The Conan the Barbarian actor had to swing a prop weapon at his co-star and told Men's Health that he was careful not to hit him.
"At the end of Bullet to the Head I had to swing this wooden axe at top speed right at Stallone and you don't really want to clip him. He's got these slabs of meat for fists," Momoa said.
Of emulating another action superstar Arnold Schwarzenegger as Conan, he added: "I wanted Conan to be like a big cat.
"I wanted to be strong but agile. They hunt and they eat. That's why they're big."
Asked about how he builds his strength, Momoa said: "I'm not a big fan of just doing weights.
"Anything more physical is infinitely better, whether that's working on my house or hitting at the holes in my driveway with a pick."
He continued: "You know when you're working out but thinking about the sh*t you've got to do for the rest of the day?
"Well your workout is going to be half-arsed. Trust me - I'm ADD as hell."
Momoa explained: "You need to eat big and lift big. But I prefer to eat as much lean meat and green veg as possible and save the calories for Guinness. It keeps you sane.
"I love cigars and booze. If you're going to pound the beers you've got to suck it up. It [the treadmill] gets you going, man. Then you can have that beer."
Read the full Jason Momoa interview in the Men's Health March issue - on sale Friday, February 1. Also available as a digital edition.