You might have wondered about those circular marks on Olympic athletes such as swimmer Michael Phelps.

United States' Michael Phelps competes in the final of the men's 4x100-meter freestyle relay during the swimming competitions at the 2016 Summer Olympics, Sunday, Aug. 7, 2016, in Rio de Janeiro, Brazil. (AP Photo/Matt Slocum)

They're the result of a practice called cupping, used to relieve pain and tension. A glass cup creates a vacuum which causes a small amount of inflammation.

According to practitioners, this tells the body to speed up the healing process.

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For Phelps, cupping could help in several ways.

"Cupping can really accelerate muscle healing and also blood profusion to muscles which allows them to have a great range of motion and also can help improve strength and improve any kind of soreness that he might have,” said Jessica Manson of Integrative Oriental Medicine

"Feels good. Like I said my back just feels warm right now and I think tomorrow I'll feel less tension in those muscles and in my upper shoulders and my back," said Briand Clark, a cupping patient.

Cupping is also used for cosmetic purposes, such as reducing the appearance varicose veins.

The marks last around 5 days.