Hunnam did one thousand push-ups a day in preparation for King Arthur: Legend of the Sword.
The 37-year-old actor plays the titular role in Guy Ritchie's medieval fantasy film, which also stars David Beckham, Jude Law and Djimon Hounsou, and the American hunk has claimed he would complete multiple sets of the common calisthenics exercise to get in shape for the movie, although "some days" he had to do much less after a long day on set.
The British heartthrob told E! News: "I mean there were some days I didn't hit the goal and I would try to recoup the next day."
"On the days where I was fighting 14-hours a day, I let myself off and went home, and said 'Okay, let's knock a zero off this equation - 100 is good enough for today."
However, the 48-year-old filmmaker believes Charlie is not being entirely truthful, and he has revealed the Sons of Anarchy star can only do a slight fraction of his whopping 1,000 and will take a "half an hour break" in between.
Guy explained: "I got to tell you, 1000 push-ups? Not so much. You'll get 50 out of him at the most! And there's like a half an hour break."
Charlie has also admitted he feels "happier, more energised and disciplined" when he's exercising.
Speaking previously about his workouts, Charlie - whose full name is Charles - said: "It's important for me to keep moving. We're supposed to be active animals. It's in our DNA. Sweating is how I change my oil."
"I just feel happier, more energised and disciplined if I work out.
Ultimately I train a lot every day because I'm f***ing crazy."
King Arthur: Legend of the Sword is set to hit the big screens this month.