As on any old Wednesday night at the gym, people were plodding on treadmills, blankly watching TV screens overhead. Twenty more minutes. Nine more minutes. But then they started turning to look behind them, trying not to stare, at a dozen people strapping on weird shoes and giggling.

Regan King leaned against the wall, snapping the buckles on black plastic boots that looked like in-line skates with a big almond-shaped spring underneath instead of wheels. She took a few tentative steps forward, wobbling, then bent her knees and jumped. The springs sent her up in the air and she grinned, eyes wide, bouncing through the cardio area like a little kid jumping on a bed.

Next to her at Gold’s Gym in Cary, N.C., other people were gasping as they started to hop, too. Some leapt forward as they jogged; some jumped straight up. People were bouncing like little cartoon characters. You could almost see the words popping underneath their feet: Boing. Boing. Boing. You could almost see the thought bubbles over their heads: “Wheeeeee!”

The latest gimmick? Maybe.

But fans say bouncing makes exercise fun instead of dreary. People can wear these “rebounding shoes” to run outside on pavement, to bounce through sand, gravel or grass, to dance or to jog on a treadmill. And it takes much of the impact out of exercise, cushioning joints.

Most people can’t keep doing high-impact things, such as jogging, their whole lives; knees get sore, ankles hurt and hips ache. When you’re running or jumping, you hit the ground with a force that’s two to six times your body weight, said Mike Huff, coordinator of the sports performance program at Duke University Medical Center. That slams your joints.

“But having (special) shoes or being on a rebounder dissipates that force … so there’s not just a sudden impact. It spreads that force out as you’re going down.”

That’s why crazy, springy shoes like the Kangoo Jumps could help. A University of Nevada at Las Vegas study found that they did dramatically lessen impact force for runners, compared with traditional running shoes.

The shoes were invented in Switzerland in 1994 and have been sold in the United States since the late 1990s. Almost three-quarters of sales have been to adult women.

Lower impact

A more established way to lower the impact of aerobics is something called rebounding exercises done on little trampolines. Trainers sometimes use trampolines for rehabilitation after an injury, along with water exercises, which are even safer. Rebounding requires coordination; you have to steady yourself some. That skill is especially important for athletes, who have to react quickly, but everyone needs it.

Even people like Rie Gau, who has had three operations on her back and open heart surgery last spring, as well as chronic pain and severe osteoporosis that has led to multiple breaks and fractures. Gau just started using a rebounder to try to build strength, bouncing lightly on it a few times a day.

“This is something I can do that doesn’t hurt me,” said Gau, who had been unable to exercise at all for years. “I feel it’s helped cardiovascularwise and also the muscles.”

Her daughter Barb Gau, a Chapel Hill, N.C., therapist and health and wellness coach, who uses the springy shoes but knew they would be too difficult for her 69-year-old mother, said the rebounder has already improved her mother’s balance and mobility.

Rebounding has its drawbacks. Some people find it uncomfortable to keep bouncing. And people can twist ankles or fall.

Some people don’t want to try the rebounding shoes, either; they look at them and immediately imagine, in vivid detail, how they will fall off the several-inches-high springs.

High price

The rebounding shoes are new to most fitness experts. Huff had never tried them but after looking at pictures said people should start slow and be careful how they land with the shoes.

“The first thing I tell people before they buy any product is that there’s no magic product,” he said. “You’ve got to be sure you’re going to use it, that you’re not just flushing $200 down the drain on some gimmick.” He looked at the picture again and laughed. “It does look like fun.”

The shoes cost $199.95, and aren’t sold in stores; they have to be fitted and adjusted for people’s height, weight and other variables, said Daniel Taylor of Jump America Inc., the California-based company that markets Kangoo Jumps in the United States.

Like most 25-year-olds, King is always looking for something new, so when she saw the bouncing class on the schedule, she had to try it. At first it felt a little bit like being on skis, she said, the boots, the unsteady feeling.

Standing still on the rebound shoes is harder than moving. That’s what made the class so tough the folks were going at it the entire time, more than half an hour of hopping around with arms pumping.