What were those red circles on Michael Phelps?

As a dripping-wet Michael Phelps celebrated his 19th Olympic gold medal on Sunday evening, viewers at home might have found themselves squinting at his shoulders and wondering about the large purple-red polka dots on his skin:

Those spots come from a traditional Chinese medicine treatment called "cupping," when large hot suction cups are left on the skin for several minutes, resulting in hickey-like marks. The prevalence of such spots on athletes at the 2016 Olympics is proof that such forms of alternative medicine are becoming increasingly mainstream, going beyond the occasional Gwyneth Paltrow sighting:

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Cupping is believed to treat everything from muscle and joint pain to the common cold and cancer. "You're like, 'Okay, I'm sore here,'" men's gymnastics team captain Chris Brooks told USA Today. "Throw a cup on, and your roommate will help you or you can do it yourself."

But others say there is no proof cupping does anything at all. "There's no science behind it whatsoever. There's some vague conceptual connection with acupuncture, and is often sold by the same people. But how could it possibly do anything? It's nonsense," pharmacology professor David Colquhoun of the University College London told The Independent, adding, "It's desperately implausible, how the hell should sucking up a bit of skin in a cup do anything to your athletic performance?"