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.”