Heres the thing. We have a stationary bike at home and it has a digital display so it needs electricity and so does the system, which controls the difficulty of your training. So in order to get the display and the difficulty control up and running, you need batteries. My thought is, why not make the bike produce energy for itself? Oh, wait, there are such bikes, Ive seen them in fitness studios. Apparently, the fitness studio owners know a lot about paying as little as they can for the energy, they dont have to. So instead of stationary bikes, that need batteries, they bought bikes, which convert the mechanic energy generated during the workout into electricity to power themselves.
Lets take it further. I suspect, that there is still much more energy being produced in the standard work out session, than is needed for the little display and the difficulty control. Why not include USB connectors in the bike, so that you can plug in your MP3 player, to charge it, while you work out. Or other small appliences. Or you could have a built in standard battery charger, and you could charge your batteries and then use them anywhere you want to.
And we could expand the idea even more. It would take a lot of engineering and careful planning, but eventualy, there has to be a way, to design a wh*** fitness studio, so it would power itself. Of course harnessing the power generated through weight-lifting would be a bit complicated, but. There is also a lot of biking (and simmilar stuff), lever-pressing and pulley-pulling going on. These could generate the power for at least the sound systems, or refrigerator. Furthermore I think, that you dont need to add weights to increase resistance in your training. You can also shift gears, which will not only provide increased resistance (you know it from mountain bikes) but also greater momentum. That way, the more "weight" you are working with, the more energy you produce.
Central computer system could then monitor the energy output and adjust accordingly using standard connection to the electric grid as a backup.