Working out at the gym takes a lot of energy — both for the patron and the gym itself. And now, new machines are being developed to capture it. A Seattle-based company has developed exercise equipment that takes the resistance generated by all that kinetic energy and sends it back into the electrical grid. "Green gyms" have popped up for like-minded exercisers to turn their sweat into kilowatts — although the net effect might be less than you'd think.