Imagine the gym of the future. It has rows of exercise devices, same as now, but the machines have sensors that can detect who is using them (maybe via RFID from your gym card) and how much poundage is being moved at any moment. For the cardio machines, your speed and distance would be measured, just as it is now.

Now imagine that each machine is networked to a server. Everyone in the gym works as a team, with their actions becoming the inputs for a wall-sized video game. Each gym would have a captain, and you'd play via the Internet against other gyms. The poundage you move on your machine might be, for example, adding speed or ammunition to the captain's guns, or making your team's avatar faster or more protected in some way. You can imagine a million game types in which the gym equipment's movements can feed into the action. The simplest game would be a Viking rowing boat, or dog sled, racing against another gym, or multiple gyms. The most complicated would be some sort of combat game where your vehicle's speed, shields, and weapons power are determined by the output of the exercisers.