When visitors to the Energia Gym in Hadley, Mass., put their feet on the pedals of the Spin bikes, they are reducing their carbon footprint at the same time.
That's because the bikes in Energia's studio have been retrofitted so that they can turn the energy of the workout into electricity.
An average one-hour workout can create enough energy to power two laptops. Over the course of a year, the gym estimates it will produce enough power to light 72 homes for a month, while reducing the amount of carbon dioxide by 5,000 pounds.
All the bikes in the Spin class are connected to a grid command center, which converts the energy from direct current to alternating current.
One Spin class participant told CNBC that watching how much energy she was producing was a good motivator.
"Seeing how many watts I'm producing is not only a gauge of how hard I'm exercising, but also how much energy I'm producing."
Jennifer Siddall, owner of Energia, said that is a good thing, for both gym members and the environment. "They are realizing how tough it is to generate energy, and might start asking themselves, 'Why are we wasting it?' Why not think twice and make an effort to conserve?'"