Influence of input device,work surface angle,and task on spine kinematics

1 Introduction

The use of tablet computers has grown exponentially each year. Due to the increased prevalence of tablet computers, it is likely in the future that their role in the office will increase as it becomes more common to bring personal technology into the workplace [1]. Modern software and touch technology have enabled tablets to perform nearly all the advanced functions of desktop computers. Their mobility allows tablets to be versatile across industries and favoured in off-site and non-traditional office settings. Coupled with the rapid growth of tablet use and lack of associated duration and posture guidelines from scientific research, the risk of developing pain and discomfort may be rising due to the postures assumed while interacting with these new devices [2–4]. Modern equipment, such as an adjustable work surface slope, is often used in conjunction with tablets and other input devices, adding to an increasingly complex workstation environment. This increase in tablet use generates a need for the evaluation of their influence on spine posture during their prolonged use in the workplace [2].

Mobility and simplicity are valued features of the tablet, as a desktop computer is not conducive for jobs that require employees to work at a number of different job sites such as factories, hospitals, meetings, or remote locations. While both devices can be used to complete similar tasks, each has their benefits and drawbacks. Desktop computer users can adjust the height, distance and tilt of the monitor, keyboard, and mouse in line with ergonomic guidelines; however, they completely lack mobility. Being a single unit device, tablets provide users with location mobility and the ability to perform tasks in standing or sitting postures with ease. As a result, neck and upper extremity postures may be different when completing the same task on a desktop computer versus tablet computer due to the location of the user interface [2, 5]. While the tablet keyboard and screen are often fixed together, external aids such as a keyboard and a monitor arm can be used in order to meet ergonomic recommendations. However, use of such aids attempts to meet guidelines developed for desktop computing and takes away from the original purpose of tablet devices. Bendix and Hagberg [6] reported a trade-off between two spinal regions, cervical and lumbar, in order to maintain an appropriate distance between users and their object of interaction/device. This possible relationship between the two spine regions, when working at a desk, may explain postures assumed by individuals when tablets are used to complete their work. In children, Straker and colleagues [3] have shown that device orientation influences cervical spine flexion with head flexion values of 111° during tablet use and 86° during desktop computer use.

Previous studies have shown that the type of task completed using a desktop has a strong influence on trunk posture during seated work, independent of the type of chair being used [7, 8]. Both studies reported that a reading task caused more postural variability and lower levels of muscle activation compared to other tasks [7, 8]. The typing task in Gregory and colleagues [8] was reported to have the lowest probability of rest and the highest thoracic erector spinae activity. A study by Dennerlein and Johnson [9] reported task dependant postures in the upper extremity, and said that this occurred because the tasks were keyboard and mouse intensive and the type of task affects posture due to the actions required by the user to complete the task.

The spine posture adopted during work is dependent on the location of the tablet or desktop computer monitor due to the gaze or viewing angle [2, 10]. Young and colleagues [2] investigated the link between three levels of tablet tilt (15°, 45° and 63°) on neck flexion values. Across the tablet tilt levels, the most neutral neck postures (15° to 25° flexion from neutral) were found while participants watched a movie with the tablet angled at 45° [2]. Studies have reported differences in neck flexion and asymmetrical postures, when using a tablet versus a desktop computer [3]. Straker and colleagues [3] commented that discomfort may increase with increased neck flexion due to the flexor moment that neck extensor musculature must oppose. Hunting and colleagues [11] found a causal relationship between constrained postures and medical symptoms reported as well as increased neck pain with increased neck flexion. Depending on the angle of cervical flexion, studies have shown both increased and decreased cervical muscle activity occurs. Increased activity occurs when participants are in a moderate range of head flexion [10, 12] with full neck flexion resulting in the flexion-relaxation phenomena in the cervical spine [13]. The effect of device on posture should be investigated to assess which device is optimal for a particular configuration.

Recent studies have shown that manipulating the slope of a desk surface and display location can position the spine in a neutral posture, reduce neck muscular demands, and decrease neck flexion [2, 16]. This was accomplished by changing the screen height, horizontal distance and screen angle of the monitor [2, 6]. Changing these elements brings the device towards the user to facilitate a closer and proper viewing angle [16]. Similarly, several studies have shown that increasing the tilt of the tablet towards the user will decrease the amount of head and neck flexion by way of decreasing the gaze angle [2, 17]. It is of interest to investigate if working with a tablet on a sloped surface also yields similar results for the lumbar spine as working on a computer. One reason why a slight desk slope is used is because a slope of 10° is enough to impact posture but still allow writing materials to sit on the desk surface and not slide off [14, 16]. Furthermore, slope, device and task are all elements that have the opportunity to influence spine posture. Once the relationship has been quantified, a foundation can be established for guidelines to be implemented into the workplace to reduce pain and discomfort at the workplace.

The purpose of this study was to evaluate the kinematics of the cervical and lumbar spine segments while participants performed simulated occupational tasks using two different input devices, a desktop computer and a tablet, on a sloped and horizontal work surface. Participants completed controlled typing, reading, and combination tasks in each of the conditions. First, we hypothesized that the cervical and lumbar spine angles would fluctuate more when completing tasks on the tablet because of its mobile nature. Second, we hypothesized that desk slope would influence spine angles for all conditions as a sloped desk surface has been shown to position the cervical and lumbar spine near to neutral postures. Lastly, we hypothesized that the type of task performed would affect posture and range of movement differently for each input device and work surface slope. Median and range of motion of the lumbar, thoracic and cervical spine angles were analyzed in each combination of input device, desk slope, and task to determine the posture and range of motion for each spine location.

2 Methods

3 Results

All spine angles are presented are the average of the median values from the APDF with respect to a neutral static standing calibration trial. Significant results are presented for the relative cervical and lumbar spine angles. There were no significant interactions (p = 0.4718) or main effects of desk (p = 0.1043), device (p = 0.1175) or task (p = 0.4530) for the relative thoracic median spine angle (Table 2). There were no significant interactions (p = 0.2452) of main effects of desk (p = 0.8264), device (p = 0.1289) or task (p = 0.1083) for thoracic ROM (Table 3).

4 Discussion

The type of device used, task performed, and work surface slope impacted the cervical and lumbar spine postures adopted. First, it was hypothesized that different devices would influence cervical and lumbar spine median angle and ROM. Using a TABLET elicited greater cervical spine flexion angles than a PC to perform the same task in both SLOPED and HORIZONTAL positions (Fig. 3). Second, we hypothesized that the cervical and lumbar spine angles would fluctuate more and experience more variability in TABLET compared to PC for the same task. It was found that ROM was task dependent; during READ and FORM, participants used a greater range of their lumbar spine motion using the TABLET versus the PC. For the cervical spine, the ROM was lower when interacting with the keyboard, on the TABLET, for the MAIL and FORM tasks (Table 4). Lastly, it was hypothesized that the work surface slope would affect spine angles for all conditions. The slope of the surface affected the posture and ROM of the cervical, but not the lumbar spine. Regardless of input device, working on the HORIZONTAL tabletop resulted in greater neck flexion values compared to completing tasks with a SLOPED work surface (Fig. 3). Results of the MAIL and FORM tasks were similar between device and work surface slope for cervical spine range of motion (Table 4). However, there was a different pattern of response for the READ task where cervical ROM was greater when using the TABLET versus the PC. Using the PC to READ on a SLOPED work surface resulted in more cervical spine ROM than in a HORIZONTAL position.

Greater lumbar flexion was recorded when participants used the PC to complete the READ and FORM tasks, compared to using the TABLET. This may be because in the TABLET trials participants were able to easily adjust the screen closer to their body position which would allow participants to maintain a less flexed lumbar spine posture while tasks were completed. In the computer trials, the monitor was 40–74 cm at a fixed distance from the participant [20]. The participants likely changed their lumbar spine angle to alter how close they were to the screen. Due to the mobility of the tablet, the user is able to place the mobile computer in various positions eliminating the flattening of the lumbar spine. By bringing the tablet closer to their body, the lumbar spine will flex forward less to interact with their device. Using the tablet resulted in less lumbar spine flexion than using the PC. Although this device decreased the amount of lumbar flexion, it is important to consider the implications that using a tablet has on the cervical spine.

The influence on the cervical spine range of motion varied between desk, device, and task. A larger ROM was recorded for cervical spine when participants completed the MAIL (16.89°) and FORM (16.82°) tasks on a PC compared to when MAIL (6.15°) and FORM (8.39°) were completed on the TABLET used the PC to complete the MAIL and FORM tasks compared to the TABLET when collapsed across desk (Table 4). The reading task does not follow this pattern. During the READ task, the largest ROM was during the HORIZONTAL TABLET (16.94°) condition and the lowest was during the PC HORIZONTAL condition (10.34°). The PC HORIZONTAL (10.34°) and TABLET SLOPED (10.37°) conditions resulted in very similar amounts of cervical spine ROM (Table 4). The current study found that the type of task influences trunk posture, which supports previous findings by Gregory and colleagues [8] and van Dieen and colleagues [7]. The READ task was found to have the highest amount of motion which is consistent with findings reported by van Dieen and colleagues [7] and Gregory and colleagues [8] who reported reading tasks as having the highest degree of postural changes, and longest and most frequent EMG gaps respectively. These findings suggest that task requirements have a noted influence of working posture, and both task and device should be considered when designing or re-designing modern workstations.

If individuals use devices without being educated on how to interact properly with the device, poor postures may negatively influence the spine. Straker and colleagues [3] reported that head and neck flexion were not significantly different while interacting with a tablet flat on a desk compared to using paper. Although using the tablet on a horizontal table is not optimal, it is noteworthy that using it in this way resulted in more postural and muscle activity variability in the cervical spine than a desktop computer, and this variability may protect from factors associated with static loading [3]. Tablet usage may have resulted in increased cervical flexion because the neck accommodates for a change in tablet position when the person brings it closer to their body requiring a steeper gaze angle (Fig. 3). Similar to the current study, previous studies have reported that tilting the work surface slope toward the user between 10° and 60° results in reduced head and neck flexion compared to working on a flat work surface [14, 15, 17]. This reduction in neck flexion is beneficial as it reduces extreme neck flexion values which have been associated with the flexion relaxation phenomenon (FRP).

These postural and muscular activation results suggest that spine posture and movement should not be considered as separate entities when attempting to describe the “best” posture for a given task. While some research has indicated that a raised laptop screen height may result in less cervical flexion moment, higher productivity and lower risk of developing musculoskeletal discomfort [21], other studies have argued that for a healthy lumbar spine, slight flexion is advantageous as it allows nutrients to enter the spine [22]. Further, it is unknown if movement variability is causative or adaptive during a prolonged period of standing when an individual reports low back discomfort. Moreover, both posture and ROM must be considered in addition to the individual when making recommendations on proper workstation configuration [23].

A recent study that investigated the influence of working at a visual display terminal (VDT) on the cervical spine did not find a difference in the FRP of the cervical spine compared to before the work at the VDT terminal [24]. There may have been no difference in the FRP because participants only worked at this terminal for 30 minutes [24]. It is possible that cervical FRP may occur when working on a tablet on a horizontal work surface slope as this combination has been shown to produce large amounts of neck flexion (3). Meyers and colleagues [13] reported that a cervical FRP was consistently found in participants performing neck flexion. Moreover, working with a tablet for a prolonged period of time is likely to generate FRP in participants’ cervical spine. This phenomenon has been established in the low back as a potential trigger for low back injury via a cumulative effect [25]. Although not yet studied, the cervical FRP may have implications on pain development when users assume a highly flexed posture for a prolonged period of time as found in this study when the participants were completing the FORM task (mimicking data entry), using a tablet on a horizontal work surface. Participants experienced more neck flexion when using a tablet on a HORIZONTAL versus a SLOPED surface, which has been found in previous studies and was consistent for both devices [3, 6, 14, 15] (Fig. 3). When looking specifically at the tablet, Young and colleagues [2] found that the orientation of the tablet, either flat or tilted, affected head and neck angles by way of viewing angles. By changing the viewing angle, neck and head flexion angles can be manipulated [2, 15]. Ariens and colleagues [26] have linked neck flexion angles above 20° with reporting of neck pain.

Based on the task specific responses of spine postures, when selecting a device, one should consider the physical nature of the task and the work surface. For example, if a worker was performing data entry as their primary task, a desktop computer with a properly positioned monitor would not be ideal as there was more lumbar spine flexion during the form task. If the worker only has a tablet, it should be fitted with an external keyboard and a tablet monitor arm to emulate a desktop computer set up. When using a TABLET to READ, the desk should be sloped or an adjustable tablet monitor arm used to properly position the cervical spine. Poor use of the tablet has been shown to greatly increase neck flexion when work surfaces do not allow for accommodations (3). To prevent this from occurring, we propose that tablets should be implemented into the workplace alongside an adjustable work surface that has the ability to slope toward the user.

Collecting data for a short period of time, such as one hour used in this study, limits the direct application of data directly to eight-hour workdays or chronic exposure. However, the responses exhibited in this study indicate the potential for increased cervical flexion that has been associated with pain reporting [27]. A study examining postural changes over a full work day found neck angles were not dependent on working time [27] indicating the results from this study can be representative of postures adapted in similar working conditions. The chair used for this study was not representative of a standard office chair found in the workplace; lacking armrests and a backrest. This limits direct comparison to previous findings about office work stations. However, the goal of this hybrid workstation is to provide an alternative working posture to a traditional western workstation for a user. Given that the work surface was aligned relative to the individual’s position, the relative location of the interface device and work surface should yield similar results independent of the seat pain. Differences that could have impacted lumbar postures was the restriction of the seat location, the ability to translate closer/further from the work surface like a chair with wheels could and the type of seat pan. A habituation period was given to the participants to allow them to familiarize themselves with the office chair; however, the length of time for true habituation is unknown and could have influenced some of the results. Presenting the tasks in random order was done to prevent the influence of time and the increased use of the chair from affecting the results.

5 Conclusions

The device used, work surface slope and task performed each had an influence on spine posture during office-based tasks at a hybrid workstation. Completing READ and FORM tasks using a PC resulted in higher lumbar flexion values than when using a TABLET. Changes in spine posture, both cervical and lumbar, were influenced heavily by the task performed by the participants: cervical spine median angle was greater during FORM than during READ or MAIL, and median lumbar angle was lowest during data-entry tasks at a PC but highest for data-entry tasks on a tablet. Reading tasks appear to be more conducive to neck posture changes, as cervical spine ROM was highly variable during READ, but remained stable across work surface and device setups for FORM and MAIL. The traditional office setup of a PC on a HORIZONTAL slope resulted in similar ROM (10.34°) to using the TABLET on a SLOPED surface (10.37°). This TABLET setup also decreased the cervical spine flexion angle compared to working with a tablet on a horizontal work surface, which suggests the former configuration may be beneficial to tablet users. Time and care should be taken when designing a task as well as choosing what device is to be used to complete the task. Further research to investigate the cervical spine flexion relaxation phenomenon during tablet use is of interest to continue to investigate the link between posture and MSD development. The increase in cervical spine ROM can be interpreted as movement to offload tissues in the neck due to uncomfortable postures assumed while interacting with the TABLET on a horizontal work surface slope while users are reading. Moreover, tablet arms should be implemented when possible to position the tablet sloped toward the reader to eliminate high flexion postures.

Conflict of interest

None to report.