Chapter 8. General Discussion and Conclusions
8.2. Conclusions
8.2.1. Major outcomes
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.
An ergonomic smart device design could improve user interface and user experience. Study 1 was on the rear interaction of non-flexible smartphones. The location of rear interaction of non- flexible smartphones was defined as 8.8–10.1 cm from the bottom and 0.3–2.0 cm to the right of the vertical center line (0.3–2.0 cm to the left for left-handed individuals). Since the horizontal and vertical strokes with rear interaction were not parallel to devices’ X- and Y-axes, the input recognition should be defined to consider the sloped angle. During TH, slopes were made in the range from −10.8° to −13.5° with reference to the X-axis, and during TV, slope was made in the range from −1.6° to −8.4° with reference to the Y-axis. Study 2 considered the best size for non- flexible smartphones. The dimensions optimizing grip comfort and design attractiveness were 140 H × 65 (or 70) W × 8 T × 2.5 R. The most preferred mass was 122 g (in the range 106–137 g).
The horizontal perimeter of 146 mm was associated with high grip comfort and design attractiveness. This value lies in the middle of the cylindrical handle circumference range that has previously demonstrated high grip force and comfort (140–151 mm). Study 3 was for defining the most preferred folding method for foldable display devices. Preferred screen size during calling was 140 H × 65 W, and for web searching and gaming it was 140 H × 130 W. The Z-type folding method was recommended, which provided small-to-large screen sizes (5.3" – 9.0" or 13.5 cm – 22.9 cm) as well as high folding/unfolding preference. Study 4 was for defining the grip region of the rollable display device. Side bezel width (horizontal grip width) was recommended to be a minimum of 20 mm, and the vertical grip part was recommended to have 123.6 mm GripFF. Side bezel width and vertical grip part were recommended as 14 and 113.6 mm, respectively for GripMM. Side bezel width and vertical grip part of the right hand were recommended as 14.6 and 95.6 mm, respectively for GripFP. Considering grip comfort, the recommendation for GripFF was the best grip region. Device thickness of 10 mm is recommended.
Study 5 was for defining the preferred screen width for rollable display devices and tasks. The preferred screen width of three device height (70 mm, 140 mm, and 210 mm) was 70.7 mm, 142.0 mm, and 207.2 mm, respectively. To cover the 95th percentile of the results, 100.0mm, 206.2mm, and 311.1mm were recommended as the maximum screen widths. The recommended (maximum) preferred screen aspect ratio of three device heights (70 mm, 140 mm, and 210 mm) was 1.4 (2.0), 1.2 (1.7), and 1.1(1.6), respectively. For TaskVideo, the screen aspect ratio of 1.4 was recommended, which was wider than for TaskSearch and TaskMail (screen aspect ratio of 1.2). The mean (SE) preferred screen width for HandS, HandM, and HandL was 127.9 (6.6), 135.9 (7.4), and 156.0 (8.4), respectively. A device (screen) height of 140 (120) mm with a width of 142 mm screen improved the overall user experience. Study 6 was for defining the preferred and acceptable pulling force for rollable display devices. Short pulling duration (0.5 s in this study) was recommended.
Comprehensively, 6 T was the best thickness in consideration of muscle activation and perceived
comfort. The preferred pulling force was 6.1 N, and the range of the 5th-95th percentile was 2.1- 12.5 N. The acceptable pulling force was 1.8 times higher than the preferred pulling force, which can be used as the threshold of pulling force.
Based on the above findings, specific guidelines were suggested for each study.
Guidelines for rear interactions of non-flexible smartphones
- The location of rear interaction of non-flexible smartphone should be 8.8–10.1 cm from the bottom and 0.3–2.0 cm to the right of the vertical center line (0.3–2.0 cm to the left for left- handed individuals).
- The range of the angle for recognizing the horizontal and vertical strokes should be defined by the range −10.8° to −13.5° with reference to the X-axis for horizontal strokes and the range −1.6° to −8.4° with reference to the Y-axis for vertical strokes.
Figure 8.2 (a) Recommended location of rear interaction, (b) range of angles for
horizontal strokes, and (c) range of angles for vertical strokes
Guidelines for smartphone dimensions
- The dimensions of smart devices should be 140 mm (H) × 65 mm (or 70 mm) (W) × 8 mm (T) × 2.5 mm (R) to optimize one-handed grip comfort and design attractiveness.
- The mass of smart devices should be 122 g (in the range 106–137 g).
- The horizontal perimeter should be 146 mm, which provides high grip comfort and design attractiveness.
Figure 8.3 The recommended smartphone dimensions and mass considering one-
handed grip comfort
Guidelines for foldable display devices
- Device size (screen size) for foldable display devices should be 140 H × 65 W (120 H×60 W) for calling and 140 H × 130 W (120 W×128 W) for web searching and gaming.
- The folding method should be Z-type, which provides small-to-large screen sizes (5.3" – 9.0" or 13.5 cm – 22.9 cm) as well as high folding/unfolding preference.
Figure 8.4 (a) The recommended device size and (b) folding method (Z-type)
Guidelines for rollable display device: Gripped region
- The size bezel width should be a minimum of 20 mm for horizontal grip width and 123.6 mm for vertical grip width with GripFF (gripping both sides of the device freely; unrestricted gripping).
- The size bezel width should be a minimum of 14 mm for horizontal grip width and 113.6 mm for vertical grip width with GripMM (gripping both sides of the device minimally;
restricted gripping).
- The size bezel width should be a minimum of 14.6 mm for horizontal grip width and 95.6 mm for vertical grip width with GripFP (gripping the left side freely and pinch-gripping the right side; pinch gripping).
- Device thickness should be 6-10 mm.
Figure 8.5 Recommended width and height of side bezel for three gripping conditions
Guidelines for rollable display device: Preferred screen size
- To cover the 95th percentile of preferred screen width, the screen width of three device heights should be 100.0 mm, 206.2 mm, and 311.1 mm as the maximum screen width. Mean screen width of three device heights (70 mm, 140 mm, and 210 mm) was 70.7 mm, 142.0 mm, and 207.2 mm, respectively.
- To cover the 95th percentile of preferred screen aspect ratio, screen aspect ratio of three device heights should be 2.0, 1.7, 1,6 as the maximum screen aspect ratio. The mean screen aspect ratio of three device heights (70 mm, 140 mm, and 210 mm) was 1.4, 1.2, and 1.1, respectively.
- To cover the 95th percentile of preferred screen aspect ratio, screen aspect ratio of TaskVideo, TaskSearch, and TaskMail should be 1.8, 1.9, and 2.0, respectively. Mean screen aspect ratio of TaskVideo, TaskSearch, and TaskMail was 1.4.
- Screen width for HandS, HandM, and HandL should be 250.7, 256.7, and 282.5, respectively (mean screen width for HandS, HandM, and HandL was 127.9, 135.9, and 156.0, respectively).
- Device size should be 140 H × 206.2 W, which improved the overall user experience, and accommodate different tasks and diverse user needs.
Figure 8.6 (a) Mean screen width for three device height levels and (b) recommended
screen width for three device height levels to accommodate the 95
thpercentile preferred
screen width.
Guidelines for rollable display device: Pulling force
- Pulling duration should be short (0.5 s rather than 1.0-1.5 s, as considered in this study).
- Device thickness should be 6 T in consideration of muscle activation and perceived comfort.
- Pulling force should be 6.1 N (preferred pulling force) or 12.5 N (to cover 95th percentile pulling force). In consideration of acceptable pulling force, pulling force should be 1.8 times as high as preferred pulling force.