LOS ANGELES – Close your eyes. Picture Mr Mark Zuckerberg. What do you see?

Perhaps you envision the Meta boss and Facebook founder in a grey 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 is unlikely you conjure anything like the image Mr Zuckerberg posted on 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 has just finished a dangerous mission.

Mr Zuckerberg wrote in a caption that he had just done the Murph Challenge, a Memorial Day workout and fund-raiser named for Navy Seal officer Michael P. Murphy, who died in Afghanistan in 2005. The challenge consists of a 1.6km run, 100 pull-ups, 200 push-ups, 300 squats, capped with another 1.6km run – all while wearing a vest that weighs about 9kg.

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

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

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

“He looks great,” said venture capitalist Sam Lessin, former Facebook vice-president and long-time 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 jiu-jitsu.”

That would be Brazilian jiu-jitsu, the gruelling grappling-based combat sport that is a fundamental part of mixed martial arts. In an August 2022 appearance on The Joe Rogan Experience podcast, 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,” he told Rogan at the time. “There’s something that’s just so primal about it.”

According to cultural critic Casey Johnston, who writes frequently about exercise and strength training in her newsletter She’s a Beast, sports such as Brazilian jiu-jitsu appeal to a desire among Silicon Valley types to reconnect with a primitive fighting spirit, but on their own, highly formalised 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 jiu-jitsu event, in Woodside, California, where he defeated an Uber engineer and won two medals, and lost consciousness. Jose Lucas Costa da Silva, a veteran Brazilian jiu-jitsu fighter who refereed one of Mr Zuckerberg’s matches, said that he halted the bout after he heard Zuckerberg start to snore, a sign of someone who has passed out in a chokehold.

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

After this article was published on Friday afternoon, Mr Zuckerberg and Camarillo reached out to say the Meta boss had not lost consciousness.

“That never happened,” Mr Zuckerberg wrote in an e-mail. According to Camarillo, Costa da Silva mistook his student’s effortful grunting for snores.

“This was his first tournament, but it didn’t seem like it,” said Danny Patalot, chief executive of BJJ Tour, which organised the day-long 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 lay-offs – it is perhaps a revealing time to start fighting back.

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

Though he entered the popular imagination as an indoor cat – the millennial arch-nerd 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 Lessin, Mr Zuckerberg goes through phases with new sports, including roller hockey and surfing, a hobby that was immortalised 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,” Lessin said.

Famously competitive, Mr Zuckerberg is the “great physical instigator” of his friend group, 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 Rogan’s podcast, Mr Zuckerberg said he had successfully encouraged many of his friends to train in Brazilian jiu-jitsu 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 5km in under 20 minutes. And though many commenters were skeptical of his Murph Challenge time, he 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 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. NYTIMES