Walk into a spinning class at the New York Sports Clubs' facility on Eighth Avenue and West 23rd Street in Manhattan and you'll find 20 sweaty people furiously pedaling their stationary bikes. Look closely and you'll notice something unusual about this workout: Each of the bikes is attached to a black box with wires running out of it. The box is a compact generator that converts the motion of the wheels into electricity, which is then fed into the power grid, offsetting some of the club's energy use. For these gym-goers, it's not just about their cardio fitness; their sweat is helping to make the planet a bit greener.
By adopting power-producing exercise machines in this way, gyms can promote themselves as environmentally friendly and also reduce their electric bills. At least three start-ups in the United States are now selling equipment to retrofit aerobic machines—stationary bicycles, elliptical trainers, and steppers—into electricity-generating gear. These companies have already converted several hundred machines at dozens of U.S. health clubs and university gyms.