NEW YORK (Reuters) - The Elliptigo, a cross between the bicycle and the elliptical trainer, is an outdoor cardio machine that is helping runners sidelined by impact injuries to reclaim the streets without pounding their joints.

The brainchild of a former triathlete and an ultra-marathoner who is also a mechanical engineer, the device resembles a long low seat-less bicycle that fitness experts say feels like running on air.

"We were trying to create a low-impact version of running," said Bryan Pate, co-founder of Elliptigo Inc, who with partner Brent Teal developed the hybrid over five years.

"The easiest way to describe it is an outdoor elliptical trainer, but the reality is a little more complicated," he said.

Pate, a former triathlete laid low by hip and knee injuries at age 32, said the Elliptigo is specifically designed with a long stride length and sharp recovery stroke to most closely emulate running, but without the impact.

"As an injured runner, I recognized that it's hard to get that exercise in a different way," said Pate, whose Solana Beach, California, company delivered its first product to its first customer in 2010.

Today Pate has more than 10,000 customers in 58 countries, including more than 100 elite runners who use the Elliptigo to cross train, or add non-impact cardio hours to their training routine.

But he said 90 percent of his clients are 45- to 60-year olds "who care about exercising comfortably outdoors" and can afford the equipment. Elliptigos cost $1,800 to $3,500.

Jane LeGore, a 52-year-old real estate agent in Phoenix, Arizona, was a casual runner for a decade, until hip replacement surgery.

She credits the Elliptigo, which she rides four times a week, for keeping off the weight she feared regaining.

"The calorie burn is between cycling and running," LeGore said.

Always a slow runner, LeGore said she often wondered what it would feel like to glide like an elite.

"Now I feel like I'm running on air," she said. "I can ride and ride and ride and I'm in better shape now than I was at 25."

Connecticut-based exercise physiologist and running coach Tom Holland recommends the Elliptigo for anyone wanting an outdoor cardio workout without the weight-bearing pounding of running.

"It's safer than biking, I think, because you're not clipped in. And because you're standing you're also using your postural core muscles," said Holland, author of "Beat the Gym."

Holland rode the Elliptigo for 150 miles (241 kilometers) from Manhattan to Montauk on Long Island. He describes the muscle activation, somewhere between biking and running.

"Inclines were challenging but I didn't have that muscle breakdown the next day," he explained.

The only downside was the sometimes derogatory comments the unfamiliar contraption elicited from passersby.

"Cars slow down, people ask about it. You get a lot of negative attention," Holland said. "But it was nice and different and I felt like a kid."

(Editing by Patricia Reaney and Gunna Dickson)