conducting common smartphone tasks (texting, web browsing and video watching) in sitting and in standing. It was found that participants of this study maintained head flexion of at least 308 from vertical when using smartphones, and it varied significantly between tasks and between posture conditions. Results of this study suggest that the use of smartphone in sitting or standing without a supporting surface could make greater physical stresses on the neck musculature compared to when using desktop or laptop PCs. Head flexion angle or
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Ergonomics
Figure 2. Head flexion angles. (a) Mean and standard deviation of three percentile values separated by task and posture. (b) Overlaid mean values.
224 Table 2.
S. Lee et al. Statistical analysis results (repeated measures ANOVA p-values). 10th percentile
50th percentile
90th percentile
0.004 0.009 0.367
0.025 0.003 0.361
0.080 0.004 0.386
Task Posture Task £ posture
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Note: Posture is sitting versus standing; N ¼ 18.
head tilt angle of PC users has been reported to be approximately 208 or less from vertical when using desktop PCs or laptop computers on a desk, and between 208 and 258 from vertical when using laptop computers in non-desk settings such as on the lap (Gold et al. 2012; Moffet et al. 2002; Turville et al. 1998). Findings in previous research have suggested that the head flexion posture of PC users, if maintained for a prolonged period of time of computer use, could contribute to the development of musculoskeletal problems and fatigue in the neck (Arie¨ns et al. 2001; Sommerich, Joines, and Psihogios 2001; Straker et al. 2009). Compared to PC users in the previous studies, participants of the current study made greater head flexion (33.3 –44.88, 50th percentile angle) while using smartphones and it suggests that biomechanical loads on the neck musculature would be greater for smartphone users than for PC users who conduct typical video display terminal (VDT) tasks. The amount of head flexion observed from the participants of the current study could also be compared with that of tablet PC users. In a recent experimental study that examined head and neck postures of tablet PC users, participants who were using tablet PCs on the lap or a table in reclined sitting maintained the head flexion of approximately 15– 258 beyond neutral postures (Young et al. 2012). Since the difference in head flexion between the current study and Young et al.’s (2012) research was mainly attributable to the differences in usage conditions (unsupported sitting vs. reclined sitting; without a table vs. with a table), it is difficult to conclude that smartphones cause larger head flexion than tablet PCs in general. When used in standing or sitting without a table, as was tested in the current study, tablet PCs may drive users to flex the head forward as much as what smartphones would do. Further research regarding usage patterns and user behaviours of smartphones and tablet PCs is necessary to determine relative risks of smartphone use to tablet PC use. In this study, it was found that the amount of head forward flexion of smartphone users could vary depending on what they do with the smartphone and in which posture they use the smartphone. Participants of this study flexed the head significantly more when they were conducting text messaging compared to when conducting web browsing or video watching. The differences in the head flexion angle between the three smartphone tasks could be attributable to the way of holding the smartphone. When typing text messages using the touch smartphone, participants might hold the phone with both hands and type texts using both thumbs. To reduce biomechanical loads on the shoulder joint and minimise fatigue development in the neighbouring muscles in the two-handed use, they might have lowered the smartphone and it resulted in the large head flexion angle to look down the smartphone. In video watching, to the contrary, participants might hold the device with one hand only and simply watch video clips without finger touch interactions with the phone. Web-browsing task might require participants to hold the phone with one hand to conduct touch interactions (tapping, scrolling) or both hands to type keywords, depending on the type of activities. The less frequent two-handed use in the web-browsing and video-watching tasks compared to text messaging could then lead to the higher placement of the smartphone compared to when text messaging, resulting in less head flexion. Head flexion angle variables were significantly different between the two posture conditions as well as between tasks. Participants consistently flexed the head significantly less in standing compared to sitting, and it might be attributable to postural instability associated with the head flexion in standing. Flexing or extending the head from an upright neutral posture in standing is known to degrade postural stability (Buckley et al. 2005). Participants of this study might have attempted to minimise the head flexion to avoid the postural instability, and it might be responsible for the differences in the amount of head flexion between standing and sitting conditions. In addition, participants might have also extended the upper body slightly backwards to counterbalance the mass of the head in head flexion. It could have resulted in the less head flexion angle compared to sitting conditions. These suppositions, however, need to be confirmed in future research with a full body posture analysis and body balance evaluation. Among the three tasks that were tested in this study, text messaging caused the largest head flexion and it may explain why the neck pain symptoms of heavy smartphone users are often called ‘text neck’. The longer usage duration and higher frequency of use, together with the larger head flexion in text messaging, may indicate that text messaging could be a key risk factor for neck problems of smartphone users. This result implies the importance of ergonomic interventions to reduce or eliminate either the long duration or large head flexion associated with text messaging. Periodic rest breaks with an aid by head posture monitoring applications could be an efficient low-cost recommendation to lessen the cumulative biomechanical stress from the intensive text messaging.
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There are several limitations that need to be addressed before making generalisable conclusions. First, direct comparison with the results of previous studies needs careful considerations of the differences in measurement methods. Head or neck flexion in the cited references was quantified by the angle of a line connecting the C7 and tragus with respect to vertical (craniovertebral angle), while the head flexion of the current study was determined from the rotation angle of the head rigid body from vertical. Since the craniovertebral angle includes an angle added by the anterior placement of the head (forward head posture), it could have recorded greater head flexion angle than what the head rotation of the current study could produce (Cheung Lau, Wing Chiu, and Lam 2009). Second, participants of this study were instructed not to lean against the chair backrest while making a reference posture and when conducting tasks with the smartphone. This requirement could minimise any confounding effects of the shape and design of the backrest, but it might not be a common condition of smartphone use and also might have produced variability in the reference posture between participants. Lastly, task duration (2 minutes per task) might not be long enough to see the occurrence of fatigue and resultant posture changes over time. Future research should include longer task duration and more task conditions such as reclined sitting and walking to cover more various situations of smartphone use. In summary, results of this study show that the amount of head flexion of smartphone users varied between task types and posture conditions, and the range of head flexion was greater than what PC users used to make when conducting typical VDT tasks. Future research should include longer task duration and more realistic usage conditions to strengthen the findings of this study. Specifically, unobtrusive recording of head posture for a prolonged period of time in daily activities may be useful to understand actual usage patterns and head posture of smartphone users.
Funding This work was supported by the 2012 Creativity & Innovation Research Fund [1.130072] of UNIST (Ulsan National Institute of Science and Technology).
References Analytics, Flurry. 2013. “Mobile Use Grows 115% in 2013, Propelled by Messaging Apps.” Flurry Analytics. Accessed April 30. http:// www.flurry.com/bid/103601/Mobile-Use-Grows-115-in-2013-Propelled-by-Messaging-Apps#.U2Bvuv6KC8E ANSI/HFES. 2007. ANSI/HFES 100-2007 Human factors Engineering of Computer Workstation. Santa Monica, CA: Human Factors and Ergonomics Society. Arie¨ns, G. A. M., P. M. Bongers, M. Douwes, M. C. Miedema, W. E. Hoogendoorn, G. van der Wal, L. M. Bouter, and W. van Mechelen. 2001. “Are Neck Flexion, Neck Rotation, and Sitting at Work Risk Factors for Neck Pain? Results of a Prospective Cohort Study.” Occupational and Environmental Medicine 58 (3): 200– 207. doi:10.1136/oem.58.3.200. Berolo, S., R. P. Wells, and B. C. Amick 3rd. 2011. “Musculoskeletal Symptoms among Mobile Hand-Held Device Users and Their Relationship to Device Use: A Preliminary Study in a Canadian University Population.” Applied Ergonomics 42 (2): 371– 378. doi:10.1016/j.apergo.2010.08.010. Buckley, J. G., V. Anand, A. Scally, and D. B. Elliott. 2005. “Does Head Extension and Flexion Increase Postural Instability in Elderly Subjects When Visual Information Is Kept Constant?” Gait Posture 21 (1): 59 – 64. doi:10.1016/j.gaitpost.2003.11.005. Cheung Lau, H. M., T. T. Wing Chiu, and T. H. Lam. 2009. “Clinical Measurement of Craniovertebral Angle by Electronic Head Posture Instrument: A Test of Reliability and Validity.” Manual Therapy 14 (4): 363– 368. doi:10.1016/j.math.2008.05.004. eMarketer. Social Usage Involves More Platforms, More Often 2013. Gold, J. E., B. Driban J, V. R. Yingling, and E. Komaroff. 2012. “Characterization of Posture and Comfort in Laptop Users in Non-desk Settings.” Applied Ergonomics 43 (2): 392– 399. doi:10.1016/j.apergo.2011.06.014. Google. Our Mobile Planet. Google 2013. IDC. Always Connected: How Smartphones and Social Keep Us Engaged. Report 2013. Kothiyal, K., and A. M. Bjornerem. 2009. “Effects of Computer Monitor Setting on Muscular Activity, User Comfort and Acceptability in Office Work.” Work 32 (2): 155–163. doi:10.3233/WOR-2009-0801. Maniwa, Hiroki, Kentaro Kotani, Satoshi Suzuki, and Takafumi Asao. 2013. “Changes in Posture of the Upper Extremity Through the Use of Various Sizes of Tablets and Characters.” In Human Interface and the Management of Information. Information and Interaction Design, edited by Sakae Yamamoto, 89 –96. Berlin: Springer-Verlag. Moffet, H., M. Hagberg, E. Hansson-Risberg, and L. Karlqvist. 2002. “Influence of Laptop Computer Design and Working Position on Physical Exposure Variables.” Clinical Biomechanics 17 (5):, pp. 368– 375. doi:10.1016/S0021-9290(02)00062-3. Psihogios, J. P., C. M. Sommerich, G. A. Mirka, and S. D. Moon. 2001. “A Field Evaluation of Monitor Placement Effects in VDT Users.” Applied Ergonomics 32 (4): 313–325. Seghers, J., A. Jochem, and A. Spaepen. 2003. “Posture, Muscle Activity and Muscle Fatigue in Prolonged VDT Work at Different Screen Height Settings.” Ergonomics 46 (7): 714– 730. doi:10.1016/S0021-9290(02)00062-3. Sommerich, C. M., S. M. Joines, and J. P. Psihogios. 2001. “Effects of Computer Monitor Viewing Angle and Related Factors on Strain, Performance, and Preference Outcomes.” Human Factors 43 (1): 39 – 55.
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Straker, L., R. Skoss, A. Burnett, and R. Burgess-Limerick. 2009. “Effect of Visual Display Height on Modelled Upper and Lower Cervical Gravitational Moment, Muscle Capacity and Relative Strain.” Ergonomics 52 (2): 204– 221. doi:10.1080/001401 30802331609. Turville, K. L., J. P. Psihogios, T. R. Ulmer, and G. A. Mirka. 1998. “The Effects of Video Display Terminal Height on the Operator: A Comparison of the 15 Degree and 40 Degree Recommendations.” Applied Ergonomics 29 (4): 239– 246. Young, J. G., M. Trudeau, D. Odell, K. Marinelli, and J. T. Dennerlein. 2012. “Touch-Screen Tablet User Configurations and CaseSupported Tilt Affect Head and Neck Flexion Angles.” Work 41 (1): 81 –91. doi:10.3233/WOR-2012-1337.
ISSN: 0014-0139 (Print) 1366-5847 (Online) Journal homepage: http://www.tandfonline.com/loi/terg20
Head flexion angle while using a smartphone Sojeong Lee, Hwayeong Kang & Gwanseob Shin To cite this article: Sojeong Lee, Hwayeong Kang & Gwanseob Shin (2015) Head flexion angle while using a smartphone, Ergonomics, 58:2, 220-226, DOI: 10.1080/00140139.2014.967311 To link to this article: http://dx.doi.org/10.1080/00140139.2014.967311
Published online: 17 Oct 2014.
Submit your article to this journal
Article views: 1735
View related articles
View Crossmark data
Citing articles: 22 View citing articles
Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=terg20 Download by: [Australian Catholic University]
Date: 10 October 2017, At: 06:04
Ergonomics, 2015 Vol. 58, No. 2, 220–226, http://dx.doi.org/10.1080/00140139.2014.967311
Head flexion angle while using a smartphone Sojeong Lee, Hwayeong Kang and Gwanseob Shin* Department of Human and Systems Engineering, Ulsan National Institute of Science and Technology, Ulsan, Korea
Downloaded by [Australian Catholic University] at 06:04 10 October 2017
(Received 13 June 2014; accepted 15 September 2014) Repetitive or prolonged head flexion posture while using a smartphone is known as one of risk factors for pain symptoms in the neck. To quantitatively assess the amount and range of head flexion of smartphone users, head forward flexion angle was measured from 18 participants when they were conducing three common smartphone tasks (text messaging, web browsing, video watching) while sitting and standing in a laboratory setting. It was found that participants maintained head flexion of 33 – 458 (50th percentile angle) from vertical when using the smartphone. The head flexion angle was significantly larger ( p , 0.05) for text messaging than for the other tasks, and significantly larger while sitting than while standing. Study results suggest that text messaging, which is one of the most frequently used app categories of smartphone, could be a main contributing factor to the occurrence of neck pain of heavy smartphone users. Practitioner Summary: In this laboratory study, the severity of head flexion of smartphone users was quantitatively evaluated when conducting text messaging, web browsing and video watching while sitting and standing. Study results indicate that text messaging while sitting caused the largest head flexion than that of other task conditions. Keywords: head flexion; smartphone; neck pain; text neck
1. Introduction Smartphones are mobile information and communicative devices that have an operating system, more powerful computing capacity and diverse software applications than low-end mobile phones that contain a limited set of functions. Since the release of Apple iPhone on the market, the number of smartphone users has continuously increased around the world. According to the recent data of market research institutions, the penetration rate of smartphone has already exceeded 56% in top 15 countries in 2013 (Google 2013). Frequency of use and reliance on the smartphone has also been on the rise. Per market research reports, more than 65% of smartphone owners in USA use their phones every day, and spend one to 2 hours daily on average (Analytics 2013; Google 2013). Specifically, a recent survey reports that mobile device users spend more than 20 hours a week emailing, text messaging and using social network services, indicating their heavy reliance on smartphones to connect and communicate with others (eMarketer 2013). With the growing use of smartphones, concerns of musculoskeletal problems associated with the intensive use of the smartphone have also increased. Recent investigations showed that mobile device users tend to report pain symptoms on the neck, shoulder and thumb, and the severity of the symptoms increased with the total time spent using their mobile devices (Berolo, Wells, and Amick 2011). Pain in the neck of smartphone users, which has often been referred to ‘text neck’, has received more public attention recently due to the growing use of mobile devices in head forward flexion postures. Smartphone users typically hold the device using one or two hands below their eye height, look down at the device and type or touch the touch-screen display mostly using the thumb (Gold et al. 2012). They tend to maintain the head forward flexion posture even with the existence of pain symptoms or discomfort on the neck (Maniwa et al. 2013), and it has been suggested that the prolonged and/or frequent use of the smartphone with the severe head flexion posture could be one of main contributing factors to the prevalence of neck pain symptoms of smartphone users. Physical stresses on the neck and resultant health concerns associated with the frequent and/or prolonged head flexion have been frequently studied previously in research about the position of personal computer (PC) displays. Placing a computer display too high or low is known to cause awkward postures of the user and produce neck muscle fatigue and discomfort after a prolonged period of use (Kothiyal and Bjornerem 2009; Seghers, Jochem, and Spaepen 2003). To avoid or minimise the occurrence of the neck pain of PC users, it has often been recommended to keep the head posture near neutral by placing the top of the display at or slightly below eye level (ANSI/HFES 2007).
*Corresponding author. Email: [email protected] q 2014 Taylor & Francis
Ergonomics
221
While head posture and movement patterns of PC users have been studied frequently, research regarding the posture of smartphone users has been relatively scarce. Quantitative evaluation of the duration, frequency and severity of the head flexion posture during the use of smartphone can produce important information for studying the associations between the intensive use of smartphone and the occurrence of neck pain symptoms. In addition, such information may be used to develop ergonomics guidelines that can reduce the risk of neck pain symptoms associated with the use of smartphone. As an attempt to explore the head posture of smartphone users, this study was aimed to quantify the amount of head flexion of smartphone users in a controlled laboratory setting where participants completed three common smartphone tasks (texting, web browsing and video watching) in two different posture conditions (while sitting and standing). The three task conditions were specifically chosen as they were known as the most frequently used application categories of smartphone use (IDC 2013). This study also sought to determine whether there were any differences in the amount of head flexion between the three tasks and/or two posture conditions. The null hypothesis was that the head flexion angle in the sagittal plane was the same between the three tasks and between the two posture conditions.
Downloaded by [Australian Catholic University] at 06:04 10 October 2017
2. 2.1.
Methods Participants
Eighteen young participants (nine females, nine males) who had no physical difficulties in using smartphones while sitting and standing were recruited (see Table 1). All participants had at least one year of experience with touch-screen smartphones. Prior to the beginning of data collection, each participant provided an informed consent on a protocol approved by the institutional review board. 2.2.
Data collection
In an isolated laboratory space, each participant performed text-messaging, web-browsing and video-watching tasks in a fixed order using his/her own smartphone, while the angle of head forward flexion was measured using a three-dimensional motion capture system. Each task continued for 2 minutes and repeated twice, once while sitting on a chair and then while standing. A rest break of at least 1 minute was given after each task to minimise any carry-over effects between tasks. While conducting each task, head flexion angle was measured using a three-dimensional motion capture system. Reflective markers were attached to the skin at the forehead, and left and right tragus, and the markers were tracked by 12 motion capture cameras (OptiTrack, Naturepoint, Oregon, USA) at the sampling rate of 100 Hz. A head rigid body was created from the three markers and its forward tilt angle from global vertical on the sagittal plane was registered as the head flexion angle (Psihogios et al. 2001; Sommerich, Joines, and Psihogios 2001; Young et al. 2012). A reference angle (08 head flexion) was captured when the participant was sitting up straight at the beginning of the experiment. The subject was instructed to sit on a chair without slouching the back or leaning against the backrest of the chair, and look straight ahead. During the text-messaging task, the participant was asked to communicate with an experimenter using a text-messaging application of his/her choice for 2 minutes continuously. In the web-browsing task, the participant conducted simple web browsing with self-selected contents and a web browser. The participant was instructed to avoid continuous watching or repetitive texting to differentiate the web-browsing task from the text-messaging or video-watching tasks. In the videowatching task, the participant chose a video clip prior to data collection and then watched the clip continuously without any touch gestures during the 2-minute task duration. While conducting a task in sitting, the participant was allowed to make minor head and upper extremity movements, but asked to remain seated without leaning backwards against the backrest of the chair to avoid any confounding effects associated with the angle or shape of the chair backrest (see Figure 1). In standing conditions, the participant was asked to maintain a comfortable standing posture without lower limb movements. 2.3. Data analysis Independent variables of this study included the three smartphone tasks (text messaging, web browsing, video watching) and two posture conditions (sitting, standing). For each of the six conditions (3 tasks £ 2 posture conditions), the 10th, 50th Table 1.
Female Male All
Mean and standard deviation of participant information. Age (yrs)
Height (cm)
Weight (kg)
19.8 (0.92) 20.7 (1.77) 20.2 (1.48)
161.1 (4.17) 175.3 (5.39) 168.2 (8.56)
55.0 (5.55) 66.4 (6.05) 60.7 (8.12)
Downloaded by [Australian Catholic University] at 06:04 10 October 2017
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Figure 1.
Text messaging while sitting on a chair.
and 90th percentile values of the head flexion angle were computed from the probability distribution. Effects of ‘task’ and ‘posture’ on the head flexion angle data were evaluated using the repeated measures two-way analysis of variance (ANOVA). Differences in the head flexion angle between the three tasks were further examined by Tukey’s post hoc analysis. A significance criterion of p , 0.05 was used for all statistical analyses. 3.
Results
Head flexion angle ranged from 37.28 (10th percentile, standing) to 46.88 (90th percentile, sitting) when conducting text messaging, from 33.48 (10th percentile, standing) to 42.58 (90th percentile, sitting) in web browsing and from 30.28 (10th percentile, standing) to 44.38 (90th percentile, sitting) in video watching (see Figure 2). In general, participants flexed the head more while using the smartphone in sitting compared to standing. Text messaging resulted in greater head flexion than that of the other tasks. Results of ANOVA show that the head flexion angles were significantly different between standing and sitting (e.g. posture) across all tasks (see Table 2). When conducting text messaging while sitting, participants flexed 10– 14% more in average than in standing. Similarly, sitting posture resulted in 4 – 6% and 23– 24% more head flexion than standing in webbrowsing and video-watching tasks, respectively. Head flexion angle also varied between the three tasks, but significant differences were found only for the 10th percentile and 50th percentile angles. Tukey’s post hoc analysis results show that the 10th percentile and 50th percentile head flexion angles of text messaging were significantly greater than that of web browsing and video watching. 4.
Discussion
The main findings of this study include the range of head flexion of smartphone users when they were conducting common smartphone tasks (texting, web browsing and video watching) in sitting and in standing. It was found that participants of this study maintained head flexion of at least 308 from vertical when using smartphones, and it varied significantly between tasks and between posture conditions. Results of this study suggest that the use of smartphone in sitting or standing without a supporting surface could make greater physical stresses on the neck musculature compared to when using desktop or laptop PCs. Head flexion angle or
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Figure 2. Head flexion angles. (a) Mean and standard deviation of three percentile values separated by task and posture. (b) Overlaid mean values.
224 Table 2.
S. Lee et al. Statistical analysis results (repeated measures ANOVA p-values). 10th percentile
50th percentile
90th percentile
0.004 0.009 0.367
0.025 0.003 0.361
0.080 0.004 0.386
Task Posture Task £ posture
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Note: Posture is sitting versus standing; N ¼ 18.
head tilt angle of PC users has been reported to be approximately 208 or less from vertical when using desktop PCs or laptop computers on a desk, and between 208 and 258 from vertical when using laptop computers in non-desk settings such as on the lap (Gold et al. 2012; Moffet et al. 2002; Turville et al. 1998). Findings in previous research have suggested that the head flexion posture of PC users, if maintained for a prolonged period of time of computer use, could contribute to the development of musculoskeletal problems and fatigue in the neck (Arie¨ns et al. 2001; Sommerich, Joines, and Psihogios 2001; Straker et al. 2009). Compared to PC users in the previous studies, participants of the current study made greater head flexion (33.3 –44.88, 50th percentile angle) while using smartphones and it suggests that biomechanical loads on the neck musculature would be greater for smartphone users than for PC users who conduct typical video display terminal (VDT) tasks. The amount of head flexion observed from the participants of the current study could also be compared with that of tablet PC users. In a recent experimental study that examined head and neck postures of tablet PC users, participants who were using tablet PCs on the lap or a table in reclined sitting maintained the head flexion of approximately 15– 258 beyond neutral postures (Young et al. 2012). Since the difference in head flexion between the current study and Young et al.’s (2012) research was mainly attributable to the differences in usage conditions (unsupported sitting vs. reclined sitting; without a table vs. with a table), it is difficult to conclude that smartphones cause larger head flexion than tablet PCs in general. When used in standing or sitting without a table, as was tested in the current study, tablet PCs may drive users to flex the head forward as much as what smartphones would do. Further research regarding usage patterns and user behaviours of smartphones and tablet PCs is necessary to determine relative risks of smartphone use to tablet PC use. In this study, it was found that the amount of head forward flexion of smartphone users could vary depending on what they do with the smartphone and in which posture they use the smartphone. Participants of this study flexed the head significantly more when they were conducting text messaging compared to when conducting web browsing or video watching. The differences in the head flexion angle between the three smartphone tasks could be attributable to the way of holding the smartphone. When typing text messages using the touch smartphone, participants might hold the phone with both hands and type texts using both thumbs. To reduce biomechanical loads on the shoulder joint and minimise fatigue development in the neighbouring muscles in the two-handed use, they might have lowered the smartphone and it resulted in the large head flexion angle to look down the smartphone. In video watching, to the contrary, participants might hold the device with one hand only and simply watch video clips without finger touch interactions with the phone. Web-browsing task might require participants to hold the phone with one hand to conduct touch interactions (tapping, scrolling) or both hands to type keywords, depending on the type of activities. The less frequent two-handed use in the web-browsing and video-watching tasks compared to text messaging could then lead to the higher placement of the smartphone compared to when text messaging, resulting in less head flexion. Head flexion angle variables were significantly different between the two posture conditions as well as between tasks. Participants consistently flexed the head significantly less in standing compared to sitting, and it might be attributable to postural instability associated with the head flexion in standing. Flexing or extending the head from an upright neutral posture in standing is known to degrade postural stability (Buckley et al. 2005). Participants of this study might have attempted to minimise the head flexion to avoid the postural instability, and it might be responsible for the differences in the amount of head flexion between standing and sitting conditions. In addition, participants might have also extended the upper body slightly backwards to counterbalance the mass of the head in head flexion. It could have resulted in the less head flexion angle compared to sitting conditions. These suppositions, however, need to be confirmed in future research with a full body posture analysis and body balance evaluation. Among the three tasks that were tested in this study, text messaging caused the largest head flexion and it may explain why the neck pain symptoms of heavy smartphone users are often called ‘text neck’. The longer usage duration and higher frequency of use, together with the larger head flexion in text messaging, may indicate that text messaging could be a key risk factor for neck problems of smartphone users. This result implies the importance of ergonomic interventions to reduce or eliminate either the long duration or large head flexion associated with text messaging. Periodic rest breaks with an aid by head posture monitoring applications could be an efficient low-cost recommendation to lessen the cumulative biomechanical stress from the intensive text messaging.
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There are several limitations that need to be addressed before making generalisable conclusions. First, direct comparison with the results of previous studies needs careful considerations of the differences in measurement methods. Head or neck flexion in the cited references was quantified by the angle of a line connecting the C7 and tragus with respect to vertical (craniovertebral angle), while the head flexion of the current study was determined from the rotation angle of the head rigid body from vertical. Since the craniovertebral angle includes an angle added by the anterior placement of the head (forward head posture), it could have recorded greater head flexion angle than what the head rotation of the current study could produce (Cheung Lau, Wing Chiu, and Lam 2009). Second, participants of this study were instructed not to lean against the chair backrest while making a reference posture and when conducting tasks with the smartphone. This requirement could minimise any confounding effects of the shape and design of the backrest, but it might not be a common condition of smartphone use and also might have produced variability in the reference posture between participants. Lastly, task duration (2 minutes per task) might not be long enough to see the occurrence of fatigue and resultant posture changes over time. Future research should include longer task duration and more task conditions such as reclined sitting and walking to cover more various situations of smartphone use. In summary, results of this study show that the amount of head flexion of smartphone users varied between task types and posture conditions, and the range of head flexion was greater than what PC users used to make when conducting typical VDT tasks. Future research should include longer task duration and more realistic usage conditions to strengthen the findings of this study. Specifically, unobtrusive recording of head posture for a prolonged period of time in daily activities may be useful to understand actual usage patterns and head posture of smartphone users.
Funding This work was supported by the 2012 Creativity & Innovation Research Fund [1.130072] of UNIST (Ulsan National Institute of Science and Technology).
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