Gunnar Peterson praises Amber Heard's work effort during her hours of training for Aquaman

Amber Heard Trained Multiple Hours a Day for Aquaman: 'She Is a True Athlete'

Amber Heard fully committed to her role as Mera in the upcoming Aquaman movie, spending hours in the gym to fit the part.

“She wanted to deliver for the character,” Gunnar Peterson, one of the multiple trainers Heard worked with during filming, tells PEOPLE.

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Peterson trained Heard, 31, four to five times a week, “for a non-stop, no-breaks hour with me, and THEN she went to her fight training, which was rigorous!” he says.

They focused on full-body fitness, similar to what Peterson features in his videos for Now health products, for Heard to fit the sea queen role — and to fit into her costume.

“Everything was athletically based. We trained movements, not muscles,” Peterson says. “Squat presses, sled work and lots of work in a rotational plane against resistance. She is a true athlete.”

Peterson adds that their workouts were “unrelenting,” but Heard handled it with ease thanks to her dedication.

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“She was so consistent,” he says. “Without her being so committed, the results would not have happened.”

And he says that Heard had the perfect attitude for getting the work done.