Olympians are covered in red circles from cupping therapy but there's no evidence it works

You may have noticed a bunch of athletes at the Rio Olympics with large red circles all over their skin, including 19-time Olympic gold medal winner Michael Phelps and American swimmer Natalie Coughlin.

The marks are the result of cupping therapy, a technique using suction cups that has been practised throughout Asia for thousands of years. It is so popular in China that it is even performed by street vendors in the Yunnan Province, according to the Associated Press.

Proponents say that cupping can help with pain, back problems, and other general aches. However, modern science has not been able to confirm the benefits beyond a placebo effect, according to Business Insider's Rebecca Harrington.

Cupping therapy can be performed different ways. Traditionally, a burning cotton bud is placed inside a glass cup, which creates a vacuum inside as the hot air cools. The cups can also be attached to a pump that creates suction. Red, swollen welts appear where the cups were placed.

The therapy is believed to mobilise flow to the skin, which creates "a mild immune response," Kathleen Lumiere, an assistant professor of acupuncture and oriental medicine at Bastyr University in Washington, told the Wall Street Journal in 2012.

Still, there is limited research to back up the purported benefits of cupping.

In the same WSJ article, Romy Lauche, a scientist at the University of Duisburg-Essen in Germany, said, "the studies are very preliminary. We cannot say it has proven its efficacy."

A 2012 study published in PLos One, which reviewed 135 trials of cupping therapy from 1992 to 2010, found that "cupping has potential effect in the treatment of herpes zoster and other specific conditions" but "further rigorously designed trials on its use for other conditions are warranted."

It is common for athletes to try new treatments that may relieve pain, whether or not they are supported by science. In 2009, some athletes began wearing copper bracelets in the belief it might improve their performance.

In 2008 and 2012, Olympic athletes were seen wearing a bright adhesive known as Kinesio tape on their shoulders, backs, abs, and quads.

The special tape was developed more than 30 years by Japanese chiropractor Kenzo Kase, who claims that the elastic tape reduces pain and supports muscles without restricting movement like other traditional sports tapes. The tape "alleviates discomfort and facilitates lymphatic drainage by microscopically lifting the skin," according to the Kinesio website.

But like cupping, there are too few studies to definitely say whether it works. It is probably more psychological than anything.

"We need more evidence. We do not have research reports. Part of the reason people are using Kinesio tape is to find the science," Kase told BBC News ahead of the London 2012 Olympics.

"Personally, I think [the tape] is more of a placebo effect," John Brewer, a sports professor at the University of Bedfordshire, told Roberts. "There is no firm scientific data to show that it has an impact on performance or prevents injuries."

Study after study has documented the benefits of the placebo effect. People who think the have been given caffeine or morphine feel less fatigue or pain, even if all they have ingested is a sugar pill, according to Business Insider's Kevin Loria.

"A psychological edge and the confidence that comes with it may be all someone needs to hurl a javelin further than their opponent or to spike that volleyball one final time," Loria notes.