Chapter 5. Study 4] Future Mobile Display Devices: Rollable Display
5.4. Discussions
Figure 5.7 Bimanual correlations for the horizontal and vertical grip widths and grip comfort according to device thickness and grip type (all of the bimanual correlations were significant, with p ≤ 0.027, except for the horizontal grip width for GripFP × DeviceThick, with p = 0.20).
significant. For the right hand, GripFF × DeviceThick provided the highest mean (SE) grip comfort of 80.8 (1.8), whereas GripFP × DeviceThin yielded the lowest mean (SE) grip comfort of 42.6 (2.8), and GripFF × DeviceThin was not in the same group as GripFF × DeviceThick (Figure 5.2). Specifically, GripFF–DeviceThick (in Group Aʹ) provided the highest mean grip comfort (80.8), followed by GripFF–DeviceMedium (75.1; in Group Aʹ/Bʹ) and GripFF–DeviceThin (69.4; in Group Bʹ), whereas the remaining six treatments provided low grip comfort (≤47.5; in Group Cʹ) (see Figure 5.2). As opposed to grip type and device thickness, hand length did not significantly affect either the grip widths or grip comfort in this study (p ≥ 0.55).
5.4.2. Grip type effects
Regarding the horizontal and vertical grip widths for both hands, the effect of grip type was significant (see Table 5.2). In the case of GripFF, the 95th percentile horizontal grip widths for both hands were 20 mm, the same as the bezel width of the prototypes. Hence, a bezel width of 20 mm appears to be the minimum necessary to accommodate 95% of individuals. Although the grip comfort for GripFF was high (75.1), a wider bezel may increase grip comfort further. The 95th percentile vertical grip widths for the left and right hands were 122.6 mm and 123.6 mm, respectively. In the case of GripMM, the 95th percentile horizontal (vertical) grip widths for the left and right hands were 14.0 (113.0) mm and 13.0 (113.6) mm, respectively. In the case of GripFP, the 95th percentile horizontal and vertical grip widths for the right hand were 14.6 mm and 95.6 mm, respectively. The required bezel width for a pinch grip was at least 14.6mm, similar to GripMM (13–14 mm). Accordingly, when designing rollable devices to accommodate 95% of South Koreans, the horizontal bezel width should be at least 20 mm (around 15 mm for a minimal design concept or for a pinch grip), and the vertical bezel width should be at least 124 mm.
Although the effects of device thickness and grip type were both significant for the right-hand grip comfort (p ≤ 0.0094), grip type was more influential (partial η2 = 0.729 vs. 0.159). The grip type levels were split into two groups (GripFF and GripMM-GripFP), and the right-hand grip comforts with GripFF/MM/FP were 75.1/45.3/45.2. Therefore, the horizontal bezel width should be 20 mm rather than 15 mm.
The gripping methods used for rollable display devices are different from those considered in previous studies. Lee et al. (2018) compared the gripping methods used for hand tools requiring high grip force with those for smartphones requiring high grip comfort and operability rather than high grip force and showed that if a degree of firmness is added, the dynamic grip defined by
Kapandji (1983) could better describe smartphone gripping methods than the power, precision, and combined grips defined by Napier (1956). The grip forces with cylindrical handles (Yakou et al., 1997; Kong & Lowe, 2005; Dianat, Nedaei, & Nezami, 2015) or span measuring equipment (Blackwell, Kornatz, & Heath, 1999; Lee, Kong, Lowe, & Song, 2009; Kong, Kim, Lee, & Jung, 2012) involved a power grip, whereas a dynamic grip was used for smart devices (Otten, Karn,
& Parsons, 2013; Lee et al., 2016; Lee et al., 2018; Chowdhury & Kanetkar, 2017; Lee et al., submitted). In contrast, rollable screen pulling requires a power that both hands are involved in holding and pulling both sides of the device. After the screen is pulled out, a bimanual dynamic grip (using both hands to grip the device and touch the screen), unimanual power grip (using the other hand only to touch the screen but not to grip the device), or bimanual power grip (using both hands only to grip the device) can be used. The three gripping methods considered in the current study (gripping both sides freely, gripping both sides minimally, and gripping the left side freely and pinch-gripping the right side) were considered for screen unrolling, but not for touch interactions. Hence, the latter may require additional gripping methods. In addition, relatively light prototypes (≤70 g) were used in this study, whereas the means (SEs) for 804 smartphone and 151 tablet PC models from the top five manufacturers are 143.3 (1.03) g and 459.4 (12.0) g, respectively. Holding a display device heavier than a smartphone requires a power grip or firmer dynamic grip rather than the non-firm dynamic grip typically used for smartphone holding.
5.4.3. Device thickness effects
Among the six dependent variables considered in this study, the effect of device thickness was significant only for the right-hand grip comfort (p = 0.009; Table 5.2). The right-hand grip comfort increased with increasing device thickness from 51.7 (for DeviceThin), 56.5 (for DeviceMedium), to 57.3 (for DeviceThick). DeviceThin was statistically different from the other two.
For the right-hand grip comfort, a device thickness of 6 mm or preferably 10 mm should be used, in addition to a bezel width that accommodates GripFF (a 20 mm width for 95% accommodation).
Mobile objects with a specific range of thicknesses provide high grip comfort. Yakou et al. (1997) showed that the optimum grasping diameter for a cylindrical object was 30–40 mm for men (and 10% lower for women). Kong and Lowe (2005) demonstrated that 41–48 mm and 37–44 mm handle diameters (23.3% of the hand length of the user) maximized the perceived comfort for men and women, respectively. Lee, Kong, Lowe, & Song (2009) investigated a grip span range of 45–55 mm and found that 50–55 mm provided high grip comfort. Kong, Kim, Lee, & Jung
device and found that the grip feeling changed from comfort to discomfort at 65% of the maximum voluntary contraction for gripping. In addition, the range of cylindrical handle circumferences associated with high grip force and grip comfort, which is 140–151 mm according to Blackwell, Kornatz, & Heath (1999) and Kong and Lowe (2005), includes the perimeter of a smartphone that provides high grip comfort (146 mm; Lee et al., 2018), indicating that the sizes of the grip apertures enclosed by the thumb, palm, and fingers for high grip comfort are similar between circular and rectangular cylinders. When hand-carrying a retracted rollable display device, one-hand grip comfort is important as in non-flexible smartphones. To use the rollable screen, however, both sides of the device should be held and pulled by both hands. Therefore, bimanual grip comfort should be considered.
5.4.4. Bimanual coupling
When identical gripping methods were used for both hands (GripFF and GripMM), the bimanual coupling with respect to the horizontal and vertical grip widths and grip comfort increased for all three device thicknesses (0.60–0.97 for GripFF and GripMM vs. 0.22–0.57 for GripFP; Figure 5.4).
In addition, the horizontal and vertical grip widths for GripFF and GripMM were similar between the two hands (bimanual differences: 0.0–0.9 mm for horizontal width and 0.2–8.1 mm for vertical width), whereas those for GripFP were different (bimanual differences across the three hand-length groups: 4.6–6.6 mm for horizontal width and 11.7–22.9 mm for vertical width) (Figure 5.2; Table 5.2).
The interactions between the two brain hemispheres during bimanual symmetric movements contribute to the behavioral coupling between the two arms (Sadato, Yonekura, Waki, Yamada,
& Ishii, 1997; Cardoso de Oliveira, Gribova, Donchin, Bergman, & Vaadia, 2001). Therefore, compared with GripFP, the symmetric conditions of GripFF and GripMM appeared to cause behavioral coupling during bimanual pulling and to contribute to higher bimanual correlations.
Bimanual actions are divided into three categories (Maes et al., 2017). Discrete bimanual actions are related to tasks that include pauses between movements (e.g., tapping with each hand). Serial bimanual actions are involved in tasks composed of multiple actions in series (e.g., opening the cap of a bottle). Continuous bimanual actions are performed during tasks that are repeated for some time without pausing between repetitions (e.g., drawing a circle with each hand separately but simultaneously). The rollable screen unrolling motion is similar to continuous bimanual action, but the roles and detailed movements of the two hands can be asymmetric. The dominant and
non-dominant hands play manipulative and stabilizing roles, respectively (Bagesteiro & Sainburg, 2002; Sainburg, 2002; de Poel, Peper, & Beek, 2007). During a bimanual task, the dominant hand moves first, and the non-dominant hand tends to follow the movement of the dominant hand (de Poel et al., 2007). Hence, asymmetric roles and initial asynchrony of the two hands are expected during bimanual rollable screen pulling, which should be investigated in a future study.
5.4.5. Limitations and future studies
This study had some limitations. First, the force of the spring for screen retraction was always 2.5 N, and the gripping method and grip comfort could change according to the required pulling force (Kong, Kim, Lee, & Jung, 2012; Dianat, Nedaei, & Nezami, 2015). Second, the weights of the three prototype devices were different, with the weights of DeviceThin/Medium/Thick being 58, 63, and 70 g, respectively. Objects of equal size but different weights can affect grip comfort and preference (Ulin, Armstrong, Snook, & Monroe-Keyserling, 1993; Lee et al., 2018). Furthermore, the mean (SD) weight of 286 smartphone models released by the top five smartphone manufacturers worldwide is 140.5 (37.0) g, and the weight range is 75–500 g. Hence, an additional study using heavier prototypes (≥75 g) is also required. Third, this study was focused on young individuals in their 20s. The hands of older individuals are less sensitive to pressure (Thornbury & Mistretta, 1981), and their muscular strength is weak (Rosenberg, 1997; Rolland et al., 2008). These age-related differences could affect grip comfort. Fourth, the gender ratios differed across the three hand-length groups. Gender-related differences exist in grip force (Nicolay & Walker, 2005; Morse, Jung, Bashford, & Hallbeck, 2006). Because the hand-length effects were all non-significant in the current study, where the HandS and HandL groups consisted of 10 women and 10 men, respectively, it is not likely to observe gender-related differences in grip comfort for bilateral screen pulling. Nonetheless, it is worthwhile to examine gender-related differences in grip comfort for bilateral screen pulling using two gender groups with comparable hand sizes. Fifth, all of the participants were right-handed, and the thickness of only the right side of the prototype was varied. Although approximately 73.1-97.5% of the population is right- handed (Llaurens, Raymond & Faurie, 2009), it is necessary to investigate the effects of handedness on bimanual coupling with respect to the grip regions and grip comfort. Sixth, only Koreans participated in this study, although a wide range of hand lengths (150–210 mm or 0.7th– 99.9th percentiles) was considered and non-significant hand length effects were observed (p >
0.55). Because each ethnic group has distinct hands in terms of size, proportion, shape, and obesity (Davies, Abada, Benson, Courtney, & Minto, 1980; Courtney, 1984), it is necessary to
Finally, only bimanual screen unrolling on a transverse plane (using the device in landscape mode), but not on a sagittal plane (using the device in portrait mode), was investigated in this study. It is therefore necessary to examine the effects of the screen unrolling direction on the grip regions and grip comfort. Despite these limitations, the fundamental findings of this study will be useful for designing ergonomic rollable display devices.