For years, many have wondered why Richard Simmons, the flamboyant fitness guru and talk show fixture, had abruptly retreated from public life.

After decades in the spotlight, the 74-year-old suddenly stopped teaching at his workout studio in 2014. He was last seen publicly over eight years ago. Simmons’ disappearance sparked a podcast titled "Missing Richard Simmons," as well as numerous conspiracy theories alleging the star was being held hostage in his Hollywood Hills home by his housekeeper.

In 2017, Simmons sued two tabloid publications that alleged he was transitioning into a woman. Attorneys for Simmons have said he supports transgender rights, but the allegations have defamed him.

Richard Simmons

Richard Simmons was the subject of a TMZ special that aired Monday night on FOX. (Harry Langdon/Getty Images)

On Monday night, FOX aired "TMZ Investigates: What Really Happened to Richard Simmons," which explored his rise to stardom and surprising departure from the public eye. It featured interviews with Denise Austin, Suzanne Somers, as well as pal Bruce Vilanch and Simmons’ longtime costume designer Leslie Wilshire.

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According to the doc, there’s a surprisingly simple reason why Simmons retreated from the spotlight.

"Doctors told him back in 2014, around the time he disappeared, that he needed a left knee replacement," the doc alleged. "And if he didn’t get one, he might never be able to exercise again. Now, this is key to the disappearance of Richard Simmons. The prospect of surgery, of becoming sedentary, caused Richard to sink into a deep depression… We know still to this day Richard still hasn’t gotten corrective surgery on his left knee. He walks with a cane and that explains a lot. He’s just not the same guy anymore."

Richard Simmons disappearance

Numerous conspiracy theories concerning Richard Simmons' whereabouts have been offered over the years. (Ray Tamarra/Getty Images)

In 2014, eight months after Simmons left the public eye, the outlet claimed that the star had "two bad knees." His operation on the first was so painful that he allegedly was delaying the second.

In 2017, a woman alleging to have known Simmons for 35 years told the outlet that knee problems have kept Simmons in "virtual hiding."

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The documentary claimed that Simmons delayed his second surgery because he was worried about being able to bounce back after the procedure and whether it would worsen any ailments.

Richard Simmons fitness

Richard Simmons, a beloved fitness guru, was a talk show fixture at the height of his career. (Moses Robinson/Getty Images)

"He wanted to be remembered as a vibrant, healthy man," the doc claimed, "not an elderly man with medical problems. Richard wanted to retreat before his image was recast as an old man. And his knee problems were a huge factor in his decision… He had a right knee replacement a few years back. And still needed a left one. He was in a lot of pain."

The special also alleged that there is one LAPD detective, who didn’t want to be identified, but who has talked to Simmons over the years. According to him, Simmons "doesn’t want to be remembered as this frail, old guy. He wanted to be remembered as a guy that millions of people loved and watched. That’s why he disappeared."

Simmons was compared to Greta Garbo, the Oscar-nominated star of the ‘30s who was fiercely protective of her privacy. She retired at age 35 and moved to New York City, where she led a private life until her death in 1990 at age 84. The special shared that "the Richard who disappeared is more authentically himself" than his "loud" persona.

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Richard Simmons doc

Richard Simmons in 2013. Before he retreated from the public eye in 2014, Simmons would greet tour buses that stopped by his Hollywood Hills home and personally call fans to support their weight loss efforts. (David A. Walega/WireImage)

"He has cleverly figured out ways to live in plain sight," it alleged. "He’s given up public life entirely… Richard is a master of disguise."

Still, Simmons has endured some heartache. Pals shared that his beloved eight Dalmatians have passed away over the years. His last dog died at age 17. Still, his housekeeper is "a fiercely loyal employee and friend."

Somers described how she noticed Simmons may have had some insecurities before he retreated entirely.

Suzanne Somers

In the special, Suzanne Somers spoke about her last interaction with Richard Simmons. (David Crotty/Patrick McMullan via Getty Images)

"One night I was on the ‘Larry King Show,’ and he was either going to be a guest or supposed to be a guest, and I heard through the Larry King people that he didn't want to be on the show with me," the "Three’s Company" star recalled.

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"I said, 'Why?' and he said, 'She'll make fun of me,’" the 75-year-old shared. "And that's not my style, I never make fun of anybody. But that's when I realized a little insecurity has gotten in there. I liked him. I was always so surprised when he thought I'd make fun of him. But doesn't that show you he's got a heartache? Something's broken inside because he did it. He had it and then let it go."

A spokesperson for Simmons told Fox News Digital on Tuesday they had no comment about the allegations made in the doc.

Richard Simmons dogs

According to the special, Richard Simmons' beloved eight Dalmatians have passed away over the years. (Ron Eisenberg/Michael Ochs Archives/Getty Images)

In 2021, Simmons revealed that he hoped a new generation of fans would be introduced to his classic exercise series, "Sweatin’ to the Oldies." At the time, it was announced that Fuse Sweat, a streaming channel, would include Simmons’ show in its line of programming.

"I am very excited that my groundbreaking fitness series of ‘Sweatin’ to the Oldies’ still is so relevant and popular and I hope many new fans will discover this timeless classic," he said in a statement.

“TMZ Investigates: What Really Happened to Richard Simmons” will stream for free on Tubi starting Aug. 30.