What is Cupping? Michael Phelps' Body is Covered With Purple Marks After Alternative Therapy

Michael Phelps took to the pool for yet another medal-winning performance in Rio Sunday, but it wasn’t his speed in the water everyone was talking about — it was the purple circles across his shoulders.

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The marks are the result of cupping, an ancient Chinese therapy where special glass cups are heated with a flame and placed on the body.

It is said to increase blood flow as it creates a vacuum on the skin. It also relieves muscle pain.

The suction from the glass causes capillaries to burst, creating the deep purple marks on the patient's skin. It is basically the same side effect as a hickey.

The most decorated Olympian of all time has posted about the technique on Instagram in the past.

Phelps proudly showed off his purple marks in a recent Under Armour ad.

Gymnast Alex Naddour also uses the ancient technique, as does his team captain, Chris Brooks.

Read: French Gymnast Snaps Leg In Sickening Mishap, Then Dropped While On Stretcher

Brooks told USA Today: “You’re like, ‘OK, I’m sore here. Throw a cup on, and your roommate will help you, or you can do it yourself.”

Mark Moshchinsky, L.Ac of Tree of Life Acupuncture in New York City demonstrated how cupping works for Inside Edition.

He said: “if you think of a massage it pushes the tissue in, this pulls the tissue out. It stretches it.”

Inside Edition producer Alycia Powers had the procedure done by Moshchinsky. She said: “it feels like a bunch of tiny little hands just squeezing your back.”

Watch: Watch Michael Phelps and U.S. Swim Team Star in Epic Carpool Karaoke Spoof