Image courtesy Elite Daily

So, you’ve been relaxing, enjoying watching the Olympic Games in Rio… when all of a sudden you start seeing spots!

No, you (probably) don’t need your vision checked.

Olympic athletes this year, and swimmers in particular, seem to all be covered with these mysterious purple-red spots on their skin. Is it a new fashion statement? A product tie-in? A strange reaction to the documented pollution plaguing Rio at the moment? Actually, it’s none of the above.

The spots are the product of a physical therapy technique known as “cupping,” a process that actually has its roots in centuries-old Chinese medicine. Suction cups are warmed up, then placed on the skin atop a sore muscle group. The suction vacuums the air out of the cup, and capillaries begin to break from the resulting pressure. Immediately, increased fluid and blood flow rush to the affected area, and recipients of this therapy insist that the resulting feeling helps to alleviate muscle pain.

There have been clinical trials — albeit with small sample sizes and some reliance on anecdotal reporting — that show cupping to work favorably on both cancer pain and lower back pain. As to whether it gives Olympic athletes a (legal) edge, you’d have to ask them… but you might just find your answer by counting the sheer number of swimmers sporting the distinctive round marks.

A lot of people watching want to know: is cupping safe? After all, it involves intentionally rupturing capillaries and subjecting muscle tissue to some pretty intense treatment. The truth is, the only major study on cupping from 2012 does seem to indicate that it is mostly harmless.

Still… no matter how cool those spots might look on Michael Phelps, you might not want to try this one at home!