The good and the bad of urban rebounding - jumping on a mini trampoline

Mini trampolines, the go-to fitness tool for people seeking a joint-friendly cardio bounce, are also adding spring to the push-ups, planks and lunges of boot camp and interval workouts.

Deborah Horton, a group fitness instructor at Crunch fitness in New York City, leads a 60-minute mini trampoline, or rebounder, class called Urban Rebounding that folds planks, jumping jacks and hamstring curls into and between cardiovascular intervals.

"Rebounding is a non-impact cardio workout on a surface so soft you can bounce a raw egg on it and it won't break," said Horton.

It offers a softer landing than high-impact cardio workouts, from running to high intensity interval training on hard ground, which can also be hard on the body.

Read: How a trampoline workout adds bounce to fitness classes 

The resistance component - just add weights

To add a resistance component, Horton added, some rebounding classes incorporate free weights and body bars.

"I do think it (rebounding) is one of most beneficial ways to get the heart rate up in a safe and healthy way that will allow your body to last," she said.

Michele Olson, professor of exercise physiology at Auburn University in Montgomery, Alabama, said rebounding is a low-to-moderate intensity form of cardio exercise that burns calories similar to brisk walking.

"The calorie burn and intensity are not as high as jogging or running," she said, "so it could be very useful for less-fit persons, (or) those with orthopoedic issues."

Read: Is walking or jogging best for weight loss?

Interspersed with regular weight lifting and more vigorous running routines, Olson said, rebounding could also supplement the workouts of fitter individuals seeking more variety and less impact.

But bouncing, she added, is no substitute for walking.

"Good ground-based walking stimulates the bones in the spine more and is important for preventing osteoporosis," Olson said. "To protect the bones the feet need to hit the pavement a few times a week."

John Hines of Bellicon-USA, a Chicago company that distributes Bellicon mini trampolines, said while rebounders are popular among seniors, sales spike among people in their 30s and 40s, and are more popular than expected among 25- to 30-year-olds.

"I tell people to leave it in the living room next to the television," Hines said, adding that initially many people are afraid they'll bounce off, but that fear vanishes very quickly.

"It's interesting how quickly we adapt to bouncing," he said. "You can really relearn balance at any age."

What are the dangers?

Exercise physiologist Dr. Mark P. Kelly suggests that for seniors mini trampolines have their ups and downs.

"Many older adults feel the trampoline is perfect for them with the soft landing. In reality, the older individual may be the worst candidate for using the bouncy surface due to weak ankles and poor balance," said Kelly, who teaches at California State University, Fullerton.

Read: How to exercise for poor balance and to prevent falls

Kelly said mini trampolines are associated with ankle sprains and strains. He urges beginners to hold on to a stable object until the ankles are strengthened and the user gets used to the bouncing action.

Watch a urban rebounding class in action, from Time magazine