What are the circles on Michael phelps?

Michael Phelps had a procedure performed on him known as "cupping". It's traditional Asian medicine. A small, glass cup is heated with a flame to superheat the air inside it, and then placed on the skin. It creates a vacuum and pulls toxins out of the body. It also brings the blood up to the surface of the skin where it stays for several days and looks like round, red or purple bruises.

The treatment is used to help knotted up muscles and painful areas on a person's body.