Another lunchtime spin class had recently been added by popular demand. But since it was new on the schedule, the session at the New York Sports Club at 23rd Street and Eighth Avenue in Manhattan wasn't crowded when I arrived on a recent Wednesday.
Just over a year ago, the club fitted a full line of Star Trac Spinner NXT cycles at this Chelsea location with generators designed to convert human energy into electricity. Since then, nationwide sales of the devices have doubled, according to Mike Curnyn, chief strategy and marketing officer at Green Revolution, their designer and manufacturer. Currently installed in 75 fitness centers across the United States, the retrofitted technology is also to be introduced in Europe later this year.
I had never ridden a spin bike. So, equally driven by curiosity about a nascent energy source that's been hiding in plain sight since the dawn of health clubs, I decided to hop in the saddle to see what all the fuss was about.
Dominic Manfredi, a district group exercise manager at the club and a spinning instructor, happened to be taking the same class. He demonstrated the Green Revolution's mechanism, a small black box fitted around the bike's flywheel that connects to a separate small control panel with 20 possible gear settings. The piÃ¨ce de rÃ©sistance, of course, is a digital readout the top of the control panel indicating the wattage produced by each cyclist.
"It's nice to see that you are putting energy back into the grid with your own leg power," said one green bike devotee, Christina Dacosta, an online media manager for a gay, lesbian and transgender nonprofit, adding that the wattage readout had the added benefit of enabling riders to measure their improvement.
Lisa Hufcut, director of media relations at Town Sports International, which owns and operates the club, has also tried out the retrofitted bikes at that location. "I personally have become addicted to them," she said. "When I go to take other spin classes, I miss having the watts read out and taking into account how many calories I've burned."
In a phone interview after my 45-minute workout, Mr. Curnyn explained that each bike's generator feeds into a cable connected to a black cabinet at the front of the studio. There, DC electricity is converted into AC electricity. This is supplied to the health club's main electrical panel "the same way Con Ed is tied to that panel and pushing electricity into club for use," he said.
Grid inverters in the panel play "traffic cop," ensuring that human energy gets used first, Mr. Curnyn added.
Given the feel-good implications of generating renewable energy, it surprised me that the phenomenon isn't even more widespread.
But in a forecast of 2011 trends last December, the International Health, Racquet and Sportsclub Association, a Boston-based trade group serving the health and fitness industry, said that a "rapidly growing number" of health clubs and cycling studios are using cardio equipment designed to produce energy.
"This ties into the overall move towards 'greener' business practices that many health clubs have adopted," the report said.
Still, some trendy spinning studios like SoulCycle and Flywheel Sports have yet to invest in cycles harnessing the pedaling energy generated by their popular and rapidly proliferating fitness centers.
"There is a lot of interest in human power, but it is in its infancy," Mike Tagget, the owner of Human Dynamo, a manufacturer of green spin cycles, wrote in an e-mail. "It's important that the machines don't cost much more than regular machines, as the energy payback" is only about 1 to 2 cents per hour.
And although Green Revolution's technology simply combines existing brand-name bikes with retrofitted generators, Mr. Curnyn admitted that it took roughly three years for clubs using his company's product to make a return on their initial investment of $200 to $300 per bike, despite savings of "up to $1,000 a month" in club energy bills and the demand from cyclists who are eager to inject their workouts with a sense of higher purpose.
A slow return on the investment accounts for only part of the wariness, however.
The bikes at Flywheel, which I subsequently tested, have a system on their control panels measuring torque, or resistance. Combined with Green Revolution's wattage readout, "there's the potential for information overload" for the cyclist, Mr. Curnyn said.
While other hubs of fitness continue to shop around, of course, there's always Europe, where Mr. Curnyn said the response to his company's technology had been "overwhelming."
"People in Europe are a lot more aware of their personal carbon footprint than we are here," said Mr. Curnyn, adding that Green Revolution expects to introduce its first pilot generator this fall in London and to offer a fully compatible European Union version of its technology by the end of the year or "very early 2012."
In the meantime, the 75 health clubs using them in the United States can claim bragging rights as well as lure green-minded customers. "Early morning classes here are always packed," said Mr. Manfredi, who teaches a 6:45 a.m. spin class at the club's 23rd Street location.
Of course, "people might argue that it's better to run and bike outside than support a gym that burns so much electricity," said Ms. Dacosta, the media manager who was exercising when I tried out a class.
But in extreme weather, it's a good compromise, she added. "In the hot summer and cold winter, spinning keeps me in shape."