Michael Phelps woke up on the morning of the first race of his fifth Olympic Games with some soreness in his right shoulder.
At 31 years old, the swimming superstar could not afford to have anything slowing him down. So he went to a Team USA athletic trainer for some cupping therapy, a treatment he has been receiving for years to help relax his muscles and ease soreness.
"The trainer hit me pretty hard with one and left a couple of bruises," Phelps said.
With large purple circles dotting his shoulder and back, Phelps delivered a performance for the ages to lead the 4x100-meter freestyle relay team to victory, giving him his 19th career gold medal.
Phelps swam the fastest 100 meters of his life, a blazing 47.12 seconds on the second leg of the relay that turned a slim deficit in the race against France into a comfortable lead that teammates Ryan Held and Nathan Adrian were able to hold the rest of the way.
The stirring victory -- televised in prime time back home in the United States — put Phelps back on the podium and thrust cupping therapy into the spotlight.
It dates back centuries and has enjoyed a bit of a renaissance over the past decade after athletes like Phelps and NFL star DeMarcus Ware and actresses Gwyneth Paltrow and Jennifer Aniston started touting its benefits.
Researchers have traced cupping's origins back to China and Greece somewhere around 1500 B.C., and Team USA gymnast Alex Naddour was among other Olympians who have been seen with the purple marks in Rio. And it's not just for star athletes.
Health spas often offer the service for a few hundred dollars and the cups can be purchased online for as little as $15 and applied at home.
Some in the medical community believe it's nothing but hocus pocus, the latest form of snake oil that tricks patients into paying for it again and again. Others insist that it aids recovery, relaxes muscles and helps an athlete maximise performance.
Phelps certainly believes in it. In the end, that may be all that matters.
"I've done it before meets at pretty much every meet I go to," he said.
The treatment involves applying glass or plastic cups to the area of discomfort and either applying heat or suction to create a vacuum. The suction pulls the skin away from the muscle and draws oxygenated blood to the area. The suction also is what causes the bruising, like giant hickey without the fun that comes with it.
"I haven't had bad ones like this in in a while," Phelps said on Sunday, a day after the relay.
Steve Hamilton, a massage therapist for the Denver Broncos, has used the technique for years in working with players. He said the biggest benefits include increased circulation, decreased muscle tension, decreased inflammation, increased range of motion and improved blood flow.
"Its effectiveness is because it's all part of the body's natural healing process," Hamilton said. "But cupping helps the body recover faster by keeping the process moving forward. It's awesome because it's all natural."
But there is a scarcity of accredited studies to prove cupping's efficacy, which has led to a fair amount of skepticism. Many doctors have called it nonsense, and others believe it is no more effective than a placebo.
Russian state TV, after watching more than 100 of its athletes become embroiled in a doping investigation that excluded them from the Rio Games, took a dig at Phelps when it branded cupping "as-yet legal doping" in a lengthy feature on the treatment.
Hamilton said cupping is "no such thing."
"It helps enhance the body's natural neurological and circulatory function, thus allowing the body to be pushed and perform at a higher competitive level," he said.
Phelps even included the process, which can be painful, in a commercial he did for Under Armour on the rigorous training he puts his body through to get ready to compete.
When he is competing in an event where victory is measured in fractions of a second, any little edge could make the difference.
"If he feels like he feels better," Hamilton said, "that's never a bad thing.