We explore how to create interactive systems based on electrical muscle stimulation that offer expressive output. We present muscle-plotter, a system that provides users with input and output access to a computer system while on the go. Using pen-on-paper interaction, muscle-plotter allows users to engage in cognitively demanding activities, such as writing math. Users write formulas using a pen and the system responds by making the users' hand draw charts and widgets. While Anoto technology in the pen tracks users' input, muscle-plotter uses electrical muscle stimulation (EMS) to steer the user's wrist so as to plot charts, fit lines through data points, find data points of interest, or fill in forms. We demonstrate the system at the example of six simple applications, including a wind tunnel simulator. The key idea behind muscle-plotter is to make the user's hand sweep an area on which muscle-plotter renders curves, i.e., series of values, and to persist this EMS output by means of the pen. This allows the system to build up a larger whole. Still, the use of EMS allows muscle-plotter to achieve a compact and mobile form factor. In our user study, muscle-plotter made participants draw random plots with an accuracy of +/- 4.07 mm and preserved the frequency of functions to be drawn up to 0.3 cycles per cm.
