Why Michael Phelps and other Olympians have big red circles all over their skin

NBC

Michael Phelps will swim his first event of the Rio Olympics on Sunday night, and he'll do it with noticeable red marks splayed all across his back. 

If you've watched any of the Olympics so far, you've likely seen similar spots on other athletes, including some American male gymnasts. The marks are the result of cupping therapy, a healing technique that's particularly en vogue at the moment among Olympians.

As NBC explained:

Basically, cupping involves a cup attached to a pump. 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 spots are bruises from the cups, and can apparently last up to two or three weeks. Here's American swimmer Natalie Coughlin getting the treatment:

Coughlin's bruises also seem to be much worse, though that's likely from right after the treatment:

Here's another angle of Phelps' back, from Sunday night:

NBC

Phelps added cupping to his recovery routine leading up to Rio. The treatment is featured in a pre-Olympics commercial from Under Armour. 

It certainly looks painful. But if it makes your muscles feel better, and gets you to the Olympics, so what?