Index Finger Zone: Study on Touchable Area Expandability Using Thumb and Index Finger

This paper evaluates how the "index finger zone" on the back of a smartphone could compensate for the limitations of the "thumb space." We conducted two experiments to investigate how to reach to a distant point comfortably with one hand. First, we gave the participants four typical tasks while using mobile phones (tapping, texting, calling, and scrolling). In these situations, we measured the position of the index finger and the thumb with the natural hand posture. Consequently, the index finger was primarily positioned in the upper left side. Second, the main experiment was to determine how a touchable area could be extended using the index finger zone on the back side. 1) Randomly selected tiles were touched 84 times with a thumb and 42 times with an index finger. 2) Each tile's preference was evaluated with a 5-point Likert scale. As a result, we found that the "comfort zone" could be expanded by 15% by using index finger zone.