The marks come from a traditional Chinese practice known as “ba guan” — or “cupping” in English — where small glass jars are heated and placed on the skin. The heat creates a seal that sucks the skin roughly three centimetres into the jar, drawing blood to the area.
It can also be applied with glass suction cups, which appears to be the method Phelps uses according to photos posted on his verified Instagram account.
Backs, arms and legs covered with red circles are already a common sight in China on men and women of all ages, but the practice has boomed in popularity since Phelps’ marks were spotted, practitioners say.
Its primary use is supposedly to relieve pain, although there is little conclusive evidence of any benefit.
But Chinese media have been cheering cupping’s appearance at the Olympics as proof of the value of traditional culture, with both the official Xinhua news agency and Communist Party mouthpiece People’s Daily touting the soft-power benefits.
“Chinese traditions and products proliferate Olympic village”, read one headline on the People’s Daily website.
Ding Hui, manager of the Lily Spring Health & Spa in Beijing, said she has seen a 30 percent jump in clients asking for cupping treatment since the Olympics started.
“Even though Chinese people have known about it for a long time, they see a great athlete does it and see it really works,” Ding said.
“For athletes, they build up harmful lactic acid in the body and cupping can help relieve it.”
But in several studies researchers have said that while patients report feeling less pain when treated with cupping, they cannot be sure of a causal link because there is no way to rule out the placebo effect.
There have been relatively few clinical trials and outside China the practice has been largely restricted to alternative medicine clinics.