Smartphone and Musculoskeletal Risk Factors: A Systematic Review

Introduction

According to the United Nations, the International Telecommunication Union reported that about four billion cell phones were produced worldwide in 2008 and it is predicted to pass 2.87 billion user by 2020 [1]. Adults, adolescents, and children have at least one smartphone (SP) or another kind of handheld device. The popularity of SPs can be attributed to their low price, light weight, and ease of use. These devices have afforded people multiple applications, on-demand access to the Internet, and convenient texting, emailing, and calling, thus motivating users, especially teenagers and young adults, to spend considerable time on their devices. A study indicated that the frequency of daily SP use has increased, with Americans spending at least 2 hours per day and 20 hours per week on their phones [2].

The use of SPs is going to have a specific posture that involves neck muscles and thumb. In this position, users usually bend their neck and thumb near to its range of motion, most of the times [3]. Working with mobile phones does not require much physical power, but holding and using the devices require a basic load and force, especially when users carry out repetitive and high- speed tasks [4]. It is revealed that, Mobile phone users hold their devices below eye level while texting, a task that requires neck flexion.

Prolonged neck flexion while using SPs increases the risk of neck pain as it injures the vertebral bodies of the neck [2, 5, 6]. Gustaffson, et al. [7] in a cohort study on 7,092 SP users showed that after one year, the participants with no history of musculoskeletal disorders experienced pain in their hands and fingers, and the participants with a history of pain exhibited susceptibility to chronic neck and upper back disorders. Additionally, the findings also indicated that the continuous use of mobile phones in an awkward posture lead to long-term effects on the neck and upper extremities of the participants. Similarly, electromyography studies of healthy users conducted by Xie, et al. [8] and Shieh [9] showed that the participants felt pain in the neck and shoulders while texting.

In electromyography studies it was determined that during working by smartphones involve muscle activity of the abductor pollicis longus (APL), abductor pollicis brevis (APB), extensor digitorum (ED), and left and right trapezius are increased. Moreover, the speed of finger movements during abduction/adduction is higher than the speed of movement during flexion/extension [10].

Keypads and typing styles are other factors that need to be examined in relation to the above mentioned effects of SP use. Kietrys, et al. [11] showed that no association exists between keypads and typing styles but also indicated that these factors affect finger muscles instead of the trapezius [11]. According to the author, typing style influences neck flexion, whereas keypad type does not. The author also found that engaging with large touchscreens involves considerable neck flexion and high muscle activity in finger flexors, wrist extensors, and the trapezius. A study focusing on posture, typing style, and gender revealed that 91% of users exhibited a flexed neck posture and that 90.3% of users use their wrists in an unnatural position. A high proportion of the men exhibited shoulder protraction, and a high proportion of the women positioned their elbows at an angle less than 90 degrees, especially while standing. Over 46.1% of the subjects typed with both thumbs, and one-third typed with only the right thumb. No difference was observed between typing style and gender [12].

The studies discussed above can serve as a guide for professionals and practitioners in defining the problems related to SP use and establishing the best approaches to preventing such problems as these studies demonstrate the potential damage mechanisms that underlie texting [13]. The aim of the current research was to systematically evaluate previous studies on the manner by which SPs are used and such musculoskeletal risk factors that explored by objective methods. The results are expected to illuminate SP-related issues that have not been considered by scientists in the fields of technology and health care.

Methodology

Articles on the level of muscle activity and the kinematics of upper extremities during SP use were searched over Google Scholar, PubMed, Research Gate, and Science Direct (2007–2017) on July 2017. Figure 1 graphically illustrates the search strategy implemented in this work. The search was conducted using a combination or string of suggested keywords by experts in biomechanics and ergonomics keywords as follows: “(musculoskeletal disorders OR pain OR discomfort OR disorder) AND (upper extremities OR neck OR hand OR wrist) AND (electromyography OR motion capture/analyzer OR electrogoniometer) AND (smartphone OR mobile phone OR cellphone OR handheld device).” The inclusion criteria were studies involving SP users and not tablet, notebook, or laptop (etc.) users; articles published in English; case studies, cross-sectional research, and cohort studies; and studies involving the use of electromyography, electrogoniometers, and motion capture. Literature reviews and pre-post interventional studies were also excluded from the review. The database search yielded 513 articles, out of which 185 were excluded because of duplication and repetition. Initial 513 articles, out of which 185 were excluded because of duplication and repetition. After the remaining articles were read, a further 287 were eliminated because these studies used only the questionnaire method, disregarded the use of SPs in their examinations, involved psychosocial investigations and mental patients, or conducted only literature reviews. This left us with a final sample of 18 articles.

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Reading and evaluating previous studies on the issue of interest require the supervision of professionals in biomechanics, anatomy, and ergonomics (posture) and familiarity with electromyography, motion capture, and electrogoniometers. We therefore asked a biomechanics expert and an ergonomics specialist to assist us in reading and selecting the full text of 41 selected articles.

Results

The results of the reviewed studies were organized under the following categories: general characteristics, SP features, environmental characteristics, postural issues of SP users, muscle activity and fatigue, and duration and questionnaire.

Discussion

The main goal of this paper was to reveal the existed study over SPs and reviewing its relationships between duration, feature of SP and MSDs. Most of the selected studies were laboratories studies that examined the effect of different tasks, SP features, condition of study like kind of SP grip, and different level of muscular activity. Selecting laboratories studies decreases chance of bias which is very common in questionnaire based studies. EMG and motion capture were two used devices in studies, also mistakes in detecting the belly of muscles and chance of cross-talk in EMG is not negligible.

The review revealed evidence that different tasks and typing styles are associated with neck flexion and musculoskeletal disorders among users of SPs. Repetitive and prolonged activity is known as the most impressive factor associated with any kind of handheld device. Users continue using their devices in the manner that they are accustomed to until they feel pain and discomfort in the neck, shoulders, hands, and fingers possibly and unfortunately no threshold for SP use has been suggested. Among the SP features examined in the reviewed literature, touchscreen size was a major factor that affected the users’ musculoskeletal systems.

It is obvious that large screen size devices contain more materials, so resulted in a heavier SP; thus, size and weight are significantly related to increased neck flexion. A previous study on older technologies identified notebook and computer size as a factor in musculoskeletal disorders among users; such factor was also considered by Szelto and Lee, who found that a reduction in display size induced higher cervical and thoracic flexion [28]. Additionally, muscle activity was highly associated with SP features, SP tasks, and postures.

Increased activity in the trapezius muscle was observed due to the growing size of touchscreens, not using of forearm support, task duration, and use of a one-handed grip [8, 11, 15, 22, 27]. The vicious circle model that muscle pain likely leads to enhanced muscle activity during muscle work, lead the muscles to increase their activity.

On this basis, one study revealed that high electromyography activity occurred in the neck and shoulder muscles of subjects with neck and shoulder complaints [10]. The association among keyboard type, neck flexion, and muscle activity suggested that physical keypads more strongly influence suggested that physical keypads more strongly influence activity in hand muscles and stimulate lower wrist flexion (angle) than do touchscreens [11]. Low performance, high FDI activity, slow tapping, and tiredness in the thumbs can also be attributed to the use of small buttons (d=3 millimeters) [20, 23]. By investigating about the effect of different tasks, among all popular tasks in smartphone users, texting and typing were the strongest factor for neck flexion [2, 26].

The design of SPs was informed by evidence regarding the risk factors associated with older technologies; now, the nature of SP use is that tasks are repetitive and require lengthy periods of holding a device [29, 30]. Repetitiveness and prolonged gripping while texting cause hand pain-a phenomenon so prevalent among BlackBerry users that that condition was accorded its own term, that is, “BlackBerry thumb” (or hand) [29, 30]. Finger and wrist positions influence deformity in the median nerve, which causes numbness and clumsiness in hand movements involved in daily routines [12, 31, 32, 33]. The SP-related neck pain known as text neck syndrome is a result of maintained and awkward posture while texting, especially when a user is holding a heavy SP for a long period as his/her neck is flexed [2, 34].

Maintained and static posture and prolonged duration of using SP show the occurrence of pain and discomfort, visual analog scale (VAS) is a simple way to evaluate effort in participants. The VAS reports of the participants in the reviewed studies indicated that increased SP usage elevated pain levels [24]; and that neck pain was greater than trunk pain when SPs were positioned on the lap than at desk level, which means working on SP at a level closer to the eye-height level provides lower neck flexion in users [5, 15].

A history of musculoskeletal disorders was also associated with discomfort scores; symptomatic subjects presented more severe pain in the neck and shoulder than asymptomatic participants, especially when the participants used a single-handed grip [8]. A one-year follow up with the symptomatic and asymptomatic subjects showed an increase in neck and shoulder pain; however, the pain vanished after a five-year follow up in both groups possibly because of changes to phone model, texting style, and phone grip [7]. Pain among users of SPs is not studies on older technologies presented the same results, indicating that computer and notebook users are at risk of acute pain in the neck and shoulder [35, 36].

Generally, posture influenced an SP user’s joint motions. Sitting or standing and a one-handed or two- handed grip exerted various effects on neck and wrist deviations. Varying degrees of neck flexion were observed during different tasks, such as typing, video watching, and web browsing, but sitting generally induced higher neck flexion than did standing in all tasks, particularly during texting [2]. A two-handed grip induced higher wrist extension, abduction, and supination, but it also enabled a higher index of performance, faster movement, and greater stability [18].

Muscle activity was highly associated with SP features, SP tasks, and postures. Increased activity in the trapezius muscle was due to the growing size of touchscreens, not using of forearm support, task duration, and the use of a one-handed grip [8, 11, 15, 22, 37].

Stability and the rest were observed in the FDI and ED muscles during SP use. Pressing small buttons while using the medial side of the thumb directly influenced in increasing activity in the FDI. Activity in the ED muscle was highly related to efforts to stabilize an SP as the thumb moved at high speeds [14, 20, 22, 23]. On this basis, then, SP stabilization and speedy typing increased electromyographic activity in the ED muscle.

Interestingly, SP users are tend to keep their neck forward flexion posture even by increasing and existence of pain and discomfort on upper extremities and neck [2].

Most of the time, therefore, they may assume unnatural postures-an issue that may have been neglected by the majority of the literature. Usually, having unnatural neck posture occurs with increased flexion in the neck; many of the reviewed studies investigated neck flexion in relation to SP use, but only one examined the condition while users are sitting and standing [2].

Trudeau, et al. [38] and Kietrys, et al. [11] amined the issue during the performance of different tasks and under varying typing styles in separate studies, but this matter requires more accurate investigations with greater intervals than that adopted in conventional studies to ensure that muscle activity is close to daily activity and accurately determine the influence of muscle fatigue on neck flexion.

After evaluating the studies, we realized that none of them devoted attention to the influence of phone cases and accessories on gripping and neck flexion (Increased phone thickness may affect muscle activity). Limitations on selected articles are important to be clarified. Authors of selected studies were faced with some limitations like: natural position was defined 0 degrees in the neck region, but this degree is never possible in real life. Also, not only neck flexion needed to be measured, but also the effect of head rotation and lumbar flexion needed to be investigated in future studies [2, 19].

Studies mostly were invited participants in their 20s and results cannot be generalized for sample groups with different ages and various characteristics. [16, 17, 21, 25]. Participants were asked to work with SPs for a short time, which is done typically longer in daily usage [2, 5, 24]. Needing follow-up studies in different occasion like in transportation systems (subway or bus) and even walking [17].

Some of the selected studies claimed that small sample size was a remarkable limitation [15, 17, 18]. Another important factor was considering different functions like net browsing, gaming, reading, and video watching [2, 8]. Additionally, Differences in using Android, IOS, and Windows needed to be investigated too [8, 26]. Working with SP with non-dominant hand was not considered. Also, selecting data in landscape and portrait mode should be considered [18]. Furthermore, selected studies claimed that all features of SP like length, depth, back and front curvature, and weight needed to be under more investigations [16, 25, 26].

Having ample time before each investigation or prior experiences with devices in the study should be considered in further studies [11] Using self-reported questionnaires were considered as a limitation in order to increase chance of bias in the answers [39]. In postural issue, comparing posture of users with natural and normal postures with choosing postures by participants was not considered [24]. Another missed factor was having no approach in detecting threshold of injuries and muscular fatigue [23]. In selecting muscle for EMG tests, there was no possibility of choosing all muscles, so for example, among 8 muscles just 2 of them had been investigated [14, 22].

Limitation

The limitations in selecting articles are worth discussing. First, we excluded investigations based on the questionnaire method (which is a very popular approach in SP studies). We endeavored to prevent biases triggered by personal comments; for example, with respect to the daily use of mobile phones, the participants may have provided durations lower or greater than actual usage times; with regard to the self-expression of pain, they may have indicated less or more pain than the actual level experienced and thereby increased the probability of errors and biases in the obtained results. Second, all the risk factors discussed in the review were physical in scope, but the cognitive and general conditions of adult SP users (e.g., being elderly or impaired) were not negligible. In the initial sample, some studies focused on the usability of touchscreens, the performance of elderly SP users, the behavioral differences in using electronic devices, and deaf or blind users, but we excluded these studies from the review to evaluate the effect of SP on healthy and normal users [40, 41, 42, 43, 44, 45, 46, 47].

Conclusion

This systematic review identified SP features, environmental conditions, typing styles, grip styles (one- and two-handed), and task durations as some of the known risk factors that affect SP users’ musculoskeletal systems. In thumb, neck, wrist and shoulders. The duration of tasks in selected studies was in the range of 5 Sec. To 90 min. but studies revealed that users of SPs Usually spend from 2-5 hours per day on their SPs [2, 11, 18]. Interestingly, this small amount of time had shown a significant pain and discomfort in users and even 15 min gaming creates pain in neck and shoulders [15].

Spending long periods of time on a repetitive task and using certain muscles lead to pain and injuries in muscular fibers [24] so, it can infer that this symptom of MSDs is related to the time of SP usage. Regarding to Kim, et al. [24] task duration and repetitive tasks have influence on muscle fatigue, in prolonged activity more muscles involved in activity and finally fatigue occurs. It is clear that more studies are needed on Smartphones use with longer task duration than current studies and also further study will be beneficial in illuminating the current gap between tasks duration and muscle activity. It is revealed, no time limit is scientifically reported as a standard time for using SP. However, it’s logical to advise users to limit and alternate time, but users tend to continue using of SP even when they feel pain and discomfort in the neck [48].

There is a need of more investigations about the characteristics of the SP and its effects on the occurrence of musculoskeletal disorders. Lack of guidelines in designing of SP can be a factor in releasing SP by companies in various shapes and features; also it is an important factor that needs to be considered in future studies.