Even as Michael Phelps piled a 23rd gold medal onto his stack, he also drew attention at the Rio Olympics for circular bruises on his shoulder resulting from a pseudoscientific medical treatment called cupping. Several ancient cultures practiced variants of cupping in order to reduce pain or heal injury. On Respectful Insolence, Orac writes "even if it does date back 5,000 years, arguably so does bloodletting." He continues "there is no compelling evidence that cupping is effective for any condition. Certainly, there is no credible evidence that it helps athletic performance." The benefit that Phelps and other Olympians perceive from cupping treatment is likely a result of the placebo effect. Treatments may also become part of a ritual for athletes or a superstition. Orac says "athletes have a distressing tendency to embrace pseudoscience, as long as they think it can give them an edge." Certainly Phelps has his edge, but there's no reason to think suction cups have anything to do with it.
See also: What's the harm? Cupping edition