All outside scenes were dubbed and re-recorded in ADR due to massive wind noise caused by shooting near downtown San Francisco.
Darren Holmquist wanted to change the martial arts trope of setting up a technique to be used later (in this case the 540 kick) normally used to turn the tide of the fight by instead having in not working in the opening of the final fight. Used to bring tension as well as show CJ is still not at the level of Gavin. Gavin used that kick to actually defeat JB earlier in the film.
Each of the 3 main characters uses kickboxing mixed with another martial art. Gavin is kickboxing mixed with Taekwondo. CJ is kickboxing mixed with Capoeria. JB is kickboxing mixed with Mixed Martial Arts (MMA).
Darren Holmquist suffered a lot of bullying growing up and that formed a lot of the story however he added a family dynamic he didn't experience but based off of some of his friend's experiences.
The final scene from the fight to the closing shot took 4 shoot days. Two of them were spent on the fight scene.