The U.S. Olympic swim team has a history of improving its times from the trials to the games.

The Americans are counting on it happening again next month in Rio after not setting a single world record during the eight-day selection meet.

Led by Michael Phelps and Katie Ledecky, the U.S. still boasts the most powerful swim team in the world.

But other countries have improved since four years ago in London and Phelps knows he’s got to be much better by the time the Rio Games begin.

Clearly, there’s cause for concern.

Many of the times swum in Omaha were slower than the 2012 trials, where no world records were set. Ryan Lochte and Missy Franklin struggled this time around and they’ll be joined by 29 rookies on the team, one more than in London.

“We’re going to have to improve for sure,” David Marsh, head coach of the U.S. women’s team, said Sunday.

Phelps has said he wants to swim another personal best at least once before he retires after Rio. He was faster at last year’s summer nationals where he swam times that would have won medals at the world championships in Russia.

“It’s been 2009 since I’ve done a best time,” he said. “It’s frustrating to not be able to go the same time or faster than I did last year, especially because I think I’m in better shape than I was last year.”

Phelps joked that if Bowman doesn’t have a plan to make him faster he’ll have to fire the only coach he’s had his entire career.

“His times this week were mediocre, and I’m crystal clear on what to do,” said Bowman, who will guide the U.S. men in Rio.

Phelps qualified in three individual events, Ledecky and newcomer Maya DiRado made it in three each, and Franklin will swim two.

Ledecky carries huge expectations into her second Olympics. In January, she lowered her own world mark in the 800-meter freestyle at a meet in Austin, Texas, where she set world-leading times in the 200 and 400 freestyles.

She’ll try to become the first woman since American Debbie Meyers at the 1968 Mexico City Games to sweep the 200, 400 and 800 free.

“I’m just going to focus on my racing and what my goals are, and anybody else’s expectations don’t really mean that much to me,” Ledecky said.

The average age of the men’s team is 23.9 years, with Anthony Ervin the oldest at 35. The women’s team averages 22.3 years, with Ledecky the youngest at 19 just as she was in 2012.

Several stalwarts of the U.S. team failed to punch tickets to Rio, including Natalie Coughlin, Tyler Clary, Matt Grevers, Cullen Jones and Jessica Hardy. All of them medaled in London, where the team won a leading 31 medals, including 18 golds.

“It’s challenging being at trials and being able to make the transition from here to the games,” Phelps said. “You got to be ready when the lights come on and if you’re not, you could fall short by a hundredth or a few hundredths, and that’s the tough part.”

Heading to his fifth and last Olympics at 31, Phelps appears more willing than he has in the past to share his experience and expertise with his younger teammates. He certainly has more time, with eight events no longer on his schedule.