Charlie Hunnam's "gonads felt horrible" after being strapped into a harness for Pacific Rim.
The actor stars in the adventure sci-fi, which also features Idris Elba and Charlie Day.
As part of the film, Charlie and the cast had to shoot in giant robots, which proved hazardous for the actor's health.
"We had to wear this huge suit while working out on this elliptical machine with high resistance," he explained to British magazine Star.
"The suit weighed 30lbs., was hot and sweaty and pinched. Out feet were strapped in and we had to wear a harness. And after wearing a harness for 14 hours, your gonads start to feel horrible!"
The action epic is directed by Guillermo del Toro, who is also behind the Hellboy franchise.
Rather than sympathise with his pained actors, the moviemaker took a relaxed approach to their suffering.
"He would be up there in this big chair and on the bullhorn when I was struggling, saying, 'I know Charlie, I am suffering, too. My cappuccino got cold!'" he laughed.
The actor sports an incredibly toned physique, which helped win him the role of Christian Grey in Fifty Shades of Grey, which he recently had to give up due to "family stuff".
Charlie isn't afraid to work hard in order to play muscley characters on screen.
"While I was shooting [TV show] Sons of Anarchy I told myself that I had to do 300 push-ups through the course of the days we were shooting. Then, I'd go to the gym every night after work and at weekends learn stick fighting!" he listed.
"Guillermo was like, 'I need to you to learn how to stick fight,' which was music to my ears. Those were gruelling long days, but fun to shoot."
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