BOULDER — Imagine you can run so fast that you feel like you’re flying.

Anti-gravity treadmills available to the public at the Boulder Center for Sports Medicine and other local physical-therapy centers can help you do just that.

The treadmill takes weight off of your lower body through an air system. Patients who have used it said it feels a bit like someone is standing behind you and pulling you up slightly with a belt while you run.

Patients rehabilitating after hip or knee surgeries seem to get the most benefit from the anti-gravity treadmills, doctors and physiologists said. People with muscular dystrophy and Parkinson’s disease also have used the treadmills. Overweight people on weight-loss regimens have been users.

Elite athletes, including Olympic triathlete Flora Duffy, a University of Colorado-Boulder student who competes for Bermuda, have used anti-gravity treadmills to gain an edge in training.

A woman who developed an “odd gait pattern” while recovering from a hip-resurfacing surgery was one of the first patients to use the anti-gravity treadmill at the Boulder Center for Sports Medicine several months ago, said Rob Pickels, an exercise physiologist at the center. The woman was an ultra runner, but she found she couldn’t run comfortably after the surgery, Pickels said.

Pickels said the woman came to the center to see if the anti-gravity treadmill could help her, after she had done weeks of physical therapy without success. The woman walked normally again as soon as she stepped into the machine and had the weight load on her lower body lowered slightly, Pickels said. Outside of the machine, he said, she reverted to her post-surgery gait pattern.

It took eight sessions with the anti-gravity treadmill before the woman was able to go back to her pre-surgery walking and running gait, Pickels said. Even though the woman said she had no pain from surgery or physical therapy, it took awhile for her body to “relearn” its former habits, he said.

Fort Collins resident Andrew Lesser had a similar experience. The cross-country runner at Colorado State University used the machine at High Altitude Spine and Sport PC in Boulder to recover from a labral muscle tear in his hip.

After a 20- to 30-minute warm-up on the machine, Lesser said he would go outside with Dr. Richard Hansen to test how his body felt when he ran on his full body weight. After several sessions, Lesser said he was back to his normal outside workouts.

Anti-gravity treadmill rehabilitation patients often are the same ones who can use a pool to exercise after surgery, said Rodger Kram, associate professor in the integrative physiology department at CU-Boulder, who has done several research studies with the machines.

But the anti-gravity treadmill can speed recovery times over work done in the pool, since the treadmill can be fine-tuned quickly, Kram said.

“In the first two days after your operation, you can go from 90 percent of weight reduction to 80 percent, then to 70 percent,” Kram said. “In the pool, you can’t really do that. You can’t increase the load rate easily, other than a walk in the shallow end.”

How does an anti-gravity treadmill workout feel to a patient?

You put on a pair of Spandex-like shorts over your running shorts or other tight-fitting clothing, step into the machine, and zip the shorts into material in the machine that looks a bit like a spray skirt on a kayak. Next, air is blown into the material until it feels like somebody is pulling you up by your belt. Your body weight can be as much as 100 percent or as little as 0 percent through adjustments made to the treadmill.

There seems to be a psychological boost from the treadmills for patients recovering from surgery, making it a good tool for the Boulder Center for Sports Medicine, Pickels said. Physical therapy in a pool also can be good, but doesn’t necessarily provide the same positive psychological effects, he said.

“The anti-gravity treadmill mimics walking in people who haven’t walked (in awhile). You’re so happy and elated that you can physically walk,” Pickels said. “It’s a novel device, and people have fun with it. It makes them want to do more.”

Healthy runners use the anti-gravity treadmill at the Boulder Center for Sports Medicine and one at the Summit Rehab facility in the Life Care Centers of America Inc. Mountain States Division nursing home in Longmont, according to Pickels and John Dean, outpatient manager at the Summit Rehab facility.

Word-of-mouth and videos on the Internet appear to be driving interest among healthy runners, Dean said. Anti-gravity treadmills cost more than $100,000, putting the price out of reach of most health clubs and home gyms so far, Pickels said.

The Boulder Center for Sports Medicine offers punch cards that run about $65 per hour for use of the device. The time can be used in increments smaller than an hour. A package of three visits is $185, or 10 visits is $552.