Heading into production on Avengers: Infinity War, where he would be portraying Norse god Thor for the seventh time, Chris Hemsworth found himself in a difficult situation. Filming of his military drama, 12 Strong, was taking longer than expected and was going to overlap with the upcoming Marvel movie.

Fortunately, the Australian leading man—and former Men’s Journal cover guy—had a secret weapon with him on set: his longtime trainer Luke Zocchi, who was able to lead him through a program that would allow him to size up quickly. “I needed to make sure that I wasn’t burning him out because he was working long hours every day,” says Zocchi. “Most of the work that we do to get Chris bulked up for Thor is done in the months before being in front of the cameras.”

The unique scenario meant that Hemsworth and Zocchi had to rely on the foundation that they had already built with his core. “There was core training in every session that we did,” Zocchi says. “People think that the core is all about your abs, but it is so much more. There is your back, glutes, and your posterior chain. I believe in functional exercises. I love bear crawls, planks, and more dynamic crunches. I want him to move more organically.”

That functional mindset began while preparing for Avengers: Age of Ultron, which is when Zocchi first started working with Hemsworth. They had met years before at a boxing gym. “Chris told me that even though he was big in the first Thor movie, he didn’t feel like he was moving well,” Zocchi says. “So we transitioned from his purely bodybuilding program to a more full-body thinking. The weights were still there, but they were just a part of the program, and not the entire program.”

For Zocchi, training the core would not just get Hemsworth looking stronger and leaner on camera, but also help him through his stunt-intensive scenes. That success also trickled into his day-to-day life, allowing the actor to better enjoy things like surfing and Muay Thai. “He loves to kickbox, so that’s incorporated into his training,” Zocchi says. “Occasionally, I have to hold the pads for him, and I really need to brace for his kicks, because he will knock me right over.”

Here, a core-variation via Zocchi:

Avengers: Infinity War is now playing in theaters.

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