Understanding grip shifts: How form factors impact hand movements on mobile phones

In this paper we present an investigation into how hand usage is affected by different mobile phone form factors. Our initial (qualitative) study explored how users interact with various mobile phone types (touchscreen, physical keyboard and stylus). The analysis of the videos revealed that each type of mobile phone affords specific handgrips and that the user shifts these grips and consequently the tilt and rotation of the phone depending on the context of interaction. In order to further investigate the tilt and rotation effects we conducted a controlled quantitative study in which we varied the size of the phone and the type of grips (Symmetric bimanual, Asymmetric bimanual with finger, Asymmetric bimanual with thumb and Single handed) to better understand how they affect the tilt and rotation during a dual pointing task. The results showed that the size of the phone does have a consequence and that the distance needed to reach action items affects the phones' tilt and rotation. Additionally, we found that the amount of tilt, rotation and reach required corresponded with the participant's grip preference. We finish the paper by discussing the design lessons for mobile UI and proposing design guidelines and applications for these insights.