Turns out he and other athletes subscribe to “cupping,” an ancient Chinese therapy that raises bruises by sucking at the skin with a heated glass. Apparently, it hurts — and what’s more, there’s no modern medical evidence that does any good.
James Hamblin, writing in The Atlantic, equates it with anti-vaxxing. “A not-recommended Google-image search will show you some horrendously purple injuries. Phelps and some other athletes undertake them because they believe this process helps muscles to heal more quickly by encouraging blood flow. A bruise is a blood clot, though, and clotted blood is definitionally not flowing. So in terms of role-model behavior, cupping may be more deleterious than a grainy bong photo, because it invites people to distrust science.”