For Chicago real estate developer Jeff Grinspoon, 43, it was the inversion table that worked for his back pain. At first, he found it stressful. But before long, he had his 78-year-old father trying it too.

“Blood flowing to the head feels weird,” said Grinspoon, who inverts once or twice a day for about five minutes. “But if you let go, it’s relaxing. I’ve actually gotten sleepy while being upside down.”

Swerdlow, 43, a marathoner and real estate attorney, suffered from a herniated disk. His physical therapist suggested an inversion table, and Swerdlow now uses it once or twice a day to help recover from long runs or lengthy plane trips. Unlike Grinspoon, he never found turning upside down to be uncomfortable.

“I’ve joked that if I can’t sleep at night, I’d use it to relax,” he said.

The shallower the angle, the longer you can stay inverted, but there is no time limit. O’Donnell said she hangs between 15 and 30 minutes a days. Grinspoon and Swerdlow, who both bought inversion tables at Relax the Back, located in Chicago and Lincolnshire, Ill., generally use it less than 10 minutes a day, depending on how they feel.

But anecdotal claims aside, does it really work? There’s little evidence for depression, and only a few studies have examined back pain.

One small study of 20 healthy men concluded that hanging upside down “significantly increased spinal length and reduced the muscle pain.” But that was published in 1978.

In 1985, a study showed 13 of 16 men with low back pain found improvement using inversion and declared it an effective means of achieving “pelvic traction” at home.

But the study, published in the Archives of Physical Medicine and Rehabilitation, found side effects included persistent headaches, blurred vision and contact lens discomfort. The study also cautioned that it should be done under medical supervision.

What studies say

Northern Illinois researchers, meanwhile, tested 19 men in 1988 and found that “gravity inversion should not be compared to or classified as an exercise” and found “no physiological adaptation occurred in any of the inverted positions as a result of the inversion training.”

Still, actor Richard Gere made gravity boots hip in the 1980s when he dangled from them in the movie “American Gigolo.” And aside from some bad publicity in 1983 when a study raised concerns that inversions could be dangerous — a conclusion that was later adjusted to say inversions were fine if participants were in good health — the market has been slowly expanding.

STL International Inc., which manufactures Teeter Hang Ups, said it has seen a 374 percent growth in sales since the late 1990s. And plenty of Internet competitors are offering inversion tables for as little as $129 as well as discounted gravity boots and chairs, something that concerns the Teeters, whose tables start at $299 and range up to $3,000 for a medical version.

The danger, said Riley Teeter, is that consumers get what they pay for.

“I fear we’re heading toward a market collapse with all the cheap copies out there,” she said. “With cheap equipment, you get catastrophic failure after just five months of use.”

That can be unnerving, given that it takes a bit of bravery to swing from your ankles on even the most trustworthy equipment. But finding the courage to try inversions is one of the benefits, Kater said.

“The idea of going upside down can be scary, especially when using the gravity boots,” Kater said. “But a person can gain confidence by doing something they aren’t used to doing. After hanging upside down, people seem to have a lighter mood and feel happier in general.”

For Chicago’s Alan Wolf, a fitness consultant who hangs upside down every day at Lakeshore Athletic Club, Kater’s yoga studio or at his outdoor back-yard gym, the daily head-rush blends body, mind and spirit. He uses it to build core strength by doing inverted crunches and squats and to clear his head.

Like O’Donnell, he uses it as a mood enhancer. “There’s just no way you can go up there, come down and still feel depressed,” he said. “It changes your world.”