# Effects of Smartphone Size and Hand Size on Grip Posture in One-Handed Hard Key Operations
## 4 Discussion  
The present study analyzed preferred grip postures by users with different hand sizes and their frequencies in one-handed operations of the smartphone hard keys (power and volume keys), which are needed to determine optimal locations of hard keys on a smartphone for efficiency and comfort. Properly designed hard key locations based on an analysis of user-preferred grip postures can enhance grip stability and reduce discomfort in the use of a smartphone [1,2]. Although one-handed smartphone operations cause more discomfort and usability problems than two-handed smartphone operations, users often prefer one-handed operations for efficiency and convenience reasons [17]. However, systematic studies on one-handed smartphone grip postures in hard key operations considering smartphone size and hand size have not been conducted. In the present study, one-handed smartphone grip postures were captured by camera and encoded by the number of fingers at each side of a smartphone. Dominant grip postures in hard key operations on smartphones were identified by analyzing the frequency distribution of grip posture, which can be used to determine the proper hard key locations of a smartphone with a particular size. The smartphone grip posture analysis method employed in the present study based on the number of fingers at each side of a smartphone was found to be effective in describing various smartphone grip postures and identifying dominant grip postures in terms of frequency. Two web cameras vertically positioned apart enabled the hand in grip position on a smartphone to be efficiently captured. Ref. [7] classified smartphone grip postures into soft and firm grips by visual inspection, while the present study quantitatively classified grip postures by the number of fingers at each side of the phone. The proposed method can be applied to measurement and analysis of grip postures for operations of not only hard keys but also graphical user interfaces (GUI) on a touch screen and other smartphone physical user interfaces such as a fingerprint sensor. The present study identified that nine grip postures are used for hard key operations, and out of those nine, three postures are dominant with a total frequency of 95.3% for smartphones of 3.0 to 7.0-inch screen size. The dominant grip postures include L3-R1-K1 (last three fingers at the left side, thumb at the right side, and index finger at the back of smartphone), L4-R1 (four fingers at the left side and thumb at the right side of smartphone), and L3-R1-T1 (last three fingers at the left side, thumb at the right side, and index finger at the top of smartphone). The dominant grip postures are similar in the aspect that the middle, ring, and little fingers are located at the left side and thumb at the right side of a smartphone, but are different in the aspect that the index finger is located at the back of a smartphone for L3-R1-K1, the left side for L4-R1, and the top for L3-R1-T1. The dominant grip postures enable the right hard keys to be easily operated by the thumb and the left hard keys by the index or middle finger while the device is being firmly held. In particular, L3-R1-K1 was found to be most dominant with a frequency of 70.0% because it enables users to switch fast between L4-R1 and L3-R1-T1 by moving the index finger to the left side and the top of a smartphone. Note that the subsequent three dominant grip postures, L2-R1-K1-B1, L3-R1-B1, and L2-R1-T1-B1 (total of frequencies = 4.7%), slightly differ from the three dominant postures in that the little finger is positioned to the bottom of a smartphone, instead of the left side, to support the base of the device more firmly. The frequency distribution of grip posture by smartphone size identified that dominant grip postures with \(>\) 10% of frequency change by smartphone size: L3-R1-K1, L4-R1, and L3-R1-T1 for 3.0 to 4.0 inches of screen size, L3-R1-K1 and L4-R1 for 4.5 to 5.5 inches, and only L3-R1-K1 for 6.0 to 7.0 inches.