What is cupping - and why are some athletes obsessed with it?

When Michael Phelps won his first gold of the Rio Olympic Games, many viewers were looking at something other than his medal – the bright purple circles on his back and shoulder.

The marks come from an ancient Chinese form of alternative medicine called cupping. The US Olympic team seems to be a particular fan of the technique, with both Phelps and gymnast Alex Naddour pictured sporting its distinctive marks.

What is cupping?

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Cupping is a form of acupuncture in which heated glass suction cups are placed on the skin. The vacuum within gradually draws the skin upwards.

The practice was first recorded in China in 1550BC and was used in Ancient Greece and throughout the Arab world and Europe in the Middle Ages.

Advocates claim it can relieve an array of maladies, including muscle and joint pain, eczema, bronchitis and even the common cold.

"Cupping is thought to draw blood to the affected area, reducing soreness and speeding healing of overworked muscles," the New York Times says.

The process can rupture the capillaries beneath the surface of the skin, causing the red or purple marks which have caught TV viewers' eyes at the Olympics. These typically fade away after a few days.

Does it work?

A 2010 review of 550 clinical studies carried out in China found no significant health risks associated with the technique and even some evidence of its efficacy as a pain reliever, MarketWatch reports, although this could be due to a placebo effect.

The majority of medical practitioners say that, as with other forms of alternative medicine, the rationale behind cupping is no more than pseudoscience.

"Cupping has, from our perspective, no medical benefit whatsoever," Dr Tony Bartone, of the Australian Medical Association, told ABC. He warned that bruising or bleeding as a result of the practice "in really bad cases, could create some deep scarring or nodules".

Even so, cupping converts are convinced they have hit on a winner. The suction cup therapy is "better than any money I've spent on anything else", Naddour told USA Today. "That's been the secret that I have had through this year that keeps me healthy."