How would future gyms work/look like ? - An Idea

This is an interesting idea I found on net.

The difference here is that the machines would all be linked to a large central generator, so if you are really working out you will be generating much more energy than is needed to power the machine display.
The swipe cards would monitor how much time you spent on each machine then - depending on the efficiency of the machine - calculate whether you were a net user or contributor of energy to the building. At the end of the year, people get a rebate on their membership depending on how much energy their use of the treadmills and cardio machines has generated for the gym.