The 12-year-old - son of Hollywood actors Will Smith and Jada Pinkett Smith - stars opposite Jackie Chan in the remake of the 80s classic, as a young boy who moves to China and is bullied by a kung fu gang, and falls for a classmate, Mei Ying.
Jaden said: "The worst I got hurt was in a dialogue scene - I actually was just walking up to Mei Ying, it's the first time I ever see her, and boom, my knee was just killing me, I was like, what the heck, you know? I had to ice it and everything."
Jaden revealed: "My dad had the idea of remaking it. He's the one who was like, 'We need to make this movie'."
Will told the ShoWest audience: "It's very scary when you go back and fiddle around with a classic."
Martial arts star Chan teaches his young prodigy kung fu in the movie but Jaden said the filmmakers kept the original title out of respect for their source material and because The Karate Kid has such name recognition among audiences.
He also revealed Chan taught him to, "stay fast and stay focused".