The following paper Using heart rate to control an interactive game (by Nenonen et al.) presented at CHI 2007 propose to use real-time heart rate information to control a physically interactive biathlon:
"Instead of interfacing the game to an exercise bike or other equipment with speed output, the skiing speed is directly proportional to heart rate. You can freely choose the form of physical exercise, which makes it easier for people with different skill levels and backgrounds to play together. The system can be used with any exercise machine or form. To make playing meaningful instead of simply exercising as hard as you can, a high heart rate impedes the shooting part of the game by making the sight less steady. This balancing mechanism lets the player try out different tactics, varying from very slow skiing and sharp shooting to fast skiing and random shooting"
Why do I blog this? This is yet another prototype of using real-world data (very internal in that case) to control a digital counterpart. I am much more interested in the players' reaction as well as challenge tuning than using this game as a way to promote exercise (although I don't dismiss that purpose):
"players reported that this was just a sprint game but it would be much more difficult to create a game where endurance would matter (...) If heart rate is used for interaction, personal characteristics could be taken into account to provide a similar gaming experience for different people. This could be done for example to have every player do specified exercises before the actual game where heart rates are measured and use this for setting the game balance. A yet another unanswered question is if heart rate interaction is more suitable for motivating people to exercise or for providing a unique interaction experience. "