Michael Phelps’s first gold medal of the Rio Olympics was momentarily overshadowed by widespread interest the large purple circles across his back and shoulders.

Viewers were taken aback by the perfectly circular marks which were revealed as Phelps shed his clothes to compete in the men’s 4x100m freestyle relay on Sunday, going on to win his 19th Olympic gold.While the guesses as to their origin offered on social media ranged from crop circles to tattoos to a symptom of the Zika virus, the real answer is they are the result of cupping therapy – a suction-based massage popular in the US Olympic team.

In the ancient form of the therapy, believed to originate in Egypt or China, the air inside the cup is heated before being place on the skin, creating suction in that cupped area. The modern form of the therapy, and the one that Phelps has been photographed receiving, uses cups attached to suction pumps.

The cup stays in place for between five and 15 minutes and is thought to draw blood to the treated area, allowing overworked muscles to heal more quickly, although there is some doubt about the efficacy of the treatment.

Phelps's fellow swimmer Natalie Coughlin has posted pictures on Instagram of herself receiving the therapy, while the US gymnast Alex Naddour has been competing at the Games with a prominent cupping bruise on his shoulder.

The interest in the mysterious marks was widespread. Google Trends reported a 2,100 per cent spike in searches for “circles on Michael Phelps” on Sunday. For a short time that phrase was being searched for more than the term “Olympic medals“, while speculation was also rife on Twitter.

(Guardian service)