Why does Michael Phelps have purple spots on his back?

After the American men’s 4x100-meter relay team won gold, Michael Phelps reached down to help anchorman Nathan Adrian out of the pool.

After the American men’s 4x100-meter relay team won gold, Michael Phelps reached down to help anchorman Nathan Adrian out of the pool.

Michael Phelps of USA dives to start. (REUTERS)

While he did it, millions of Americans watching at home saw several purple dots on his back and shoulder.

Some kind of new tattoo? Did he take a nap on a bed of tennis balls?

What is that purple dot? 

No, the circles are the result of cupping, a therapy technique that athletes use to help their muscles recover and perform at their best. It involves a therapist heating small glass cups, then placing them on the skin and pulling them from the body to loosen and relax the muscles.

With infant son Boomer watching from the second deck, the American superstar summoned a performance for the ages to propel Team USA to gold — the 19th of Phelps’ career.

7 dots in total !