Is this feasible?

At my gym if you are on a cardio machine you may plug in your headphones to a box that you power with your movement or you can plug in a iPhone or iPod or device and charge it.

Most people don’t use these as they bring their own. So how hard would it be to wire this power into the electrical system to supplement the power companies.

I doubt there would ever be enough supplied power to ever run elliminating the need of a storage battery or to fully supplement that of the electrical company. But is there a economical way to convert the traditional cardio machines and even possibly the weight machines to supply power rather then use it?

Could we make the machines more efficient so that less power could result in more power?