Michael Phelps' assault on destroying the Olympic record book has been overshadowed by a cosmetic quirk.

If you watched Phelps notch another gold medal on Sunday (his 19th overall, adding to his total as the most decorated medalist in Olympic history), you couldn't help but notice a series of discolored circles on his body. So, what exactly were they?

Cupping is a practice also done by the U.S. men's Olympic gymnastics team. According to USA Today, "The technique uses suction in cups to pull the skin away from the body and promote blood flow. It can be done with heat by lighting flammable liquid in a glass cup and suctioning that to the back once the flame goes out, with the decrease in temperature creating the suction."

Getty Images

Phelps has practiced cupping before and posted about it on social media.