At the start of the Rio Olympics, several members of Team USA arrived to compete with what some people have described as odd-looking marks on their bodies. Although the red circles on the skin of such athletes as swimmer Michael Phelps and gymnast Alex Naddour may appear accidental, they are actually markers of cupping, a Traditional Chinese Medicine (TCM) technique used for performance improvement and healing.

The ancient practice of cupping has been used for more than 2,000 years in Oriental medicine and traditionally uses a flame to heat a cup that is applied to the skin. The air inside the cup cools, creating suction that stimulates blood circulation. Depending on the patient’s condition, cups are typically left on from one to 10 minutes, and they are sometimes placed and moved according to TCM meridian points, which those who practice the technique believe are channels through which the body’s energy flows.

Thanks @arschmitty for my cupping today!!! #mpswim #mp 📷 @chasekalisz

A photo posted by Michael Phelps (@m_phelps00) on

“When we use heat, it dilates the vessels and improves blood flow to certain areas,” Dr. Melinda Ring, executive director of the Osher Center for Integrative Medicine at Northwestern Medicine, told FoxNews.com. “Because of the way cupping works, it accelerates the process and is very localized.”

Studies have suggested cupping can help relieve pain when combined with other therapies like acupuncture and spinal manipulation. While there isn’t data on its efficacy for athletic performance, Ring noted it’s commonly used for this purpose.

“Our Olympic athletes are straining their muscles and working them hard, and this idea of stimulating blood circulation can benefit their muscles,” Ring said.

Depending on an athlete’s need, he or she can undergo the technique once or multiple times daily. Ring said few people have reported adverse consequences from cupping, but at worst she has heard of mild skin scalding. Sometimes, people report a tingling sensation where the cup was applied.

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The National Certification Commission for Acupuncture and Oriental Medicine is the only national certifying body in the United States, and all certified acupuncturists can practice cupping.

“Generally it’s a pretty safe thing to do and can potentially be beneficial, but it should not be done at home by somebody who doesn’t have any experience,” Ring said.

On Sunday night, Michael Phelps made his Rio debut for the men’s 4x100 freestyle relay sporting the red marks around his right shoulder as the U.S. team won gold. In March, the technique was featured in a video Phelps made with Under Armour.

The marks may indicate that Phelps’s shoulder has been under strain and a single application of the cups for even five to 10 minutes may have relieved his pain, Ring said, noting that she has not treated the swimmer.

As for those unsightly red circles, they’re simply a direct result of the suction.

“It shows how much suction is actually happening and is not a painful experience, even though it looks somewhat brutal,” she said. “It’s more of a cosmetic side effect rather than any indication something inappropriate happened.”