A radiographic investigation of cervical spine kinematics when reading a tablet in a reclined trunk position
Introduction
In the past five years, tablet ownership in the United States has risen from 4% to over 50% (Pew Research Center, 2015). Sixty-two percent of elementary through high school students use tablets regularly in school (Pearson, 2015). In elementary school, tablet usage has surpassed laptop usage (Pearson, 2015) and 49% of U.S workers use a tablet occupationally (Gallup, 2015). An observational study found that 68% of mobile device users report neck pain (Berolo et al., 2011), and a 5-year cohort study found a strong association between neck/upper back pain and high mobile phone use (Gustafsson et al., 2017). This may be because the screen and keys are on the same plane, and thus larger neck flexion angles are necessary to effectively interact with the device while still viewing the screen (Gustafsson et al., 2017). Shan et al., 2013 found that 44% of adolescent tablet users report neck or shoulder pain, and that number increases to 52% for users who spend more than 2 h daily with their tablets. Laboratory studies have found that handheld computer use is associated with higher neck flexion levels and muscle activity than seen with standard desktop computer monitors (Douglas and Gallagher, 2017; Straker et al., 2008; Young et al., 2012). Prolonged neck flexion is associated with increased symptom reporting, such as neck pain or fatigue (Ariëns et al., 2001), and studies show a positive correlation between a more forward head posture and neck pain (Silva et al., 2009; Lau et al., 2010). A limitation to many of these previous tablet studies is that they on only assessed an upright seated posture; however, studies have shown that people interact with their devices in a variety of seated postures (Steelcase, 2015). Given this limitation, the purpose of our study was to determine how a commonly reported sitting posture when using a tablet computer, semi-reclined sitting (Shan et al., 2013) influenced cervical spine angles.
A global posture study found that tablets have allowed users to back away from their desks and recline while using the device, something previously not accounted for in the design of workplace stations (Steelcase, 2015). The second most common position when using a tablet was seated with the hips in front of the shoulders (rotating the trunk posteriorly), in a “semi-reclined” posture. (Shan et al., 2013). Douglas and Gallagher (2017) found that reading a tablet with a semi-reclined trunk produced lower levels of muscle activity in the cervical extensor muscles. Weston et al. (2017) also found that participants who used a tablet for an hour in a reclined chair exhibited less discomfort than participants who used the tablet in a standard chair. A caveat to this is that the reclined chair also had arm rests that allowed the participants to support their arms and allowed them to position the tablet with a more neutral head position. A previous study that measured internal cervical spine angles during tablet usage (Vasavada et al., 2015) found that the gravitational demand (ratio of the gravitational moment to the muscle moment capacity) is higher when reading a tablet on a desk compared to a neutral posture. With extended tablet usage, this larger gravitational demand could lead to faster fatigue by the cervical extensor muscles (Enoka and Duchateau, 2008). It is possible that by positioning the head in a more neutral posture, which is accomplished by a reclined reading position, the risk of muscle fatigue could be minimized.
A potential risk of the semi-reclined tablet reading position is that it has been found to exhibit greater neck flexion angles compared to reading with an upright trunk (Douglas and Gallagher, 2017). Weston et al. (2017) also found larger neck flexion with tablet usage compared to a standard computer, but did not assess neck flexion angles between different trunk angles. This neck flexion may be initiated at the lower cervical and upper thoracic spine. As a result, the cervical extensor muscles, many of which originate on the lower cervical spine (Vasavada et al., 1998), may be passively stretched, ultimately leading to pain after extended use. Further, these muscles may experience changes in their moment arms, potentially diminishing their ability to counteract the moment produced by the head (Vasavada et al., 1998). Despite an increase in neck flexion, head posture in the semi-reclined position was shown to be similar to that when using a monitor (Douglas and Gallagher, 2017), which means that the upper cervical spine may remain extended while the lower cervical spine demonstrates a more flexed posture. This could change the muscles required to keep the head in an upright position. Douglas and Gallagher (2017) also found that neck extensor muscle activity was lower in a reclined position than in an upright tablet reading position. Decreased muscle activity could be due to a decrease in the moment arm of the head in this position, or a greater load being placed on deeper muscles and passive structures of the cervical spine.
The purpose of this study was to use radiographic measurements to determine how the cervical spine was influenced when reading a tablet in the lap during a reclined seated position. A radiographic study was used to provide a more detailed analysis of the cervical spine kinematics compared to what can be obtained using external motion capture techniques and builds on previous work (Douglas and Gallagher, 2017) that assessed semi-reclined sitting with motion capture. We hypothesized that:
- (1)
Participants viewing a tablet with trunk rotation would exhibit greater flexion in the lower cervical spine when compared to a neutral trunk position
- (2)
Increased trunk rotation would lead to a decreased moment arm of the head, and a more neutral skull angle relative to horizontal
- (3)
There will be greater extension in the upper intervertebral joint angles in the reading postures with trunk rotation, to compensate for the predicted lower cervical spine lordosis.
Section snippets
Approach
In this study, cervical spine radiographs were taken of each participant in five different postures, three of which were when reading a tablet from in the lap. Measurements of lower cervical lordosis, moment arm, skull angle, and intervertebral joint angles were compared between the five different postures.
Participant data
Twenty-two participants (11 male: height 181.6 ± 8.6 cm, weight 78.5 ± 12.1 kg, age 21.8 ± 1.3 years; 11 female: height 144.2 ± 6.0 cm, weight 65.4 ± 10.8 kg, age 20.7 ± 0.9 years) were
Results
For the centroid cervical lordosis angle and the moment arm measure, ten radiographs (2 participants * 5 conditions) were removed from the analysis because the C7 vertebra was not clear on the radiograph. As a result, 100 total radiographs were analyzed (20 participants * 5 radiographs each). The remaining measures included all 110 radiographs. The ICC for each outcome measure can be found in Table 1. Each variable had a normal distribution.
Discussion
The purpose of this study was to examine how the cervical spine is influenced by changes in trunk position when reading a tablet. Our first hypothesis was supported. Both the reclined and semi-reclined tablet reading positions saw increased flexion in the lower cervical spine compared to reading the tablet with an upright position. Our second hypothesis was also supported. Moment arm decreased as trunk rotation increased from the upright, to the semi-reclined, to the reclined position.
Conclusions
Trunk angle influenced lower cervical flexion, gravitational moment arm of the head, the skull angle relative to horizontal, and the C1-C2 intervertebral joint angle. Reading a tablet while seated with the hips in front of the shoulders in a semi-reclined or reclined position could see some benefits in reducing the force required of the cervical extensor muscles to support the head. However, using a tablet with a reclined trunk could result in increased passive tension in the cervical extensor
Acknowledgements
This work was supported by an Arkansas Department of Higher Education Student Undergraduate Research Fellowship (SURF) & University of Arkansas Faculty Development Grant. Special thanks to the Pat Walker Health Center at the University of Arkansas for assistance with the radiographs.
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