When Michael Phelps took to the water to swim in the 4x100-meter relay Sunday, many noticed weird purple circles covering his right shoulder. The dots are the result of cupping, the latest alternative therapy elite athletes are using to try to recover faster and perform better.

But as with a lot of alternative therapies, the science on these medicinal hickeys is pretty inconclusive, suggesting you may not need to sprint off to a cupping practitioner to try it out on your sore muscles.

Cupping comes from traditional Eastern medicine

Phelps got those marks from glass or plastic vacuum cups that were placed on his skin by his personal trainer.

The practice involves very simple tools: plastic or glass cups and a vacuum pump. The process is very simple as well: The cups are placed over muscles, and then, using the pump, you create an area of vacuum pressure that draws blood to the surface, breaks capillaries, and forms the perfect purple circle on the skin.

A mechanical vacuum isn’t always needed; sometimes the cups are heated and then placed on the skin. As the cups cool, the air inside them contracts, forming the negative pressure needed to bring blood to the surface.

Cupping has been around for centuries in traditional Chinese medicine; it’s also used in the Middle East. Chinese medicine practitioners would say it helps open up channels of qi, or the body’s life force. Athletes claim it aids healing, recovery, and blood flow and reduces pain.

Phelps has apparently been cupping for some time. This photo on his Instagram page was taken about a year ago.

Thanks @arschmitty for my cupping today!!! #mpswim #mp @chasekalisz A photo posted by Michael Phelps (@m_phelps00) on Sep 10, 2015 at 12:29pm PDT

Phelps isn’t the only American athlete adorned with the spots — Alexander Naddour, an American gymnast, has been seen with the purple blotches too.

Cupping seems to be safe. But does it work?

Unlike many alternative therapies, cupping has been studied enough for meta-researchers to do several systematic reviews of the scientific evidence. Individual studies can exaggerate effects or suffer from design flaws, so researchers use systematic reviews to cut through hype and understand where, on the whole, the bulk of the evidence lies.

For cupping, the systematic reviews I read all suggested the practice isn’t harmful — but the studies on it are too weak to come to solid conclusions about whether it really works.

"We included 550 clinical studies in this review ... 78.1% of these [randomized clinical trials] were with high risk of bias," read one such review, published in a 2010 edition of BMC Complementary and Alternative Medicine. Papers with high risk of bias either don’t blind the researchers to the treatment groups or have their methods compromised in another way.

The review also noted a big limitation to evaluating cupping: There’s no standard method to measure its effectiveness.

The review concluded:

The current evidence is not sufficient to allow recommendation for clinical use of cupping therapy for the treatment of above diseases of any etiology in people of any age group. The long-term effect of cupping therapy is not known, but use of cupping is generally safe based on long term clinical use and reports from the reviewed clinical studies.

Another 2012 review in PLOS One looked at 135 studies and also found "a lack of well-designed investigations." The PLOS One review did find some evidence that cupping works, but it was for specific applications like "herpes zoster [a.k.a. shingles], acne, facial paralysis, and cervical spondylosis [an age-related degradation of spinal disks in the neck]." Not exactly conditions that affect an athlete’s performance.

A red flag for a treatment like cupping should be that no one can explain exactly how it is supposed to aid athletic performance. "The mechanism of cupping for pain remains largely unclear," a 2015 systematic review of systematic reviews on cupping reports in the Journal of Traditional Chinese Medical Sciences. (A review of reviews is as meta as science gets.) If you can’t explain what’s going on, it’s hard to know what variables need to be closely studied.

It could be that cupping brings more blood to an area and this promotes healing. But that’s just a guess. Some say it helps relieve stress in the muscles by pulling them upward. Overall, "larger well-designed trials are needed to validate the therapeutic efficacy of cupping therapy," the 2015 review reads.

This is the space where a lot of fad health trends thrive: There’s no good data to prove cupping helps, but, likewise, there isn’t data to disprove it either. Meanwhile, you have celebrity endorsements to propel the fad forward.

There’s one way it could help, however: the placebo effect.

Athletes are superstitious folk. If they try something once — like cupping, or wearing an "energy" bracelet, or what have you — and perform well, they may get scared about what will happen if they stop. Studies suggest that caving in to these superstitions can ease athletes’ minds and help them maintain confidence in their abilities.

And since cupping is probably harmless, that would not be the worst thing.