Mobile communication devices, such as mobile phones and networked personal digital assistants (PDAs), allow users to be constantly connected and communicate anywhere and at any time, often resulting in personal and private communication taking place in public spaces. This private -- public contrast can be problematic. As a remedy, we promote intimate interfaces: interfaces that allow subtle and minimal mobile interaction, without disruption of the surrounding environment. In particular, motionless gestures sensed through the electromyographic (EMG) signal have been proposed as a solution to allow subtle input in a mobile context. In this paper we present an expansion of the work on EMG-based motionless gestures including (1) a novel study of their usability in a mobile context for controlling a realistic, multimodal interface and (2) a formal assessment of how noticeable they are to informed observers. Experimental results confirm that subtle gestures can be profitably used within a multimodal interface and that it is difficult for observers to guess when someone is performing a gesture, confirming the hypothesis of subtlety.

Mobile devices are generally used in public, where the user is surrounded by others not involved in the interaction. Audible notification cues are often a cause of unnecessary disruption and distraction both for co-located people and even for the user to whom they are directed. We present a wearable peripheral display embedded in eyeglasses that delivers subtle, discreet and unobtrusive cues. The display is personal and intimate; it delivers visual cues in the wearers' periphery without disrupting their immediate environment. A user study conducted to validate the design reveals that the display is effective and subtle in notifying users. Experimental results show, with significance, that the cues can be designed to meet specific levels of visibility and disruption for the wearer, so that some cues are less noticeable when the user is not under high workload, which is highly desirable in many practical circumstances. Hence, peripheral notification displays can provide an effective solution for designing socially acceptable notification displays, unobtrusive to the user and the immediate environment.

Using a mobile device in a social context should not cause embarrassment and disruption to the immediate environment. Interaction with mobile and wearable devices needs to be subtle, discreet and unobtrusive. Therefore, we promote the idea of "intimate interfaces": discrete interfaces that allow control of mobile devices through subtle gestures in order to gain social acceptance. To achieve this goal, we present an electromyogram (EMG) based wearable input device which recognizes isometric muscular activity: activity related to very subtle or no movement at all. In the online experiment reported, the EMG device, worn on an armband around the bicep, was able to reliably recognize a motionless gesture without calibration or training across users with different muscle volumes. Hence, EMG-based input devices can provide an effective solution for designing mobile interfaces that are subtle and intimate, and therefore socially acceptable.