So, what are those marks on Michael Phelps' body?

EPA/Bernd Thissen

RIO DE JANEIRO -- Not since Oscar-winning actress Gwyneth Paltrow walked down a red carpet with red marks on her back has "cupping" received this much attention.

Michael Phelps, the winningest Olympian of all time, walked out on the pool deck for his first race in Rio sporting dark reddish-purple circles on his upper body. What were they exactly, you ask?

It's back to Paltrow and that cupping thing.

Cupping is a recovery method where suction cups pull back skin to loosen muscles and tendons. Many U.S athletes, including other swimmers and gymnasts, are using the treatment, which also increases blood flow to speed up recovery.

You can see a snippet of Phelps receiving the treatment in his Under Armour ad that came out earlier this year. We also remember wincing a bit when Natalie Coughlin posted images of her cupping experience late last year:

Laughing because it hurts so bad. Gonna leave a mark! #AthleteLife

A photo posted by Natalie Coughlin (@nataliecoughlin) on

Either way, Phelps won his record 19th Olympic gold medal Sunday night.

Hey, whatever works, right?

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