Phelps: What were those purple body spots at the Rio Olympics ?

RIO DE JANEIRO, Aug. 8 (UPI) -- American Olympic legend Michael Phelps is a maven in the unexplainable.

But one thing we can explain is why he is sporting purple dots on his skin while competing in the Summer Games.

Phelps, who has 19 career gold medals, debuted for Team USA Sunday in Rio. He helped Team USA secure a gold medal in the 4x100-meter relay.

The dots? They were from 'cupping.' Cupping is an old Chinese healing practice.

"Because this particular recovery modality shows blemishes on his skin, he walks around and looks like a Dalmatian or a really bad tattoo sleeve," Phelps's personal trainer Keenan Robinson told the New York Times. "It's just another recovery modality. There's nothing really particularly special about it."

Phelps had a practitioner place special cups on his skin that were either heated or pumped with air to create suction onto his skin. The skin is separated from the muscles underneath, allowing them to relax and heal.

Last year, Phelps posted a photo to his Instagram account, with a practitioner cupping the back of his legs.

"Thanks @arschmitty for my cupping today!!! #mpswim #mp @chasekalisz," Phelps wrote.

Sports performance expert Ralph Reiff explained the process to USA Today's Josh Peter.

"Think of a traditional suction cup that you might put on a wet window,'' Reiff told Peter. "It stays there and it creates suction underneath it. It's the same principle of what the cupping does. It creates a vacuum and lifts the skin up in that space and therefore creates a lift of all the soft tissue."

"Depending on how long you leave it on one particular of place, you get an infusion of fluid in that one area. That's why you see the marks on some of the athletes. There's an increase in blood flow to that area," Reiff told Peter.

"Sometimes it actually it breaks up some of the capillaries on the surface of the skin. So that's why you see the discoloration," Reiff told Peter.

Team USA silver medalist Chase Kalisz and bronze medalist Dana Vollmer also 'cup.'