Now, you can not only build muscle power, but electric power as you pedal on fitness bikes which are capable of lighting up the health club.
"Electrify your workout! Later this month you will be able to shrink your waistline and your carbon footprint all at the same time," a motivational poster says at New York Sports Club, urging clients to use "pedaling power to help create a healthier planet."
Some 20 clients pedaled furiously under the poster on stationary bicycles turning sweat equity into sweat electricity.
"It's a great idea, really fun. It allows students to monitor themselves, the monitor tells you how many watts you're creating. It's a win-win situation," said gym instructor Rick Meadows.
The mechanism is simple, using dynamos on the bicycles to transfer 12 volts of electricity produced by the cyclists' pedaling to a generator which creates alternating current of 110 volts.
From there, the current helps power the gym building and reduces its bills -- although not the subscriptions for members.
"I invented the system in early 2007 in Connecticut, where I live," said Jay Whelan, 46, CEO of The Green Revolution.
The commercial debut was made in 2009 in California. "We began in Washington two weeks ago and here in New York," Whelan said, adding that there are 40 customers -- so far only using spin bikes.
It takes 20 people to create about three kilowatts in a one-hour session.
In other words, with four sessions a day, the gym creates 300 kilowatts a month, which is the same as the power needed to light a typical home for half a year.
Over a year, the gymnasts can power the equivalent of 72 homes for one month, according to the company. A small gym might even be able to run its lights entirely on human power, Whelan added.
Green Revolution employs 45 people already and is looking to expand by putting the technology into other machines, including ellipticals, cross-trainers, stepping machines and recumbent bikes, the company says.
Each individual bike generator costs 1,300 dollars.
The dual-purpose gym session appeared to have won over clients at New York Sports Club.
"You are burning energy and at the same time you use it for something positive," said architect Richard Kronick.
"It's fun, challenging and efficient," said Felicia Rubin, a housewife.
Meadows shouted encouragements while green lights indicated how many watts were being generated, so that each client can see just how much electricity his legs can create.
"I created 69 watts, but I have to stop, to change the music," Meadows said. "Somebody created 105 watts. Competitiveness in the class encourages people to do better. And people love the fact that they are creating energy."
"All my members want to come back to see how many watts they create," said Jennifer Carswell, fitness service manager.
Green Revolution has several dozen clients in the United States and Canada, including at schools, universities and a homeless shelter. Inquiries have come in from four prisons, Whelan said.