Optimal user experience or flow is a theory with great impact on user experience. Promoting flow has become a competitive advantage for interactive systems, including rehabilitation. This can be achieved through an engaging interface that provides a rewarding experience and motivates the user to use the system again. This theory sustains that promoting a state of flow and improving task performance depends heavily on the balance between the challenges posed by the system and the skills deployed by the user. We further claim that balanced mental and motor skills demanded by the task improve flow and task performance. This paper presents an experiment supporting these claims. For this, we built two movement-interaction rehabilitation systems called SIBMER and Macoli (arm in Náhuatl). Both systems have two versions, one with a balanced load of mental and motor skills, and the other with an unbalanced one. Both versions are compared in terms of their potential to promote the state of flow and to improve task performance. Results show that a balance demand of mental and motor skills promotes flow, independently of the task complexity. Likewise, the experiment shows a correlation between flow and performance.