In the recent years many touch screen interfaces have been designed and used for musical control. When compared with their physical counterparts, current control paradigms employed in touch screen musical interfaces do not require the same level of physical labor and this negatively affects the user experience in terms of expressivity, engagement and enjoyment. This lack of physicality can be remedied by using interaction elements, which are designed for the exertion of the user. Employing intentionally difficult and inefficient interaction design can enhance the user experience by allowing greater bodily expression, kinesthetic feedback, more apparent skill acquisition, and performer satisfaction. Rub Synth is a touch screen musical instrument with an exertion interface. It was made for creating and testing exertion strategies that are possible by only using 2d touch coordinates as input and evaluating the outcomes of implementing intentional difficulty. This paper discusses the strategies that can be employed to model effort on touch screens, the benefits of having physical difficulty, Rub Synth’s interaction design, and user experience results of using such an interface.