# Effects of Smartphone Size and Hand Size on Grip Posture in One-Handed Hard Key Operations
## 1 Introduction  
Greater understanding user-preferred grip postures in one-handed operation of hard keys on a smartphone is needed to determine their optimal locations for better operational efficiency and comfort. Hard keys are used on a smartphone for volume adjustment, power on/off, and screen on/off. The operational efficiency of the hard keys can be improved if they are properly located based on the information of user-preferred grip postures. Improperly designed locations of the hard keys may lead to discomfort and inefficiency in finger motions due to awkward hand posture [1, 2]. Since one-handed (single hand) hard key operations have more usability problems than two-handed (both hands) hard key operations in terms of mobility of the fingers, comfort, efficiency of key operation, and stability of grip [3, 4, 5], the locations of the hard keys need to be determined by considering user-preferred grip postures in a one-handed hard key operation.  
No research has been conducted to examine the effects of task type, device size, and hand size on user-preferred grip posture to determine the proper locations of hard keys on a hand-held mobiledevice. Various tasks such as checking an email, searching on the internet, taking a photo, sending a text message, making a call, and answering a call require different grip postures with different frequencies [6]. For example, in a one-handed operation, a light grip is used for searching on the internet by moving the thumb on the touch screen while supporting the back of the smartphone with the four fingers, while a firm grip is used for locking/unlocking the phone by grasping the sides of the phone firmly to apply a significant force to the power key. Next, [PERSON] et al. analyzed the effects of finger location on antenna performance parameters in various grip postures for talk and data modes in mobile phones: soft grip (the sides of a phone are held by the distal phalanges) and firm grip (the sides of a phone are held by the intermediate phalanges) in one-handed operations [7]. [PERSON] et al. investigated various arrangements of capacitive proximity sensors on a mobile phone for recognition of grip position [8]. [PERSON] et al. compared four one-handed grip postures (thumb-on-front, thumb-on-back, index-on-front, and index-on-back) in terms of task completion time and selection error rate in a target area selection task on the touch screen of a personal digital assistant (PDA) [2]. Lastly, [PERSON] et al. investigated user-defined gestures on a tablet PC depending on three holding postures [9; 10]. However, the existing studies do not examine user-preferred grip postures for smartphone hard key operation tasks, which are important to determine the proper locations of hard keys on a smartphone for operation efficiency and comfort.  
The present study was intended to identify grip postures preferred by users with various hand sizes in one-handed hard key operations for major smartphone tasks with smartphones of various screen sizes. Grip postures captured by web cameras were encoded by the locations of a smartphone and the number of fingers at each contact location and the frequencies of grip postures were analyzed. Lastly, the effects of smartphone size and hand size on user-preferred grip posture were analyzed.