Michael Phelps

As Michael Phelps won his 19th Olympic gold medal Sunday by helping the U.S. take the 4x100 freestyle relay in Rio, lots of fans were buzzing about the red spots on his body.

Well, it's not a skin condition or a bunch of perfectly circular hickeys. It's the result of cupping, an ancient technique that uses suction cups to help the body recover faster.

Michael Phelps

"An awesome treatment to reduce inflammation, pain, stiffness; improve mobility and flexibility, and remove toxins deep from the muscles," says Luriko Ozeki, owner of Iyashi Wellness in Los Angeles. "Also great for upper-respiratory problems like coughs from colds/flus. This is a must at my office and at home."

Cupping is even featured in an Under Armour commercial with Phelps. Check it out at the 0:44 mark of this video:

Here's more of Phelps getting the cupping treatment:

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

A photo posted by Michael Phelps (@m_phelps00) on

Natalie Coughlin, who enters Rio with 12 Olympic swimming medals, is also a big believer in cupping:

Gee, I hope my #GoldenGoggles dress is open-backed.

A photo posted by Natalie Coughlin (@nataliecoughlin) on

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