In the hours before Diana Nyad completed her record-breaking swim of more than 100 miles from Cuba to Florida, her team blogged that she was "really hurting" and that her lips and tongue were swollen. But she didn't stop.
Psychologists and sports experts alike agree that there's something special about Nyad that's pushed her through the ocean for more than 50 hours as she made her way to Key West.
"It is a very high level of confidence," said Kathryn Olson, CEO of The Women's Sports Foundation, who knows Nyad personally. "It's not a confidence you and I might have in our ability to do something. It's a confidence so deep that it conquers."
Nyad, 64, swam the distance, which is 35 miles more than anyone has ever swam before, without a shark cage, according to her website. She's attempted the feat four previous times, and had to stop in 2012 because jellyfish stings burned her face and limbs.
"There's the thrill actually of accomplishing something very few people would do," said Judy Kuriansky, a psychologist at Columbia University's Teachers College, better known on the radio as Dr. Judy. "There's both an inner satisfaction and also the thrill of everybody else saying, 'I would never do that.'"
Kuriansky, who has never met Nyad, said she wondered what Nyad's childhood was like. People who accomplish extreme athletic feats usually were either praised for their over-the-top accomplishments early on in life or they were "scaredy-cats" as children and want to prove themselves as adults, she said.
After learning that Nyad began striving for the Olympics as a young girl but was sidelined at 17 with endocarditis, a virus in her heart, Kuriansky said she thought the goal of swimming from Cuba to Florida was more internal than external for Nyad.
"This is a personal conquest for her over her body and to push her body," Kuriansky said. "It's about survival because her body was broken. That's a very interesting thing that she's had a disability, and she's overcome it."
It also takes a certain level of persistence to keep going when your body is tired and in pain, Kuriansky said, adding that she knows it well. She's climbed Mt. Kilimanjaro.
"Something inside you just takes over," she said. "You train your mind to be absolutely determined. It's both for yourself and to prove yourself. You will do it despite all odds."
Athletes who take on grueling efforts learn to focus on something other than the pain or, in Nyad's case, the fear of a shark attack, Kuriansky said. They do not think about failing, and sometimes put themselves in a "bubble" of faith in their ability to accomplish their goals safely.
Ann Rosen Spector, a psychologist with a private practice in Philadelphia, said there are thrill-seekers in almost any sport who take on risky challenges willingly, constantly amping up their goals. Even people who have been injured and return to a football field or a basketball court do it because they like being the one on the team to play through an injury.
The amount of training and determination it takes to meet a difficult goal also says something about an extreme athlete's ability to sometimes go without short-term happiness in favor of long-term satisfaction. These people will often pass on hanging out with friends and relaxing in favor of training because their goal outweighs their need for immediate pleasure.
"Not everybody has that kind of stick-to-it-iveness," she said. "It has a lot to do with how some people need to be gratified in the short term."
Olson said she has no doubt that Nyad's training was hard, but Nyad also spoke and wrote about how the dawn light on the water was so beautiful that it carried her forward.
"It's not all pain," she said. "There's a lot of joy."