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 U.S. health clubs and university gyms.
But does the technology make sense? Each machine generates a small amount of electricity, and the costs of converting the equipment are steep. In the July issue's "Turning Sweat into Watts," IEEE Spectrum discusses the benefits and challenges of electricity-producing exercise equipment, dissecting the economics behind the technology and, as a thought exercise, estimating how many gym-goers it would take to power a home or the entire United States.