When the London Olympics get underway this week, much of the intrigue will again surround Usain Bolt and Michael Phelps. The two icons from the Beijing Games are back, their place in history secure, but each faces a fresh set of questions.

Thought to be invincible four years ago, Bolt and Phelps have been exposed as vulnerable by challenges from younger competitors.

“When you’re the king of the jungle, there’s always some young lion who wants to hit you in the head and take your prize,” said Ato Boldon, a four-time Olympic medalist and now a track and field analyst for NBC.

Bolt has lost to countryman Yohan Blake the past two times they’ve squared off in the men’s 100 meters. And Phelps’s standing in the pool — if not his celebrity — has been challenged by Ryan Lochte, who is seemingly engineered for magazine covers and medal podiums alike.

Bolt will try to defend his track titles in London, making history in the process. Phelps, meanwhile, needs three medals at these Games to claim the Olympics career record from Larisa Latynina, the Soviet gymnast who won 18 medals from 1956 to ’64. But Phelps’s journey to London wasn’t without impediments, as his motivation and training post-Beijing have been scrutinized.

“It’s impressive that he’s here at all . . . that he would feel desire to come back and lay himself out there,” said his coach, Bob Bowman.

After winning a record eight gold medals in Beijing, Phelps is scheduled to compete in seven swimming events in London, beginning with Saturday’s 400-meter individual medley, one of two events in which he and Lochte will meet head-to-head.

Like Bolt, Phelps might seem beatable, but also like Bolt, he’s not going to London aiming for silver.

“I feel like I’ve set myself up to do something that I can look back at when I retire in 20, 30, 40 years down the road and say, ‘That was a pretty good career,’ ” Phelps, 27, said recently.

‘Going through the motions’

Following Beijing, Phelps was easier to spot on gossip Web sites than he was in any swimming pool. At the 2010 national championships, he took gold in just two of his five individual events. A year later at the world championships, he won two golds in four individual events.

“I actually just didn’t want to put in the work,” Phelps said. “There were times I didn’t come to practice. I wasn’t interested. It didn’t excite me. I was just going through the motions.”

Phelps will compete in four individual races and three relays in London. In each of those four individual races, his times at the Olympic trials that concluded July 2 were slower than his times from the 2008 trials.

“He wasn’t sharp here,” Bowman said this month in Omaha, where the trials were held.

In 2008, Phelps’s times showed marked improvement from the trials to the Summer Games, where he set four world records in the individual events. Observers say there could be a similar, perhaps even bigger, jump this year.

“I’m sure they will tell you that they were not holding back in Omaha,” said Rowdy Gaines, the three-time Olympic gold medalist and swimming analyst for NBC, “but I guarantee you, they were holding back. There’s no question in my mind.”

While seven gold medals for Phelps would be a difficult task, Gaines said he thinks the swimmer might medal in all seven events, likely still bringing home a handful of golds. Regardless, Gaines said, he’ll almost surely return from London as the most decorated Olympian ever.

Swimmer Natalie Coughlin, owner of 11 Olympic medals, called Phelps’s heavy London load “insane.”

“I’m still in awe of the fact that he came back to the sport after Beijing. . . . He’s under a tremendous amount of pressure,” said Coughlin, who’s appearing in her third Olympics, “and I don’t think I’d be able to handle that.”

‘He can be beat’

Bolt had no peers in the months and years that followed the Beijing Games. He set world records in the 100 and 200 meters with times once thought unreachable and blazed his way across the track like no one before him.

“Now that he’s lost, it actually makes it more interesting,” said Olympic historian David Wallechinsky, author of “The Complete Book of the Summer Olympics.” “I think that it will be an electric atmosphere. . . . We don’t know really what is going on with Usain Bolt. Was he injured in a car accident? Is he sandbagging? We don’t know.”

The track world knows this much: The 25-year-old Bolt is not the favorite in London. Blake, 22, won the world championship in 2011 after Bolt’s false start, but the younger man solidified his standing in the sport at the Jamaican trials last month, when he bested Bolt in the 100 and the 200.

“It just proves that he can be beat,” said Wallace Spearmon, one of the top American challengers in the 200.

In Kingston, Bolt had his hamstring stretched following his loss in the 200. He was scheduled to run the 200 at the Monaco Diamond League meet Saturday but pulled out to have treatment before London.

“I think I am a little bit weak, but three more weeks should be good enough to get back into shape,” Bolt said at trials.

Blake, meantime, won a race last week in Switzerland with a time of 9.85 seconds and appears to enter these Olympics in peak shape. His time of 9.75 that beat Bolt at the trials was his personal best in the 100 and his 19.26 in the 200 last September is the second-fastest ever run — trailing only Bolt’s 19.19.

“The question now is, how does Bolt race with somebody he knows he has to run down, somebody he has not always been able to catch?” said Boldon, who sprinted his way to four Olympic medals for Trinidad and Tobago in 2000 and ’04. “That, to me, is the biggest psychological baggage from trials. . . . It’s straight from his nightmares.”

Repeating in the Olympics isn’t easy for sprinters. Only one man has ever won gold in the 100 meters in consecutive Olympics: Carl Lewis in 1984 and ’88. Bolt is trying to become the first Olympian to win the men’s 200 in consecutive Olympics. Bolt has always been more dominant in the 200, flying around the curve like a slingshot.

Blake has spent the past several years with an up-close view of the rewards and accolades that accompany the title of World’s Fastest Man.

“The attention, the money, the adulation, all the trapping and trimmings — he sees it every day,” Boldon said. “A part of him sees that and says, ‘I want all of it.’ So in a lot of ways Usain Bolt is responsible for creating the young cub — and now it’s biting him in the butt.”

What’s next?

For Bolt and Phelps, these Games represent chances to add to their legends. Bolt will be 29 during the 2016 Games, so London might represent a final chance to fend off the younger runners.

“If it is over in London — and maybe it isn’t — but if it is, it’s been the best run ever,” Boldon said. “There’s no doubt he separated himself from Jesse [Owens], Carl and everybody else.”

Phelps already has begun talking about what he wants to do following London. He keeps saying he’s finished with swimming. He wants to travel the world, to see everything he missed during a lifetime spent underwater.

Phelps will be 31 when the Summer Games hit Rio de Janeiro in 2016, which gives him four years to gauge his competitive fire.

“There’s no doubt in my mind, we haven’t seen the last of Michael,” Gaines said. “I think he will take two years off and realize this is what he’s good at. He’ll miss the competition. It’s really hard to walk away, especially knowing you have more good years left in you.”

Staff writer Amy Shipley contributed to this report.