They were hard to miss — the dark, purply circles dotting Michael Phelps‘ back and shoulders and running all the way down his leg. Perfectly circular, they look like some kind of rash.
In Phelps’ case, they’re completely intentional. “I’ve done cupping for a while before meets,” says Phelps. “But I haven’t had a bruise like this for a while. I asked for a little help yesterday because I was a little sore and I was training hard.”
Cupping is a Chinese practice of sucking the skin away from the underlying muscles for a short period of time in order to stimulate blood flow. For swimmers, you’d want to do that to promote faster recovery and to swish away the build up of any lactic acid in the muscles that can lead to soreness, not a good thing if you’re racing heats in the morning and finals in the evening of the same day and have more than one race over a week.
Traditionally, cupping involves wiping the skin with something flammable like alcohol, and then lighting a match in an inverted small cup to build up heat. The match goes out, and the cup is placed over the skin, where the heat builds up pressure and starts pulling the skin away. Over several minutes, the blood vessels start to expand and tiny capillaries near the surface break, which causes the redness.