Using energy exerted in Gyms

All over the world, millions of people spend huge sum of money to exercise in gyms. How about some way by which we (humans) can make effective use of that energy? For example, people do treadmill, cycling etc in the gym. May be we could find some way to convert that rotational movement(energy) into some other useful form of energy (like electricity) and may be use the same for powering the gym itself?! Think about it...