# Effects of Smartphone Size and Hand Size on Grip Posture in One-Handed Hard Key Operations
## 4 Discussion
The change in grip posture frequency distribution by smartphone size can be explained by the transition from holding small smartphones with a low height (e.g., 95 mm) by L3-R1-T1 or ones with a narrow width (e.g., 56 mm) by L4-R1 to holding large smartphones with a high height (e.g., 175 mm) or ones with a wide width (e.g., 93 mm) by L3-R1-K1, because users tend to move their index finger on the top or left-side for small smartphones and to the back for large smartphones for secure grip and support. Next, the frequency distribution of grip posture by hand size identified an increase in L4-R1 by 12.9% and 16.3% and a decrease in L3-R1-K1 by 13.1% and 13.2% as hand width and hand lengthincreased from small to large, respectively. The change in grip posture frequency distribution by hand size can be explained by the observation that the index finger can be located more naturally at the left side of a smartphone as hand size increases because users with a large hand usually grasp a smartphone along a diagonal direction of their hand with a straight wrist posture while keeping the smartphone display vertical. The findings of the present study need to be verified by users of a wider range of ages and having hand sizes beyond the hand size range of the present study and in use contexts other than standing. Users younger or older than their 20s and having smaller or larger hands than the hand size range of the participants in the present study need to be included to generalize the grip posture analysis results of the present study to the global user population because possible effects of age and hand size on grip posture beyond the ranges of age and hand size not explored in the present study. Next, the present study measured comfortable grip postures while users simulated smartphone tasks while standing in a laboratory environment; thus, grip postures of a smartphone in other use contexts such as sitting, lying, and walking need to be examined. Although the standing use context was selected in the present study for ease and efficiency in grip posture measurement, other use contexts need to be considered to examine if the distribution of grip posture changes depending on use context. For example, the grip posture of L3-R1-K1 preferred in the standing use context can be changed to L4-R1 in the walking use context to grasp a smartphone more firmly due to an increased likelihood of a drop and therefore damage to the device. Novel methods of grip posture measurement in natural and dynamic use contexts need to be developed to investigate user-preferred grip postures in various use contexts. Moreover, grip postures with real smartphones need to be investigated since mockups were used in the present study. Users can use different grip postures when holding the mockups made of white Acrylonitrile Butadiene Styrene (ABS) by 3D printer compared to holding real smartphones manufactured by molding and coated by various finishes of high quality. In addition, users can grasp in a safer way when using real smartphones to prevent dropping and damage. Lastly, research on the determination of optimal locations of hard keys on a smartphone using the identified dominant grip postures is needed. The proper locations of hard keys can be determined by identifying preferred areas for hard key operations with users with different hand sizes while the identified dominant grip postures are used. For example, the preferred area of the left hard key for L4-R1 would vary by hand size and be located higher than that for L3-R1-K1. Thus, the optimal locations of hard keys on a smartphone of a particular size can be determined by considering hard key locations preferred by the user population with various hand sizes while the smartphone is held by the identified dominant grip postures. For example, the locations for hard keys can be determined by finding the comfortably reachable locations in the grip postures of L3-R1-K1, L4-R1, and L3-R1-T1 for users with various hand sizes.