Rachel L. Davis still considers herself an avid bicyclist, but as a working mother of two with a third on the way, her biking has changed. “As I’ve become more pregnant, more tired with more children, I don’t have as much energy to be biking 40 miles,” said Davis, 36, who owns a marketing strategy company based in Washington, D.C.

She found a solution in e-bikes, bicycles with electric motors to help with pedaling.

“The e-bike is a fantastic option if someone has a little less energy but wants to get some exercise, burn calories and still get around,” Davis said.

Electric bikes are widely popular in Europe and China and are steadily making their ascent in the U.S. bike market. Erik Saltvold, founder and owner of the Midwest-based Erik’s Bike Shop chain, said the e-bike is the fastest-growing specialty bike in the industry.

But is using one cheating on getting exercise? Researchers from the University of Tennessee found in a study of walkers, e-bike riders and traditional bike riders that those who completed a 2 ¾-mile hilly route on an e-bike used 22 percent less energy than traditional bikers and 64 percent less than walkers.

“Sometimes people who pedal a lot kind of scoff and say, ‘Oh, I don’t need an electric motor.’ Quite frankly, that’s right,” said Dave Abadie, owner of E-Bike KC, an e-bike conversion company. “If you ride a lot for exercise and even for competition, an electric bike is something you wouldn’t be interested in getting.”

But e-bikes offer a gateway to exercise for people who are recovering from injuries or have been inactive, said Thomas Whitaker, director of marketing for Faraday Bikes, a San Francisco-based e-bike manufacturer. “It’s a familiar and fun way to exercise without being hard on the body,” he said. Once people get hooked on being more active, they “do more and bike more.”

Saltvold said that many of his customers say they switch off the pedal assist on flat stretches to get more exercise, then turn it back on when going up a hill.

There are safety concerns. Because the bikes are heavier than traditional bicycles, balance can be a problem. And e-bikes allow people to cycle at higher speeds than usual. “A normal rider will average 12 to 15 miles per hour, where an e-bike rider will average 15 to 20 miles per hour,” Abadie said, which gives a rider — especially one who hasn’t been biking a lot — less time to react to something like a car pulling out in front of them.

If you’re interested in an e-bike, you have a couple of options: The average price of new e-bikes is about $2,500, with top-end models going for up to $10,000. An existing bike can be converted to an e-bike, but in addition to adding a motor, the transition also requires stronger brakes, wider tires and, sometimes, strengthening the frame. Most conversions run $800 to $1,200, Abadie said.