Most game interfaces restrict the player to a place in front of the computer screen. In mixed reality applications the point of interaction seems to be moving away from technical devices towards a more natural user interface. Actions of players are no longer indirect and mediated by devices: i.e. a jump is a jump. A mixed reality enhanced game is presented, where the traditional interface is replaced by a new interaction paradigm, in which the player uses his body to control his virtual avatar, without encumbering him in obstrusive equipment. Olympic Run is an example of a new kind of interface called Body I/O, which enables the player to feel the bodily action of the game first hand. The implicit feedback of the interface creates a deeper sense of immersion than traditional interfaces and at the same time has a positive impact on the fitness of the player.