Why JK Simmons has been on the gains

Fitness fanatic JK Simmons is seen working out in a photo taken by his personal trainer. Photo/Instagram

Academy-Award winning actor JK Simmons is undergoing a serious body transformation for his next role.

The 61-year-old, who won his first Oscar in 2014 for his supporting role in the film Whiplash, is believed to be hitting the gym hard for his upcoming role as the police chief of Gotham in Warner Bros' Justice League.

Simmons' trainer, former Marine Aaron Williamson, has uploaded a few snaps of the actor in action to his Instagram in recent weeks, and the ridiculously buff pics put men half Simmons age to shame.

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Every time we train together I forget he's 61 years old. So much respect for this guy. #JKSimmons #OldManCranking

A post shared by Aᴀʀᴏɴ Wɪʟʟɪᴀᴍsᴏɴ (𝐔𝐒𝐌𝐂) (@aaronvwilliamson) on Jun 2, 2016 at 8:52pm PDT

Williamson, a fitness trainer with many celebrity clients, told The Hollywood Reporter he first started working with Simmons when the actor was filming last year's Terminator Genisys.

"For Terminator, he wanted to just keep himself healthy while filming. And somehow during the course of him getting ready for Whiplash he wanted to take things to the next level," says Williamson, who notes that Simmons has another trainer in Los Angeles.

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Saturday morning, 61 years old & making the young generation jealous. This is what dedication looks like. #JKSimmons

A post shared by Aᴀʀᴏɴ Wɪʟʟɪᴀᴍsᴏɴ (𝐔𝐒𝐌𝐂) (@aaronvwilliamson) on Apr 23, 2016 at 3:06pm PDT

"He started to fall in love with the fitness lifestyle. Where I come into play is now that he's at this point we are giving him muscles he didn't have before.

"His motivating factor with a lot of it is to do something with his arms and shoulders to be able to possibly down the road play a character who can wear a tank-top."

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He's making me look bad. #JKSimmons #AintPlayinNoGames

A post shared by Aᴀʀᴏɴ Wɪʟʟɪᴀᴍsᴏɴ (𝐔𝐒𝐌𝐂) (@aaronvwilliamson) on Apr 30, 2016 at 10:15am PDT

And Simmons' secret to achieving incredible rippling muscles in this, his 61st year?

"A lot of low-carb stuff, lean proteins, a lot of vegetables. Every day might be a little different for him. He eats every three hours.

"What he calls his "secret weapon" is a little piece of dark chocolate. He eats one square every day," says Williamson.