As U.S. swimmer Michael Phelps glided through the water en route to his 19th Olympic gold medal this weekend, fans noticed a strange pattern of circular bruises on his toned shoulders. The perfectly round marks, a concerning shade of liver-purple, have also been spotted on the bodies of other swimmers and gymnasts on Team USA. Are U.S. athletes just bruised after sleeping on stacks of Olympic medals, as Twitter conjectured? No, but they’re the result of an ancient therapy that might keep the medals piling up.

The technique, known as cupping, uses hot cups to concentrate blood in certain areas of the body to suck out impurities with a vacuum. It’s been a mainstay in traditional Chinese medicine for centuries but has gotten traction in Western athletics as a recovery tool for elites. To create suction, practitioners — or sometimes the athletes themselves — turn over the circular cups onto their skin, then either heat the cups or apply an air pump to them. Overall, the effect is somewhat like receiving a hickey: The vacuum effect breaks the delicate vessels beneath the cup, allowing blood to seep out and pool under the skin’s surface. The result is a perfectly round bruise — and the belief that it’ll speed up recovery.

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

A photo posted by Michael Phelps (@m_phelps00) on

The idea is that bringing blood to overworked areas — on Phelps, they’re concentrated on his massive shoulders and thighs — speeds up recovery by increasing blood flow and reducing soreness. Physiologically, it makes some sense: To rebuild muscle that’s torn apart during strenuous workouts, the body needs nutrients, such as energy-rich glycogen and muscle-forming amino acids, which travel via the blood stream. Soreness is caused by a buildup of lactic acid in the muscles, which is naturally whisked away by rushing blood. Bringing more blood to certain areas and rapidly releasing it using cupping should, in theory, speed up the process.

A handful of scientists have attempted to explain the ancient technique. In a small pilot study published in the Journal of Integrative Medicine this year, Ukrainian scientists suggested cupping might manipulate the body’s immune response by decreasing levels of natural killer cells, which have been associated with increased pain. One commentary, published in the Journal of Bodywork and Movement Therapies in 2015, discussed research suggesting that cupping mitigated pain by stimulating the body’s mechanosensitive fibers, similar to the way massage therapy works.

A comparison of the effects of cupping with those of a healing technique known as progressive muscle relaxation, published in 2012 in the journal PLoS One, suggested that the ancient practice was better at improving “well-being and decreasing pain,” but the uncontrolled study, involving only 61 people, wasn’t exactly conclusive.

Despite the technique’s persistence throughout centuries and cultures, the research on its physiological effects remains thin. Western scientists mostly believe that the ancient technique’s benefits are a result of the placebo effect — that is, that Team USA’s athletes are merely tricking themselves into believing they’re being healed. The psychological benefits are, no doubt, enhanced by the inescapable visual reminders that the technique leaves behind.

Is the placebo effect a good enough reason to subject Olympic athletes to two straight weeks of looking like humanoid pepperoni pizzas? Quantifying its usefulness won’t be easy, but there’s no doubt that during the Games, which are as much a test of mental strength as they are of physical prowess, every little bit helps.

Photos via Getty Images / Al Bello

Yasmin is a writer and former biologist living in New York. A Toronto girl at heart, her writing also appears in The Last Magazine and SciArt in America. You might recognize her as a past host of Scientific American's YouTube series.