Usain Bolt says a bet with friends that he'll 'get fat' motivates him to keep working out in retirement

• Usain Bolt still works out religiously even though the esteemed sprinter retired four years ago.
• The 8-time gold medalist tries to stay fit because his friends bet he'd "get fat" in retirement.
• Bolt told Insider the bet originated because "I always bragged about my abs when I was competing."

Usain Bolt hasn't competed on the world stage in four years, but he's still working hard in the gym.

With eight Olympic gold medals, multiple world records, and the unofficial title of "World's Fastest Man" under his belt, it's fair to wonder why the Jamaican sprinter still won't take a break now that he's retired. But Bolt still has something to prove: that he can stay in shape.

"I've always tried to stay fit because I have a bet with my friends," Bolt told Insider. "They were like, 'As soon as you retire, you're going to get fat.' And I was like, I can't let them win this."

"So I've been trying to do my best to stay in shape," he added.

Bolt said that the bet originated "because I always bragged about my abs and stuff when I was competing." And even though he's motivated to prove his friends wrong, he knows that there are lots of other merits to exercising and taking care of his body.

"I'm enjoying it," Bolt said. "It's good to stay fit. It's good for your mental health also. It's something that helps me to stay focused and just be happy."

He has multiple workout options to choose. Bolt is an athlete ambassador for Peloton and says that the brand's cycling classes really help him with his fitness "because it's just right there beside my bed. When I wake up, I can just get on my Peloton and go."

Bolt also has a new challenge fueling his fitness routines. Bolt has been training for a half-mile race against a CarMax agent as they complete an appraisal with one of their clients. The company claims the entire process takes less than two minutes, and Bolt will put that assertion to the test for the whole world to watch on Facebook Live Tuesday, July 13.

Bolt is taking his CarMax challenge more seriously than you might expect. He's been getting out to the track at least twice a week to train, he says, as well as working on specific exercises "to get my lung capacity up."

"They told me they're one of the fastest appraisals for car companies online now - in two minutes, you can be approved," Bolt told Insider. "So I was like, 'Okay, cool.' And they're like, 'we'd like you to hit 800 meters.' I was like, 'Now that's interesting.'"

"I've never done something like this before, and for me, I love a challenge," he added. "So I'm very hyped to compete."