With the wag of a triumphant finger, Michael Phelps solidified his place as the most decorated Olympian of all time Tuesday night at the Rio summer games. The swimmer’s prowess is hard to deny, even to South Africa’s Chad le Clos, Phelps’ rival who scooped up the 200-meter Olympic butterfly title four years ago. Phelps set the record straight this year, propelling the U.S. team in the 4×100-meter relay team to a gold medal, the 25th of his career. Yet amidst all the gold and silver, this year’s audience could only see purple: giant purple circles dotting Phelps’ chest and shoulders, remnants from an ancient Chinese healing practice known as ‘cupping.’

After Tuesday night, the question remains: is cupping what’s propelling Phelps?

“There is science behind it, it’s just way, way, way behind it, probably over the horizon,” Medical Ethicist Art Caplan said in an interview Wednesday with Boston Public Radio. “It’s ancient, it’s Chinese, it has a tradition, it has proponents, it just is all woo. That is to say, there is no evidence that this kind of intervention does anything.”

The practice of cupping dates back to 3000 BC. Special cups are placed on the skin, creating a vacuum and drawing blood to the surface, creating those polka-dot bruises. Some athletes, including Phelps, swear by cupping to treat pain, deep scar tissues in the muscles and connective tissue, muscle knots, and swelling. According to Caplan, that’s most likely just a placebo effect. “If somebody makes you sit still and puts hot cups on you for awhile, or pumps pressure in and leaves you all marked up, I guess you want to believe it’s doing something,” he said. “But it really is sad that our best olympian ever is kind of a self-advertisement for pseudo-science. A lot of kids watch this, and a lot of people are talking about it.”

Ultimately, it comes down to hard science. In traditional Chinese medicine, cupping is believed to influence the flow of energy or “qi” through the body. “The problem [with] the ancient Chinese theory of all these energy flows and stuff,” Caplan said, “is that there’s just nothing to back it up in the body as we understand it.”

That’s of course limited to what we understand. From a scientific perspective, Caplan says we should stick with the facts. “Part of the problem is this— all of the coverage was basically, ‘what a great intersection of a tremendous athlete and an ancient healing technique,” Caplan said. “It didn’t say, ‘what a tremendous athlete and a kind of psychological effect that you get by doing weird things that you believe in.’ That’s what bothers me.”

To hear Art Caplan’s full interview with Boston Public Radio, click on the audio link above