Close your eyes. Picture Mark Zuckerberg. What do you see?

Perhaps you envision the Meta boss and Facebook founder in a gray T-shirt, spotlighted on a Silicon Valley stage. Maybe his cartoonish metaverse avatar hovers before you — with legs, or without them. Possibly Jesse Eisenberg, the bony actor who played the fledgling tycoon in “The Social Network” (2010), appears in your mind’s eye.

But it’s unlikely you conjure anything like the image Mr. Zuckerberg posted to Instagram (which Meta owns) and Facebook on Monday.

The mirror selfie shows the social-media billionaire wearing a camouflage flak jacket while glistening faintly with sweat. His neck swells wider than his jaw. His shoulders are capped with muscle. His forearms bulge.

He looks — to use a scientific term — completely shredded. He also looks completely focused, like a guy in a Michael Bay movie who just finished a dangerous mission, or at least the Raya profile picture of the actor who plays that guy.

Mr. Zuckerberg wrote in a caption that he had just done the Murph Challenge, a Memorial Day workout and fund-raiser named for the Navy SEAL officer Michael P. Murphy, who died in Afghanistan in 2005. The challenge consists of a mile run, followed by 100 pull-ups, followed by 200 push-ups, followed by 300 squats, capped off with another mile run, all while wearing a 20-pound vest.

Just as eye popping as Mr. Zuckerberg’s arms was the time he said the brutal routine took him: under 40 minutes. That’s an elite time according to Dr. Joshua Appel, the Veterans Affairs physician who helped popularize the workout, which has become a staple in CrossFit gyms.

So when, exactly, did Zuck decide to go Rambo? And why?

Mr. Zuckerberg did not respond to a request for comment about his physical fitness.

“He looks great,” said Sam Lessin, a venture capitalist, former Facebook vice president and longtime friend of Mr. Zuckerberg’s. “But I don’t think he’s doing it for looks. I think he’s doing it because he’s really into jujitsu.” (Though Mr. Zuckerberg’s photo suggests that he may be feeling himself, at least a little.)

That would be Brazilian jujitsu, the grueling grappling-based combat sport that is a fundamental part of mixed martial arts. In an August 2022 appearance on “The Joe Rogan Experience,” Mr. Zuckerberg said that he had taken up martial arts during the pandemic, training with Dave Camarillo, a well-known coach in the Bay Area.

“The crazy thing is it really is the best sport,” Mr. Zuckerberg told Mr. Rogan at the time. “There’s something that’s just so primal about it.” (Mr. Rogan is a martial arts enthusiast and Ultimate Fighting Championship commentator.)

According to Casey Johnston, a cultural critic who writes frequently about exercise and strength training in her newsletter She’s a Beast, sports like Brazilian jujitsu appeal to a desire among Silicon Valley types to reconnect with a primitive fighting spirit, but on their own, highly formalized terms.

“It’s like being on a playground with a bully but in this new framework,” she said. “It’s not quite choreographed but the stakes and the rules are unambiguous.”

On May 6, Mr. Zuckerberg competed in his first Brazilian jujitsu event, in Woodside, Calif., where he defeated an Uber engineer and won two medals, and lost consciousness. José Lucas Costa da Silva, a veteran Brazilian jujitsu fighter who refereed one of Mr. Zuckerberg’s matches, said that he halted the bout after he heard Mr. Zuckerberg start to snore, a sign of someone who has passed out in a choke hold.

“This is something we are trained to know,” said Mr. Costa da Silva, who added that Mr. Zuckerberg was a good sport who was “enjoying the moment.”

“This was his first tournament, but it didn’t seem like it,” said Danny Patalot, the chief executive of BJJ Tour, which organized the daylong event at a local high school. “He was aggressive using technique. There was no fear. That’s not typical for a white belt competitor at a first tournament.”

For Mr. Zuckerberg, who has absorbed a number of metaphorical body blows over the past several years — including an election meddling scandal, a ghost town metaverse and widespread layoffs — it is perhaps a revealing time to start fighting back.

Mr. Zuckerberg is, of course, not the first tech titan to add significant amounts of muscle. Jeff Bezos, the Amazon founder and chairman, has, in his 50s, developed remarkable biceps, linebacker shoulders and impressive vascularity. But some of Mr. Zuckerberg’s friends say the new regimen is not exactly a surprise.

Though Mr. Zuckerberg entered the popular imagination as an indoor cat — the millennial archnerd outsmarting the strapping regatta princes of Harvard — the 39-year-old has been devoted to physical activity for much of his life. He was the captain of his prep school fencing team and became an avid runner in the 2010s.

According to Mr. Lessin, Mr. Zuckerberg goes through phases with new sports, including roller hockey and surfing, a hobby that was immortalized in an image of the magnate afloat in Hawaii, a haunting mask of sunscreen slathered on his face.

“He sees them as super-fun side quests,” Mr. Lessin said.

Famously competitive, Mr. Zuckerberg is the “great physical instigator” of his friend group, Mr. Lessin said, and frequently proposes physical challenges. “We’ve been on trips where he’ll say, like, ‘I’ll race you up that mountain.’” And on Mr. Rogan’s podcast, Mr. Zuckerberg said that he had successfully encouraged many of his friends to train in Brazilian jujitsu with him.

Perhaps unsurprisingly, Mr. Zuckerberg has a passion for benchmarking. In April, he posted a photo of a stopwatch appearing to confirm that he had run five kilometers in under 20 minutes. And though many commenters were skeptical of his Murph Challenge time, Mr. Zuckerberg clarified that he had done a special, easier-but-still-painful version, in which the exercises are performed in circuits.

Also performing a modified version of the Murph challenge: Two of Mr. Zuckerberg’s three daughters. He posted a picture of the girls mid-push-up, in the midst of a “quarter-Murph (unweighted)” — a clarification that the children were not wearing flak jackets.