It was these dark, round splotches that raised eyebrows at the Olympic games in Rio.
The circles are from cupping, an ancient healing treatment.
In New York City, licensed acupuncturist Erika Weber said cupping therapy is when heated jars are placed on the body to create suction. And the suction stimulates blood flow.
ERICA WEBER, LICENSED ACUPUNCTURIST:
"An athlete can benefit from cupping because it moves blood stagnation in the muscle layer. When there's blood stagnation, it causes range of motion issues, inflammation, so this helps bring it to the surface of the skin, so free flow of blood can move through the muscles and help move muscles more freely."
In the West, the many health organizations are skeptical of the benefits of cupping and consider it an alternative treatment.
But people who've tried it, say it works for them.
COLETTE MAGNAM, CUPPING PATIENT
"So I have a neck and a back injury that can cause very, very tight muscles. It's a chronic condition, so with that, I find a lot of knots around my shoulder blades and I would say definitely the cupping helps to heal that process."
While the effects of cupping are disputed, U.S. Olympic swimmer Michael Phelps did win his 19th Olympic gold medal, but individual results may vary.