Chapter 8. General Discussion and Conclusions
8.1. General discussion
The major goal of this study was to develop ergonomic design guidelines for current non-flexible smartphone and future smart devices with flexible displays, by evaluating a variety of factors related with UI/UX.
Six studies were conducted in a laboratory environment. Studies 1 and 2 were about current non- flexible smartphones. Study 1 (Chapter 2) examined the effects of task type (neutral, comfortable, maximum, vertical, and horizontal strokes), phone width (60 and 90 mm), and hand length (small, medium, and large) on grasp, index finger reach zone, discomfort, and muscle activation during such interaction. Study 2 (Chapter 3) examined the ergonomic smartphone forms by investigating the effects of hand length, four major smartphone dimensions (height, width, thickness, and edge roundness), and smartphone mass on grip comfort and design attractiveness. Studies 3-6 were about future mobile devices with flexible displays (foldable and rollable). Study 3 (Chapter 4) was conducted to determine ergonomic forms for mobile foldable display devices in terms of comfort and preference. Study 4 (Chapter 5) examined the effects of gripping condition, device thickness, and hand length on bimanual grip comfort when using mobile devices with a rollable display. Study 5 (Chapter 6) examined the effects of device height (70, 140, and 210 mm), task (web searching, video watching, and E-mail composing), and hand length (small, medium, and large hand groups) on various UX elements associated with using rollable display devices. Study 6 (Chapter 7) examined the effects of gender (15 males and 15 females), device thickness (2 T, 6 T, and 10 T), and pulling duration (0.5 s, 1.0 s, and 1.5 s) on preferred and acceptable pulling forces, muscle activities, and perceived discomfort of the upper limbs associated with unrolling rollable displays.
In this general discussion, the three types of independent variables, object, task, and hand, are further considered.
8.1.1. Shapes and dimensions of smart device (object)
Over time, the dimensions of smartphones have changed. Starting with the iPhone (Apple, Inc., USA), known as the first smartphone, a number of smartphone models have been released around the world. Among the smartphones released from 2007 to 2018, the screen sizes of 804 smartphones launched by the top 5 manufacturers are summarized in Figure 8.1. Among the tablet PCs released from 2010 to 2018, the screen sizes of 170 tablet PCs released by the top 5
manufacturers are also summarized in Figure 8.1.
Figure 8.1 Change of screen size of smartphones and Tablet PCs by year
As mentioned in the previous chapters, and as seen in Figure 8.1, the screens of smartphones are growing. If the screen is larger than a certain size, the grip comfort and portability become very poor. As shown in the results of Studies 2 and 6 however, a certain standard smartphone is suitable for usability. Through Study 2, it was found that the specific size of the smartphone provides high grip comfort (140 H × 65 (or 70) W × 8 T × 2.5 R). In Study 6, users preferred a specific size of smart device with rollable display (140 H×142 W; 140 H×206.2 W recommended in consideration of 95th percentile of preferred screen width). It is expected that next-generation displays such as the foldable or rollable display can compromise the contradiction among the needs of large screen size and those of portability and grip comfort.
In Study 1, subjective discomfort increased with the 90-mm-wide device, and the FDI was highly activated with the 60-mm-wide device. Finally, the specific range of input (best location for rear interaction) was suggested. In Study 2, the size of the smartphone affected the grip comfort. The dimensions optimizing grip comfort and design attractiveness were 140 H × 65 (or 70) W × 8 T
× 2.5 R. In Study 3, the structure of mock-up (folding methods) affected the preference, and the best folding method was Z-type. In Study 4, device thickness affected grip comfort (for the right hand with GripFF, the thicker the device, the higher the grip comfort). In Study 5, device height affected preferred screen width (mean (95th percentile) preferred screen width of three device heights were 70 H×70.7 (100.0) W, 140 H×142.0 (206.2) W, and 210 H×207.2 (311.1) W).
Comprehensively, 140 H×206.2 W was recommended. In Study 6, device thickness affected the
0 1 2 3 4 5 6 7
2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018
Smartphone screen size (inch)
Year
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Table PC screen size (inch)
Year
muscle activation and perceived comfort, and a device thickness of 6 T was the best. Smart devices were the main topic of all six studies, but the details of device shapes were different.
Nonetheless, the dimensions or shapes of the devices affected muscle activation and perceived discomfort in all studies, and the specific value or range of preferred dimensions or shapes were defined in all cases. Therefore, it is necessary to carry out the experiments considering the dimensions or shapes of the product, not only for the improvement of existing products but also for the development of next-generation products.
8.1.2. Smartphone as a multi-tasking device (task)
The smartphone is a versatile device that performs a variety of tasks in addition to phone functions. It has evolved into a hand-held multi-functional device that contains computers and other devices, such as cameras, e-books, and document work, which are unrelated to the main phone function of calling. The tablet PC is a device that can work effectively with a larger display than the smartphone, which is a portability-based display device. Therefore, smart devices should be improved in a suitable form in consideration of various tasks.
This study covered various tasks from simple grip or touch to practical tasks such as E-mail sending or web searching. In Study 1, task was composed of five basic motions that make up practical application usage (neutral stroke, comfortable stroke, maximum stroke, horizontal stroke, and vertical stroke). The range of index finger reach zones (touch area) varied by the task. In reality, horizontal and vertical stroke were not parallel to the device’s X- and Y-axes. As mentioned earlier, different tasks used different muscles. In Studies 2 and 3, task was not an independent variable. In Study 4, grip regions and grip comfort were significantly different depending on the gripping methods. In Study 5, three practical tasks (web searching, video watching, and E-mail sending) were used. TaskVideo was recommended to use the screen aspect ratio of 1.4, which was wider than that of TaskSearch and TaskMail (screen aspect ratio of 1.2), indicating that it will be the good function to change the screen aspect ratio for each application in the rollable device. In Study 6, the tasks were the same in terms of pulling motion, but the pulling duration was considered at three levels, short (0.5 s) to long (1.5 s). The longer the pulling duration, the lower the muscle activation. However, the longer the pulling duration, the lower the perceived comfort. Therefore, in the unrolling motion, users preferred short pulling duration with less considering muscle usage.
8.1.3. Should hand anthropometry be considered for ergonomic experiments? (hand)
The hand is the body part that directly touches the mobile device. In this study, one-handed grip postures used for a static task (calling; Study 2), grip postures used for simple input tasks (Studies 1 and 3), and bimanual grip postures involving shoulder external rotations (Studies 4-6) were considered.
In Study 1 (about rear interaction of non-flexible smartphone), the size of the touch area by the index finger during each task varied by hand length levels. During the comfortable stroke, the reach envelope was the widest for the small hand group. In contrast, during the maximum stroke, the reach envelope was the widest for the large hand group, mostly due to longer index fingers at higher hand length levels. In addition, after using a 90 mm-width (wide) smartphonephone, the small hand group reported higher discomfort ratings than the other two groups did (50.24 vs.
38.85) and also reported discomfort ratings in the range of 10.2 (neutral stroke) to 62.8 (maximum stroke) for the 60mm-width (narrow) smartphonephone in the range of 22.1(neutral stroke) to 81.0 (maximum stroke) for the 90mm-width phone. In Study 5, hand length significantly affected the preferred screen width. The larger the hand length, the wider the preferred screen width. In Study 6, gender significantly affected the perceived comfort. %MVC of PD, FCR, and ECR of the female group was 1.4-2.4 times higher than that of the male group. Perceived comfort of the female group (3.6-4.2) was 1.1-1.3 times higher than the male group. In Studies 2, 3, and 4 however, hand length did not show any significant difference. As a result, hand length effect could exist depending on the specific topic (purpose) of each study and the experimental environment (such as subject ethnicity and age). Hand is the body part directly in touch with the device.
Therefore, usability studies should consider the effect of hand length as one of the main factors.