The dark purple spots on U.S. swimming star Michael Phelps' shoulders during the Rio Olympic Games have origins in a traditional Chinese healing practice dating back thousands of years.

Phelps, who won the 19th Olympic gold medal of his career on Sunday and helped the U.S. men's team to victory in the 4x100 meters freestyle relay, is one of a number of Olympic athletes practising "cupping".

During the therapy cups are placed on the section of the body in need of repair to create suction. The method is said to draw out any toxins, mobilise blood flow, soothe sore muscles and even help cure insomnia.

"This is our traditional way and method of treatment. Cupping therapy relies on suction pressure," said Sun Huanjiang, who has worked as a Chinese medicine practitioner for 26 years.

"For example, if (this person's) blood circulation has problems, the suction pressure brings the blood here. Then their metabolism will gradually pick up. That's the effect, taking the illness away." Sun explained.

Cupping therapy can be traced back over 6,500 years to ancient China, and also has a long history in the Middle East and Egypt. But it has been less popular among more sceptical Western medical societies.

"Gradually, not only Chinese but also people from around the world can understand this ancient and traditional Chinese therapy. With more understanding of China, I think our Chinese medicine, especially acupuncture and cupping, will be understood by people from around the world," said Tong Xun, a 70-year-old retired professor who specialized in religion.

Wang Shufeng, 75, said the treatment quickly alleviated symptoms she had just before visiting a traditional Chinese medicine clinic.

"When I came in, my head felt dizzy, my eyes were blurred, and my neck was stiff. After the massage and acupuncture, I'm feeling much better," said Wang.

Chinese social media has exploded with photos and posts of Phelps' circle-covered back since his debut swim this week.