Abstract: Childhood health issues related to sedentary behavior have risen dramatically over the last 20 years. One of the key factors in this rise is that children are increasingly spending more time on sedentary leisure activities such as game consoles and the Internet. Research turned to interactive exertion interfaces, or exergames, to try to compensate for the lack of physical activity in children and teenagers. Part of this research, especially in HCI, has focused on game design to truly guarantee that children are motivated to play with exergames and hence increase physical activity. Other research, essentially medical, has focused on determining whether these existing exergames foster sufficiently high levels of physical activity as recommended by health experts and compared to sports activity. An important part of research, which has almost not been addressed, is that of finding an automated system to control the amount of physical activity (APA). Such system would ultimately allow health and physical education experts to draw intensity curves for play sessions to guarantee that children perform a healthy activity. Such system clearly needs a way to control this APA. We have defined a game system variable, dubbed Interaction Tempo, which has been empirically proven to be directly related to the APA performed by children in an exergame platform we have designed, the Interactive Slide. In this paper we define and justify what Interaction Tempo is and how it is related to the game control system. We describe two studies (independent factorial designs) that we have designed and undertaken with over 420 children. Our current results do not quantify the APA performed by children in an absolute manner. This will be part of future stages of our research with the support of medical experts and relating APA to Energy Expenditure. However, we have now proven that we can control and modulate the change in APA through the change in Interaction Tempo. These results provide a solid ground on which to design new exergames, as well as the underlying mechanism for developing adaptive systems that automatically control gameplay. This way the APA will always be at the level defined by physical education or health experts for the duration of a play session.