If you were watching Michael Phelps swim his first event of the Rio Olympics on Sunday night, you may have been left wondering what all those red circle marks splayed all across his back were.
The marks are actually the result of cupping therapy, a healing technique that is particularly popular among Olympians.
“Basically, cupping involves a cup attached to a pump,” NBC News explains. “Athletes put the cup on their skin and create suction with the pump. Some say the technique increases blood flow and helps a person’s sore muscles heal.”
The circular spots are bruises from the cups that can last up to two or three weeks.
American swimmer Natalie Coughlin shared an Instagram post of her getting the treatment:
Some studies have questioned whether or not there are truly benefits to the cupping therapy.
USA Today posted a video explaining the “cupping” therapy: