If a user holds the device while pointing, the hand has to solve multiple tasks and direct pointing becomes more challenging due to the hand’s bio-mechanics. Thus, in addition to occlusion (Siek et al., 2007), a second problem of direct touch is the accessibility of targets that are further away or very close. The center of the tablet is hard to reach if the device is held in landscape format with both hands (Odell and Chandrasekaran, 2012). Moreover, for one-handed pointing on mobile phones it was found that the thumb performance varies with its posture (Trudeau et al., 2012). Poorest pointing performances results from excessive thumb flexion. When tapping on targets closest to the base of the thumb in the bottom right corner of the screen the performance is low. The highest performance is achieved when the thumb is in a rested posture, neither significantly flexed nor fully extended.
The device’s form factor and resulting grip of phones and tablets are different. Therefore, we cannot directly transfer knowledge about ergonomics in phone touchscreen interaction to tablet’s touchscreen and back-of-tablet interaction. Furthermore, whereas, back-of-device interaction has been shown to allow for extending the design space for hand-held devices (Wigdor et al., 2007) and others (Holman et al., 2012; Karlson and Bederson, 2007; Wobbrock et al., 2008) investigated one-handed phone interaction with grasping hands; no research (to the authors’ best knowledge) exists that investigates how usable tablet interaction with grasping hands is.
We think that it is mandatory for proposing design guidelines for tablet touchscreen and back-of-device interaction to understand biomechanics of the hand. Thus the question addressed here is if tablet pointing performance depends on the digits’ configurations of the grasping hand.