Joel Edgerton tore his MCL in the cage during production, halting fight scenes for six weeks. Tom Hardy suffered a broken toe, broken ribs, and a broken finger.
Anthony Tambakis and Gavin O'Connor selected the song "About Today" by The National to close the movie, before writing the final scene. The scene was written with the song playing on a continual loop at O'Connor's house while the writers worked.
The role of the promoter, played by director Gavin O'Connor, was originally written for TapouT, founder "Charles 'Mask' Lewis." Lewis was killed by a drunk driver just before shooting began. O'Connor spoke at Lewis' memorial service on April 14, 2009, six days before principal photography began. The film is dedicated to him.
While filming in Pittsburgh, Gavin O'Connor, Anthony Tambakis, Tom Hardy, Joel Edgerton, and Frank Grillo all lived together at the Cork Factory Lofts.
When the announcer is quoting Koba's credentials, he states that he is an Olympic Gold medalist in wrestling. Kurt Angle, the actor playing Koba, won a gold medal in the 1996 Olympics in the 220lbs/100 kg weight class for freestyle wrestling. He was also a world champion in the same style and weight class in 1995.
On the first day of shooting, the crew gave Nick Nolte a standing ovation after the first take of a scene shot at a local diner. The scene was later cut but appears as a DVD extra.
Instead of going to his hotel after his flight from England, Tom Hardy appeared at director Gavin O'Connor's door at midnight the evening before his audition. The pair ended up living together for five days.
Frank Grillo based his character on famed MMA trainer Greg Jackson. Grillo and Joel Edgerton trained and lived with Jackson at his New Mexico gym during pre-production. All of Grillo's fight scene dialogue while "cornering" Edgerton was suggested by Jackson.
The role of Paddy was written for Nick Nolte by Anthony Tambakis and Gavin O'Connor, who are neighbors with the actor in Malibu. The studio was resistant to casting Nolte, but the writers held firm and Nolte's portrayal has won him global critical praise.
Actual UFC fighters Anthony Johnson and Nathan Marquardt each play the role of Sparta competitors.
Brendan Conlon may be loosely based on the life of real-life UFC fighter Rich "Ace" Franklin. Like the character, Franklin was also a high school teacher who took part in cage-fights to make extra money.
The footage shown of Tommy Riordan wrestling in high school is actually high school footage of Pennsylvania high school standout, and later NCAA Division I champion and US Olympic team member Cary Kolat. Kolat was also unbeaten during his high school career as it is suggested Tommy was.
The location for the scenes in Iraq was actually an abandoned parking lot on the outskirts of Pittsburgh, dressed to look like the desert by production designer Dan Leigh.
Koba's entrance song, "The Fastidious Horses," is one of the trademark songs of the Soviet cult singer-poet Vladimir Vysotsky.
An alternate opening shot at Moundsville State Prison in West Virginia, and featuring Tom Hardy's character cage-fighting in jail, was cut from the film.
The character "Koba" seems to loosely resemble real-life Mixed Martial Artist Fedor Emelianenko, who was the #1 ranked Heavyweight in Mixed Martial Arts for over seven years and was considered unbeatable. He also was Russian and a Sambo Champion multiple times over. Moreover, there is even a physical resemblance, as Fedor was white, bald, and about Koba's size. However, the name "Koba" was a moniker for Josef Stalin. Emelianenko's reputation as a professional athlete, as well as his character, are highly positive, and his physique is pudgier than that of Angle's, so the similarity is only in their professional achievements.
The original script location for the Conlon family was Long Beach, CA. It was moved to Pittsburgh due to Pennsylvania tax breaks. The scenes set at Boardwalk Hall in Atlantic City were originally written for the MGM Grand in Las Vegas. Gavin O'Connor moved the action to New Jersey to fit with the gritty east coast aesthetic of the film.
Olympic gold medalist Kurt Angle played the Russian fighter "Koba," whose nickname was taken from the moniker given to "Josef Stalin." In the original script, "Koba" was named "King Kong," but it was changed for legal reasons. The brother of Gavin O'Connor's assistant, Samantha Ellison, suggested the name "Koba."
One of the commentators references Kurt Angle's character, Koba, as being a World Sambo Champion. Sambo is a martial art known for leg locks. Angle employed an ankle lock as a signature move during his wrestling career.
Writer Anthony Tambakis portrays a fight official in the film, and Gavin O'Connor plays the fight promoter. O'Connor agreed to play J.J. Riley only after his friend and co-writer Tambakis agreed to make a cameo himself.
Sadly, this was the second sports movie that Gavin O'Connor directed in which a main influence was tragically killed in a car accident during production. Herb Brooks died in a car accident during the making of "Miracle" (2004), and Charles "Mask" Lewis was killed by a drunk driver just before the making of "Warrior" (2011).
The film is dedicated to Charles 'Mask' Lewis. If you look in the background of Frank Campane's Gym you can see a quote written on the board from 'Mask' as a tribute
Co-writers Anthony Tambakis and Gavin O'Connor were introduced by comedian Bryan Callen, who plays himself in the movie alongside writer Sam Sheridan. Callen dated director Patty Jenkins for 9 years. She is now married to Sheridan.
Brothers (2015), a Bollywood movie, is a remake of this movie. It became a blockbuster in India.
Frank Grillo and Maximillian Hernandez would go on to act together in Captain America: The Winter Soldier (2014)
Frank Grillo, who plays MMA trainer Frank Campana, and Bryan Callen, who plays an MMA commentator, later appeared together in Kingdom (2014), where they played main character, MMA trainer Alvey Kulina, and MMA promoter Garo Kassabian respectively.
North Hills High School where Brendan Conlon works is actually located in Pittsburgh and not in Philadelphia.
This film stars both Maximillian Hernandez and Noah Emmerich who would go on to play FBI partners in The Americans (2013).