Amber Heard takes on 'relentless' athletic training for Aquaman

Actress Amber Heard is training like a professional athlete to get in shape for Aquaman.

The blonde beauty portrays Mera, the queen of the undersea Atlantis kingdom, in the forthcoming Marvel movie and to physically prepare for her role she trains four to five times per week with celebrity fitness guru to the stars Gunnar Peterson, who describes her workout sessions as "relentless".

"(Amber worked out) for a non-stop, no-breaks hour with me, and then she went to her fight training, which was rigorous!” Gunnar explained to People, noting she worked with multiple trainers in preparation for the superhero action film.

“She wanted to deliver for the character,” the fitness expert added.

Heard, 31, focused on full-body fitness during her sessions and Peterson is deeply impressed with her stamina and strength.

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

Gunnar was also struck by Amber's commitment to the arduous exercise, revealing she never slacked off even for a moment.

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

Gunnar went on to joke he wishes all of his celebrity clients had Amber's tenacity when it comes to working out.

“She could not have been better!” he gushed. “If I could bottle her drive and conviction I’d sell that as a pre-workout drink!”

Aquaman, which also stars beefcake Jason Momoa in the titular role, hits theaters on 21 December, 2018.