Being an Olympian is difficult.

For four years, you push your mind and body to its limits in an effort to conquer a few hours, a few minutes or even a few seconds. Qualifying for one Olympic Games is amazing, win a medal and it defines a career, but for American athletes Michael Phelps, Kerri Walsh Jennings and Allyson Felix, their Olympic dominance — the trio has combined for 39 medals so far — has set the standard in their respective sports that others aspire to achieve.

Unfortunately, even the best can’t stay at the top forever. Phelps, the greatest swimmer of all time, announced his retirement following the end of the swimming competition. Walsh Jennings, widely considered the greatest women’s beach volleyball player ever, hinted Rio was her last Olympics. Phelps previously retired following the 2012 Games in London but said he felt like he had unfinished business.

“I always said to myself, when I hang up my suit, I don’t want a ‘what if?’ ” Phelps said. “And I know that if I had stayed away from the sport after 2012, I would have 100 percent regretted that. And that’s what I didn’t want.”

Phelps won six medals in Rio, five of them gold, to claim the all-time record for gold medals with 23 throughout his career. But Phelps also made personal strides between the London and Rio Olympics. He got engaged and had a baby. He invested more time in helping the sport of swimming grow through the Michael Phelps Foundation, and the Michael Phelps Swim School has taught more than 15,000 kids how to swim.

Walsh Jennings, a 38-year-old wife and mother of three, has made similar peace with her potential retirement and like Phelps, wants to use her achievements to help grow her sport and help young people become better leaders.

“I want to build up my presence in our sport,” she said. “My husband and I want to have a leadership academy. Volleyball will kind of be the foundation of it, but we want to talk about character and team-building and what it takes to be great in life.”

While Walsh-Jennings didn’t win the gold for the first time since her first Olympics in 2000, she had the perspective to appreciate the grit it took to win the bronze and potentially end her career with medals in four of five Olympics.

“Once you win a semifinal, the gold medal match is easy,” Walsh Jennings said after the bronze medal match. “No disrespect to win the gold medal, it takes everything you have, but you’re going to go home with a medal. The bronze medal match is the gnarliest match I’ve ever played in in my career, because you either go home with a beautiful medal or you get nothing.”

The only one of these three competitors who hasn’t broached the subject of retirement is Felix, who is still competing in the Games. Like Phelps, Felix felt a sense of unfinished business in these games and wanted to defend her 400-meter title. Felix, 31, took the silver, which might prompt one last Olympics in 2020.

“I’m just a competitor,” Felix said. “I love to win. So I have had success before, but I think your goals just kind of shift. For me, that’s like really what this year’s about.”

Graham Watson is the editor of Dr. Saturday on Yahoo Sports. Have a tip? Email her at dr.saturday@ymail.com or follow her on Twitter! Follow @YahooDrSaturday