What's up with the purple circles on Olympic athletes?

Olympic swimming sensation Michael Phelps is certainly making his mark in Rio, sporting large circular bruises on his body over the weekend.

They were the tell-tale signs of a healing technique called "cupping" used by a growing number of athletes to loosen tight muscles and speed up recovery time.

Phelps and several other athletes at the Games swear by the ancient therapy — which involves using cups and either heat or an air pump to create a suction. The cups are placed on sore muscles, typically the back, and the vaccum sucks up the skin. This is believed to enable the blood vessels to expand and increase blood flow to the affected area.

Cupping time lasts between five and 15 minutes. The tugging and rupture of tiny blood vessels is what creates the deep red bruises, which tend to disappear in a few days.

Cupping is an ancient therapy used in Greece, Egypt and China. It was in the spotlight Sunday as Phelps, with visible marks on his back and arms, swam in the 4 x 100 men's relay, winning yet another gold medal.

The giant polka dot scars also have been spotted on U.S. Olympic gymnast Alexander Naddour.

Closer to home, Gustavus Adolphus College swimmers have been using the technique too, according to Swimming World magazine.

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Allie Shah • 612-673-4488

@allieshah