It leaves dark red welts on your skin, but U.S. swimmers say the temporary marks are worth it.

They were hard to miss the dark, purple circles dotting Michael Phelps' back and shoulders and running all the way down his leg. Perfectly circular, they look like some kind of rash.

Cupping is an ancient Chinese practice of creating a vacuum using a plastic, bell-shaped cups to lift 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 flush away the buildup 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.

The U.S. swimmers, including silver-medal winner Chase Kalisz and bronze medalist Dana Vollmer, have also adopted the practice in Rio, but they’re using a modern version of the therapy with a hand-held pump to create negative pressure to slighly lift the skin away similar to a vacuum.

Keenan Robinson, Phelps’ longtime strength-and-conditioning coach, says he first introduced him to cupping in 2014, just before the pan-Pacific championships. The previous year, Phelps’ coach Bob Bowman held a training camp at altitude, and several Chinese swimmers attended. They brought with them their physical therapists and their own recovery methods, which included cupping (they did it the semi-old-fashioned way with bamboo shoots).

In 2015, Phelps started increasing his workouts and the intensity of his swimming to prepare for Rio, and finding a quick way to recover and relax his muscles was a high priority. Robinson says he doesn’t use cupping to heal, per se, but rather to help keep the fascia lubricated.  Fascia is the knitting in-between and on the surface of muscle that allows muscles to move more freely and easily.

The intent is to minimize fascial restriction, muscle adhesions, and knots in the muscle so the motion is smoother. Instead of pushing down on muscle and fascia, as massage does, cupping pulls the layers of muscle and fascia apart, much like separating the layers of a flaky pastry, so fluid can flow more easily in between them to keep them well-oiled.

For Phelps, that’s getting cupped around twice a week. Even if it does leave a mark.
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