Bar Harbor, ME - Gordon Goodwin and his wife are rediscovering their passion for bicycling in their senior years thanks to new electric-assist bikes. The electric motors provide a gentle kick, making it easier for them to pedal up hilly roads around Maine’s Acadia National Park.

“We’re getting older,” said Gordon, who turns 69 Wednesday. But, he added, “It’s nice to go somewhere and have the breeze in your hair, like you did when you were young.”

They can’t get that experience on Acadia’s bicycle paths, though.

The National Park Service classifies their so-called e-bikes as motorized vehicles, relegating them mostly to paved roads clogged with summer traffic. Nationwide, the agency and others that manage public lands keep thousands of miles of trail off-limits to e-bikes.

Across the country, regulators are sorting out how to classify the two-wheelers, which are something more than a bicycle, something less than a motorcycle. The National Park Service, for its part, may soon loosen its restrictions.

“I’m hoping that they’re going to come to reality,” said Janice Goodwin, Gordon’s wife. “This will get more popular as time goes on.”

E-bikes are the fastest-growing segment of the bicycle industry. They’re popular with commuters and baby boomers who might not otherwise be able to get out on a bicycle.

The bikes, which can cost $2,000 or more, combine the frame of a regular bike with lightweight batteries and electric motors for extra zip.

Their sales jumped 72% to $144 million in the U.S. last year, helping to breathe life into bicycle sales that have been relatively flat, according to the NPD Group, which tracks retail bike sales nationwide.

Their popularity has led to conflict.