Green Ideas

What about a gym where people go excersise, and run on tread-mills - BUT! - these tread-mills generate power to return to the electric grid (???).
Everyday many people use running machines and cycles at the Gym, the energy that is created by the use of these machines could be put back in through the electric meter at each gym thus slowing the meter down and using less energy, the more energy each person creates the less they could pay to the Gym.